Year 11 GCSE Physics Unit 1 - WordPress.com 11 GCSE Physics Work, Power and Efficiency 1.2.4 recall,...

Year 11 GCSE Physics

Work , Power and Effic iency 1.2.4 recall, understand and use the equation

useful output energy Efficieny

total input energy

Unit 1

1.2.5 describe and explain various ways of making b e t t e r use of energy; 1.2.6 review primary and secondary sources relating t o the effic iency of

domestic appliances (w - (ii)b); and 1.2.7 recall and use the equation

work = force x distance and t h a t t h e work done equals the amount of energy transferred .

1.2.8 recall and use the equations

Power = energy transferred

time taken and Power = work done

time taken

t o calculate power, work done, t ime taken or energy t ransferred ; 1.2.9 plan and carry out experiments t o measure personal power and the

output power of an electric motor, and evaluate t h e validity and rel iabi l ity of the i r data (w - (i)a): and

Work When energy is changed from one form t o another, work is done.

Work Done = Energy Transfered

Work depends on two things: r l i ^ n n ^ ^ w o J g r i

pare -r-o

Work = Force x Dis tan ce

Where: Work is in Joules (J )

Force is in Newton's (N)

Distance is in meters (m)

dh

Complete t h e memory triangle

Example:

I f John l i f t s up a box of

apples weighing 120N a

distance of 1.25m, how

much work does he do?

= 110x1 .25)

91

Year 11 GCSE Physics Unit 1

pages 107-9

Work and energy Extension .Sheet

When a force moves an object, energy is transferred and work is done. Work done = energy transferred It is measured in joules. Work done = force x distance moved (in joules) (in N) (in metres)

Example A man lifts a parcel, mass 4 kg, from the floor to a shelf 2 m high. a) What is the weight of the parcel? b) How much work is done on it? c) Where does this energy come from?

Answer a) Weight = 4 kg x 10 = 40 N (see page 75) b) Work done = force x distance moved

= 40N x 2m = 80 joules (from his food)

500 N

Questions For each question show all your working clearly.

1. How much work is done in these situations: a) A man pushes a van against a friction ^

force of 300 N for 10 m. b) A mother pushes a pram with a force of

30 N for a distance of 100 m. 3cxz=> S c) A weight-lifter lifts a weight of 500 N

through a height of 2 m. I O O O J

2. A worker pushes a barrow at a steady speed of 2 m/s for 10 s, using a force of 100 N. a) How far did he travel? b) How much work is done? 2 o o o T c) Where does the energy come from? chsrr->icaS c-vfcjg-j

mSLcka. i_>-x̂ "t_c_/ 3. A boy with a mass of 60 kg climbs 10 m

vertically up a ladder. a) What is his weight? b) How much work is done? t y c o o n c) What are the energy changes here? ICeS -» Ci^E

4. An archer pulls back the arrow in his bow a distance of 0.5 m against an average force of 200 N. a) How much work is done? l o c t l b) What are the energy changes here? Kj= S ^ e l

A car is travelling along the road with 40 000 J of kinetic energy. The brakes are applied and it comes to rest in 20 m. a) Calculate the average braking force.

2 m

b) What happens to the kinetic energy? ^ _ cr^ 20 m

92

Year 11

Power

Power is

GCSE Physics Unit 1

I f you move a force of I N (which is the same as a mass of 1kg on Earth) a

distance of lm you will have done 1J of work. I f you do t h a t work in Is , you will

have done i t with a power of 1W.

Power = work done

time taken Complete t h e memory triangle

Where: Power is in W a t t s (W)

Work is in Joules (J )

Time is in seconds (s) Remember: Work Done = Energy Transferee!

Examples:

1. A crane does 5 0 0 0 J of work in 2 seconds. What is the output power?

p-. &**>A

2. Find the power of the man who pushes the box 8m with a force of 15N in a

6seconds. .

p . ^

p . i S x S .

93


W O R K A N D P O W E R

1. A cyclist moves along a flat road against a resistive force of 100K. If the cyclist travels 1000m calculate the work done by the cyclist.

ICO. QQqTJ.

Z. Ponna lifts a parcel of weight 100N onto a shelf that is 2m above the ground.

a) Calculate the work done in lifting the parcel onto the shelf.

b) What type of energy does the parcel gain? . QP£........

