CHAPTER 7

PREPARED BY: Mdm Heiana Nadia Hamzah

DYNAMICS

7.1 FORCE

1. A force is a push or a pull acting on an object.

2. When we push or pull on an object, we are exerting a force on it.

7.1 FORCEIdentify whether the activities below are push or pull actions:

a) Pressing the plasticineb) Stretching the plasticinec) Climbing a treed) Ride on a bicyclee) Walk on the groundf) Chopping onion with a knife

push

push

push

pullpull

push

7.1 FORCE

plasticine

PUSH

PULL

EFFECTS OF A FORCE

A B C

State the effect of force on each of the following activities:

A : Press & stretch the lump of plasticine

B : Shifting a wooden block to the side of the table

C : Blow gently & hard at the ping-pong ball in the opposite direction

Changes the shape of the plasticine

Changes the position of the wooden block

Changes the speed & direction of movement of ping-pong ball.

Let‘s do Activity 7.1

ACTIVITY OBSERVATION

A

B

C

D

E

F

G

Force change the shape & size of the spring.

Force change the shape & size of the span.Force move a stationary trolley.

Force increase the speed of a moving trolley.Force slow down a moving trolley.

Force stop a moving slotted weight.

Force change the direction of the magnet.

EFFECTS OF A FORCE3. Applying forces to an object can cause;

• A change of shape of an object

EFFECTS OF A FORCE3. Applying forces to an object can

cause;

• Move a stationary object.• Change its position or direction.• Increase or decrease the speed

of a moving object.• Slow down & stop a moving

object.

Types of forces

a)GRAVITATIONAL FORCE

• It is the force that pulls object to the Earth’s surface

A stone that is thrown upward will falls down because of gravitational force.

• The force of gravity acts equally on all objects.

• In a vacuum, all objects fall with the same velocity.

GRAVITATIONAL FORCE

Figure 1: objects fall with the same acceleration in a vacuum (left) but with different accelerations through air (right)

Air resistance.

Different parts of the Earth experience different gravitational forces.

Equator

North pole

South pole

Explanation

•This is because the Earth is oval. •The force of gravity is stronger at the poles & weaker at the equator.•Gravity is stronger when you are closer to the centre of the Earth.

Types of forces

b) FRICTIONAL FORCE

•A force that opposes the movement of one surface over another.•Produced when two surfaces touch one another.

TYPES OF FORCES

c) ELECTROSTATIC FORCE

• It is the force of attraction between unlike charges or the force of repulsion between like charges.

TYPES OF FORCES

c) ELECTROSTATIC FORCE

• It is produced by charged objects, which are non-conductors of electricity. This may be a plastic, glass & rubber.

• When a plastic ruler is rubbed with a dry, woolen cloth, the ruler becomes charged. It can attract small pieces of paper that are not charged.

Types of forces

d) MAGNETIC FORCE

• It is the force of attraction or repulsion between magnets or magnetic material.

• Like poles of magnets repel each other.• Different poles of magnet attract each other.

Types of forces

e) ELECTROMAGNET

• An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current.

• An electromagnet is a temporary magnet.

• The magnetic field disappears when the current is turned off.

solenoid

7.2 MEASUREMENT OF FORCE.

1. The unit of force is the newton (N)2. A spring balance is used to measure the

magnitude.3. If a spring is 5 cm long and a load

stretches it to 8 cm long, then the extension of the spring is…

8 – 5 = 3 cm

7.2 MEASUREMENT OF FORCE.

• The extension of a spring is directly proportional to the weight of a load.

• The extension of the spring increases when the load on the spring increases.

Try this out...!

QUESTIONFigure shows the extension of a spring by 2 cm when a load of 4 N was suspended on it.Calculate the increase in length when a load of 6 N is suspended on it.

SOLUTION

4 N extend 2 cm6 N extend ? cm

6/ 4 x 2 = 3 cm

Extension of the spring = 3 cm

Try this out...!

QUESTION 2

A spring without any load has the length of 10 cm. This spring stretches to 35 cm if a load of 25 N is attached to it. What is the extension of the same spring if a load of 10 N is attached to it.

ANSWER 10 cm

7.3 APPLICATION OF FRICTIONAL FORCE

A cup that is placed on a table does not move.

WHY?

The cup does not move because of the friction between the table & the cup.

7.3 APPLICATION OF FRICTIONAL FORCE

The magnitude of the frictional force that acts between two surfaces depends on the…

a) Nature of the surfaces

• If the surfaces are rough, the frictional force will be greater.• If there is oil or grease between the surfaces, the frictional force will be smaller.

7.3 APPLICATION OF FRICTIONAL FORCE

Rough surface will generate more friction compare to the smooth surface.

A

B

7.3 APPLICATION OF FRICTIONAL FORCE

b. Weight of the moving objectThe greater the weight, the greater the frictional force.

A

B

Frictional force does not depend on