3. fit ear is driven up a mountain pass. Ft gains a vertical height of 300w. The weight of the car and its passengers is 10,000N.

a) Calculate the work done by the car against gravity.

3 CCCs. OCG.ZF.. b) What is the gain in potential energy of the car? 3 o c o .coo. .d

Matt cycles a distance of 2000m against a resistance force of 150N. He travels this distance in 400s.

a) Calculate the work done by Matt.

( 5 r % 3co..cxx>.J. .. b) What is Matt's power output?

. . " ^ t S a u J

5. The diagram shows a pumped storage system used to store water in a dam. a) Calculate the work done in pumping 10,000N of water

from the lower to the upper reservoir.

100m o . c o ..cm."J.

b) If it takes 10s to move 10000N of water from the top to the bottom calculate the power output of the pump.

lOO. ...CCO. LO.

6. The output power of a crane Is 1.6kW. Calculate how long tt will take to lift a load of 5000N through a distance of 8m.

b . s f . . . 2 S : • 9 4

Year 11 GCSE Physics U n i t l

page 118

Power Extension Sheet

Power = (in watts)

work done (in joules) time taken (in seconds)

or Power = energy transferred (in J) time taken (in s)

1 watt = 1 joule per second

Example A force of 100 N moves a distance of 5 m in 2 seconds. a) What is the work done? b) What is the power?

Answer a) Work done f force x distance moved (see p. 107)

- 100N x 5m = 500 J

,„ work done 500 J b) Power = = = 250 W

time taken 2 s


1. A boy does 500 J of work in 10 seconds, ̂ q^^ What is his power output?

2. A mother pushes a pram with a force of 30 N for a distance of 100 m in 50 s. What is her power output? t o c o

An electric lamp is marked 60 W. How much energy does it transfer a) in 1 second? b) in 100 seconds? i&oooZ What are the energy transfers here? eveeWicc^ -=> W«j*v

4. An athlete runs a 100 m race in 10 s against a friction force (drag) of 100 N. What is his power output?

5. A weightlifter lifts an object of mass 30 kg through a height of 2 m in 3 seconds. a) What is the weight of the object? 3co«o

(Hint: see p. 75.) b) What is the work done on the object? t o o t l c) What is his power output? '2oo<—>

6. A boy weighing 600 N runs up the stairs, a) in 3 seconds, and then Soo«-~> b) in 4 seconds. boo L - O The vertical height of the stairs is 4 m. What is his power output in each case?

7. A lift containing 6 people is raised through a height of 20 m in 10 s. The total weight of the lift and passengers is 6000 N. What is the power of the lift motor, a) in watts? '^ooci—> b) in kilowatts? 12jc.ua

60 w

9 5

http://12jc.ua


Personal Power

Plan and carry out an experiment to measure the amount of power you can

develop when running up a f l i ght of stairs .

Method:

Results: H e ^ * c u r b e d

(uS) (uS)

What are the sources of error in th is experiment and how can you make the

measurement more reliable?

r\n>

9 6


Power of an Electric Motor

Plan an experiment t o determine t h e output power of an electric motor by

plotting a graph of work done against t ime taken.

Method :

Results:

we.

Graph:

9 7


Effic iency

A machine changes energy f rom one type t o another, but not all t h e energy

produced is useful - usually some is lost as heat.

When building a machine engineers want i t t o produce as much useful energy as

possible - they want i t t o be eff i c i ent .

The effic iency can be calculated from either of these equations:

useful output energy gr useful output power Efficieny = — • Efficieny =

total input energy total input power

Why will t h e effic iency of a machine always be less than 100%?

What does an effic iency of 0.8 mean?

ill L

-

Example:

Find the effic iency of the ramp shown.

Weight = SOON

Physics f o r CCEA Questions 17 - 24, Page 51

Physics f o r CCEA Questions 3c, 4 - 6c + 8, Pages 56

Physics f o r You Questions 18b, 19b, 20b, 23 - 26c

-• 1 X 0 0 3

cx-><-

n o d

27 - 29, Pages 150 + 151

98


pages 112, 122 Energy transfer and efficiency

An Energy Transfer Diagram (Sankey diagram) shows what happens during an energy transfer:

for a torch

Efficiency = useful energy transferred total energy input

chemical energy

stored in X 100% * e battery

K light energy r (useful energy)

energy heating u| torch + room (wasted energy)

10O3


1. A lot of energy is wasted in a car. For every 100 J of chemical energy in the petrol, only 25 J are transferred to useful kinetic energy. The rest just heats up the engine and the air. a) Draw an Energy Transfer Diagram for this,

to scale. b) Calculate die efficiency. 167.

2. The diagram shows the energy transfers for a Bunsen burner heating a beaker of water. What is its efficiency as a water heater? L\(fl0

In a solar cell, for every 80 J of solar energy shining on it, only 4 J is transferred to useful energy (as electricity). a) What happens to the oflier 76 J? U3qab}jJ QS koctV-b) What is its efficiency? S°/ 0

c) Draw a Sankey diagram of this, to scale.

4. A pulley system lifts a load and gives it 6000 J of potential energy. The person pulling on the rope gives it 8000 J of energy. "̂ -tSTo What is the efficiency?

5. An electric kettle has a power rating of 2 kW and is switched on for 100 seconds. While heating up, it loses 60 000 J to the surroundings. a) How much energy is supplied to the kettle? cccrX

(1 kW = 1000 W = 1000 joules per second) b) How much is given to the water? ' ̂ o 0 0 0 3 c) What is the efficiency of heating water?

6. An electric motor on a building site has a power rating of 400 W and lifts a load of bricks weighing 600 N through a height of 10 m in 20 seconds. a) How much energy is needed to lift the bricks?

(See page 107 or page 116.) too° 3

b) How much energy is supplied to the motor in 20 seconds? S^cotJ

c) What is the efficiency of the motor in doing this job? "}SY 0

energy heating up the room 600 J

i Useful energy > heating water

Y 400 J

b3Wr p£>^T eWcW>cc\

motor

9 9


Making Good Use of Energy

List as many ways as you can t h a t we unnecessarily use energy in our homes and

some ways in which we could r e c t i f y th i s waste:

e.g. leaving TV on standby - t u rn o f f completely

fori uoa^M>^vg rK-ievcK..—JL. u j d L p._>U loo^ci . s

<S^s_v-e cy«a.op £r«Le.-2_«_v.3 rv./e i^sVoA

The Effic iency of Domestic Appliances

All electrical appliance in the UK must display information regarding its

effic iency . There are various ways of displaying th is information:

Task: Outline how each of these systems work and l ist the info they contain.

EU Label Energy

€ 3

XYZ

^ 3

3'

info ^ o . s c le_oeJ p=

0

SEDBUK

SEDBUK Rating

9 0 . 0 % 9 1 . 3 %

8 6 . 0 % 9 0 . 0 %

8 2 . 0 % 9 6 . 0 %

7 8 . 0 % 8 2 . 0 %

7 4 . 0 % 7BJO%

7 0 . 0 % 7 1 0 %

7 0 %

- A rv>osf -e^p / £ - lensr e ff

!abe- v p o - ^ c * c v ~ e-Ln-a>V\.Ocy.\ . spaces O o - ' g C - ^podb)

&<^&>cy^ Lc^oeA ^:or b>e*_Lejs

I O O ^ V J J J S . or-« . oVc- ^)

100


Kinetic and Gravitational Potential Energy

1.2.10 recall and use the equations kinetic energy = j mass x velocity 2

= imv* potential energy = mass * acceleration due to gravity x height

= mgh

Kinetic Energy

KE is the energy t h a t a body possesses because i t is moving.

KE = i m v 2

Where KE = k l , ^ g h r ( ^

Complete the memory triangle

Rearranging KE= £ mv

To get m: - 1 l £ X/ 1

To get v:

Work done and Kinetic energy

A car is moves along a road at 60mph what has t o be done t o bring i t t o a stop?

Explain the connection between the KE of the moving car and t h e work done to

Stop i t . leg 1 o£>- = (>-3<^ V=x̂ lp^o\Co_S, -V UgiQr- l o s k >K-> W - Q V I ^ S

Example: A bullet of mass 10g travels at a speed of 200m/s. Calculate i ts kinetic energy.

- Sj2. v O.OV x l o o V

= Z o o T

101


K I N E T I C E N E R G Y # Energy

L a) What is kinetic energy? , g r ^ ^ . . . \ c ^ o 3 j j ^ . . . . v ^ . . . Q f € o o o ^ ^ c ^

b) A truck of mass ZOOOkg and a car of mass 1000kg are travelling down a motorway at the same speed.

(i) Which one has the greatest kinetic energy?

(ii) Explain your answer. 1 p/Qjgdr:.. rv^aso

c) Two cars of the same mass are travelling down a road. Explain how one car could have more kinetic energy than the other.

one. brc9s^k\n.c\.... £Q,s*c-ey ... fa\o.o.... .-^WrA,... .c^Us-a-

1. For the following pairs of objects state which has the most kinetic energy,

al A car of mass 1000kg or a lorry of mass SZOOkg, both moving at lOm/s.

b) A car of mass 1000kg moving at lOm/s or a car of mass 1000kg moving at ZOm/s.

3. A car of mass 1000kg moves along a road at a constant speed of ZOm/s. Calculate it's kinetic energy.

2QO.. . 0 0 0 . 3 .

4. A -truck of mass 3Z,000kg moves along a road with a speed of lOm/s. Calculate the kinetic energy of the truek

lbcx> ODD. T.

5. A skier of mass 90kg is skiing down a hill at a speed of 15m/s. What is the kinetic energy of the skier?

6. The kinetic energy of a cyclist moving along a road is 5000J. If the mass of the cyclist is 100kg calculate the speed of the cyclist.

7. A motorcyclist and motorcycle have a combined mass of 900kg. If they have 140,000J of kinetic energy calculate their speed.

• 102


Gravitational Potential Energy

GPE is the energy t h a t an object has gained because of a change in its vertical

position.

GPE = m g h Complete the memory

Where GPE = Cm^\nY\nrA \ h h ^ a \ g n O ^ ( T ) t r i Q n 9 ' e

m= r/Y l^S ( V - r ^

h - I A ^ I O W V ( rv\)

Potential Energy and Work bone

When you l i f t your school bag onto t h e table i t gains PE. Where did th is energy

come from? J Q j ' rJ^pnrMml ( rrv^ri)

Explain t h e connection between the work you did l i f t i ng your bag and the energy

i t gained. , v i -to hpl- » f.P£ ggnrxpd

Conservation o f Energy

A ball is dropped f rom a height, h.

?

o

PE = mgh KE = 0

PE = KE

PE at the Top = KE at the bottom

A t the t o t a l amount of energy must remain the same.

When the ball h i t s the ground the KE will be converted into heat and sound energy.

PE = 0 KE = £mv 2

Physics f o r CCEA Questions 25 - 33, Pages 53 + 54

Physics f o r CCEA Questions 3b, 6d • 7, Pages 55 - 57

103


page 117 Potential energy kinetic energy

Change in gravitational PE

change in o r gravitational PE

(in J)

Kinetic energy (inJ)

= weight x change in height

— „„„„ v „ ~ change in = mass x g x . •» 6 height (in kg) (N/kg) (in m)

= VJ x mass x speed squared (kg) (m/s)J

since weight = mass x g (see p. 75) (N) (kg) (N/kg)

g = 10 here on Earth

See the examples on page 117.


1. A diver, of mass 40 kg, climbs up to a diving platform 1.25 m high, a) What is his weight, in N? te-o

What is his change in P.E.? SooJ Where does this energy come from? <̂ Ke»v>.c He walks off the platform and falls down. f o o d

What is his K.E. as he hits the water? S o o tJ e) What is his speed as he hits the water?

b) c) d)

2. The same diver now climbs to the 5 m platform, four times as high. a) What is his change in P.E. now? ^ o c o j b) What is his speed as he hits the water? to i ls c) What do you notice about this answer?

3. Another diver, of mass 80 kg, climbs to the 5 m platform. a) What is her speed as she hits the water? /cvnte b) What do you notice about your answer? $ a ,wa as ^ojcg dnje/

(sps=d readied *\ (yee. fed I 4. A stone is dropped from a window 5 m high.

At what speed does it hit the ground? iorr»\s

5. A tennis player hits a ball vertically with a speed of 10 m/s. How high does it go?

5 m

6. A car of mass 600 kg is travelling at 10 m/s. When the brakes are applied, it comes to rest in 10 m. What is the average force exerted by the brakes?

3CCCM 7. A car of mass 800 kg is at rest

The engine exerts a resultant force of 2000 N for a distance of 5 m. t o d - fd a) What is then its K.E.? ooooT . S

Of. mass)

10 m/s

fo^eookg^

10 m

3c»oco = f * ' 0

b) What is then its speed? . i o o c o 3

104


Kinetic Energy and Potential Energy 1. A tourist's Fiat is driving along a mountain road. The combined mass

of the car and luggage is 2920kg. The car is powering uphill at 23m/s.

a) How much kinetic energy does the car have? '^'wlSuO'J

At the top of the road, the car has gained a total height of 1200m. b) Calculate the potential energy the car has gained. 3 5 ^ 0 0 0 3 ~ 3

As the car rounds a bend at the top of the mountain, a suitcase falls from the roof into the valley below. The suitcase has a mass of 20kg.

c) Work out the potential energy the suitcase lost when it had fallen a distance of 60m. 12 OCOj d) If all of this potential energy of the suitcase is converted into kinetic energy, how fast will

it be travelling when it has fallen 60m? 3U.lo rV>ls e) Explain why it will not actually be travelling as fast as this. • f ^ ' , c r l £ r (

Some workmen are using a rope to lower a bucket full of bricks from a window. They tie off the rope when the bucket is just above the ground. As they are making their way downstairs to unload the bucket, a strong wind sets the bucket swinging. Draw a diagram of the path of the swinging bucket. On your diagram:

mark with the letter A — where the potential energy is greatest. mark with 'he letter B — where the kinetic energy is greatest. mark with the letter C — where the bucket is travelling fastest. mark with the letter D — where the bucket's velocity is zero.

A bouncy ball has a mass of 0.3kg. It is dropped from a height of 3.0m. a) How much potential energy has the ball lost when it hits the ground? ^ 3

Ignoring air resistance, how fast will the ball be travelling?.-) , The ball rebounds vertically at a speed of 7.0m/s. What kinetic energy does it now have? " ^ . j S J What height will it reach on the rebound? 2 . ^ 5 ^ Explain what has happened to the energy that the ball has lost. (Yichcry

Three students carry out an experiment to compare their own personal power. They measure their mass, then time how long it takes them to run up a flight of stairs 12m high. Their results are shown in the table below. Copy and complete the table.

Name Weigh! (N) Time (s) Potential Energy Gained (J) Power (W)

Alex 520 14

Billie 450 16 o L ' O O

Jack 600 15

105

Year 11

Assessed Homework

Unit 1

Karl takes Katie on a date to a theme park. They decide to have a go on the

rol ler coaster. The rol ler coaster car weights 9000N and is brought to a height

of 45m above its start ing point in 30 seconds.

a) What is the work done in raising the car to this height (point B)? [4 ]

b) What is the power of the motor used to pull the car up the track? [4 ]

c) What is the gravitational potential energy gained by the car at point B? [4 ]

d) I f t he rol ler coaster is let run down f rom point B to a point 15m above its

initial start ing position (point C), what kinetic energy will i t have gained

assuming t h a t the roller coaster is 100% eff ic ient? [4 ]

e) What is the velocity of the rol ler coaster car at point C? [ 5 ]

f ) I f the roller coaster is actually only 8 2 % eff i c i e n t what velocity will t he car

have at point C? [4 ]

Total [25 ]

106

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Momentum By the end of th is section you should be able to : 1.1.18 recall t h a t

Momentum = mass x velocity 1.1.19 recall and understand t h a t

Change in momentum - force x time and apply th i s t o the solution of mathematical problems;

1.1.20 apply t h e principles of momentum, forces and time to an analysis of safety features of modern cars, t o include car air bags, car seat belts, car crumple zones and crash barr iers ;

1.1.21 investigate, using data loggers or computer simulations, one-dimensional inelastic collisions and, through mathematical modelling, use the data obtained to show t h a t the momentum is conserved in such collisions; and

1.1.22 recall and use the principle of conservation of momentum t o solve simple problems involving one-dimensional inelastic collisions.

Momentum

Momentum is a useful quantity t o consider when objects collide.

Think about a Year 8 pupil colliding with a Year 14 pupil. Who comes o f f the

worst? What factors does th is depend on?

Momentum is product of the mass of a body and its velocity.

P = m v

Complete the memory triangle

m -

Example: An object has a momentum of 15.90kgm/s and a velocity of 9.04m/s. What is

t h e mass of the object?

I I S . ^ ' Y~Y^ K<=\ , O U

107

Year 11

Impulse

GCSE Physics Unit 1

Consider a force F acting on a mass m f o r a time t j so t h a t i t accelerates f rom

initial velocity u t o final velocity v.

The force x t ime is called the impulse.

v — u

Impulse = F t

As a =

Then F = m

And F = ma

(v-u^

v t J

mv - mu I . e . Force = change in momentum

time taken

Force x time = change in momentum = Impulse

Questions:

1. While playing basketball in PE class, Logan lost his balance a f t e r making a

lay-up and colliding w ith the padded wall behind the basket. His 74kg body

decelerated from 7.6m/s to Om/s in 0.16 seconds.

a) Determine the force acting upon Logan's body.

b) I f Logan had h i t t h e concrete wall moving at the same speed, his

momentum would have been reduced t o zero in 0.008 seconds. Determine

what the force on his body would have been f o r such an abrupt collision.

2. A boy kicks a stone of mass 1kg, accelerating i t f r o m

rest t o lOm/s. The stone is r ig id so t h e force acts f o r

only 1/100 of a second. He then kicks a footbal l of the

same mass to give i t the same final speed. The footbal l

is s o f t in comparison t o t h e stone so t h e force acts f o r

1/10 of a second th is t ime. Which kick hurts less?

Physics f o r CCEA Question 11, Page 20

108


Conservation of Momentum Udr»'

The law of conservation of momentum states t h a t : ^ m o m = n h Vyzfrr-e. o -tdrol

Cr?lll,1\Qp CC ^-yfdr iS lno

There are no exceptions to th is law.

When two objects collide the i r changes in momenta will be equal in size but

opposite in direction. The momentum gained by one is equal t o the momentum

lost by the other.

Tackling Momentum Problems:

1. Choose a positive direction.

2. Draw before and a f t e r sketches of the objects involved.

3. Calculate every momentum you can

4. Apply the law of conservation of momentum f o r collisions involving two

bodieS. Pbefore = Pafter

m i u i + m2U2 = m i V i + m2V2

Questions:

1. A railway wagon of mass 800kg moves at a steady speed of 2.5m/s. I t

collides with another wagon of mass 1000 kg. The two wagons couple

together a f t e r the collision. Calculate the final speed and the loss in KE i f

the second wagon was stationary initially.

2. Rex (86kg) and Tex (92kg) board the bumper cars at the local carnival. Rex

is moving at a fu l l speed of 2.05m/s when he rear-ends Tex who is at rest in

his path. Tex and his 125kg car lunge forward at 1.40m/s. Determine the

post-collision speed of Rex and his 125kg car.

109


3. A candy-filled pinata is hung from a t r e e f o r Matthew's birthday. During an

unsuccessful attempt to break the 4.4kg pinata, Hayden cracks i t with a

0.54kg st ick moving at 4.8 m/s. The stick stops and the pinata undergoes a

gentle swinging motion. Determine the swing speed of the pinata immediately

a f t e r being cracked by the stick i f i t initially was at rest .

4. During an in-class demonstration of momentum change and impulse, Mr . H

asks Jerome (102kg) and Michael (98kg) to s it on a large 14kg skate cart .

Mr . H asks Suzie (44kg) to s it on a second 14kg skate cart . The two carts

are placed on low f r i c t i o n boards in the hallway. Both carts are initially at

rest and Jerome pushes o f f of Suzie's cart . Measurements are made to

determine t h a t Suzie's cart acquired a post-impulse speed of 9.6m/s.

Determine the expected recoil speed of Jerome and Michael's cart .

5. Jaclyn plays singles f o r South's varsity tennis team. During the match

against N o r t h , Jaclyn won the sudden death tiebreaker point with a cross-

court passing shot. The 57.5g ball h it her racket with a velocity of 26.7m/s.

Upon impact with her 331g racket, the ball rebounded in the exact opposite

direction (and along the same general t r a j e c t o r y ) with a speed of 29.5m/s.

a. Determine the pre-collision momentum of the ball.

b. Determine the post-collision momentum of the ball.

c. Determine the momentum change of the ball.

d. Determine the velocity change of the racket.

6. To Mr. H's disgust, a 450g black crow is raiding the recently-fi l led bird

feeder. As Mr. H runs out the back door with his broom in an e f f o r t to

scare the crow away, i t pushes o f f the 670gram feeder with a takeoff speed

of 1.5m/s. Determine the speed at which the feeder initially recoils

backwards assuming the crow and feeder were stationary initally.

110

o A w /

V- \.\\ m\2>

v >̂ _x l o o o x ( o Y - ]/x. f5?Qo + t o c ^ x (\,\\ s)

<?

3

Li) KV"»A 1. -1- l Y ) - i J - - r W V / . -»- rv->, \ A

k^) P =- r r w / - o.oST-5 x 7Sa^

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Elastic and Inelast ic Collisions

I n some collisions kinetic energy as well as momentum is conserved.

The tota l kinetic energy of the bodies before the collision is equal t o the tota l

kinetic energy of t h e bodies a f t e r the collision.

No energy will be lost in the collision as heat or sound or in the permanent

deformation of t h e colliding bodies.

T TuUi2 + i 1TI2U22 = £ miv i 2 + | m2V22

I f t he collision is elastic there is no loss in kinetic energy.

Collisions in which t h e kinetic energy is not the same before and a f t e r are called

inelastic collisions.

Although kinetic energy may or may not be conserved in a collision, momentum is

always conserved and so is to ta l energy.

Total momentum before is equal to tota l momentum a f t e r , providing no external

force is applied.

Types of collision;

• Elastic collision

• Inelastic collision

• Completely

inelastic collision

No loss of kinetic energy on impact

Some kinetic energy lost on impact

Objects stick together on impact

NB: Remember t h a t momentum is always a vector quantity and so magnitude and

direction are important.

Hence i f two objects are travell ing towards each other the tota l momentum is

found by subtracting the individual momentum of t h e objects.

Physics f o r You Questions 1 -7 , Page 147

111

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Year 11

Car S a f e t y

GCSE Physics Unit 1

Make a poster presentation detailing how one of the following car safety

features keeps passengers safer during a collision.

Crumple zones

Seat be Its

I I I I «

Revision Questions

Phys

Phys

Phys

Phys

Phys

Phys

cs f o r CCEA Questions 1 - 4, Pages 11-12

cs f o r CCEA Questions 12 -16, Page 22

cs f o r You Questions 12 - 22, Page 121

cs f o r You Questions 1 - 6, Page 128

cs f o r You Questions 8 -17, Page 141

cs f o r You Question 39, Page 153

113

Car S a f e t y

All of the safety features have one aim in mind, to increase the time to stop and to

increase t he distance over which the passengers stop.

• By increasing the time to stop, you are decreasing the rate of change of velocity,

i.e. decreasing the acceleration, so decreasing the force experienced.

mv - mu F = m

V

• By increasing the distance over which the force is acting, the average force is less.

Other information

Crumple Zones

Crumple zones are part of a car designed to collapse during a collision - usually the

f r o n t end. The f r o n t of the car crumples and stops, but the passengers continue to

move a crucial half metre or so.

Seat belts

I f you were not wearing a seat belt and the car came to a sudden stop, you would

continue to move due your inertia. Your body would most likely be stopped as a result

of the force of the windscreen or other rigid part of the car.

Airbags

Airbags need to be used in conjunction with seat belts. They perform the same

function as a seat belt and should be ful ly inf lated before you h i t them. I f you were not

wearing a seat belt then there would not be enough time f o r the airbag to inf late

before you h i t i t . Airbags are designed to inf late in 0.05s, and to def late within 0.3s.

The quarter of a second between these two times is suff ic ient to slow you down.

A f lex ib le nylon bag is fo lded into the steering wheel or

dashboard.

When t h e f r o n t end of the spring is suddenly stopped, the

mass continues forwards t o make contact with the switch,

start ing a chemical reaction. This occurs when the

acceleration is around -lOg.

An inflation system in which a spark ignites a violent chemical

reaction between sodium n itr ide (NalSb) and potassium n i t rate

(KNO3) producing nitrogen gas to inf late the airbag.

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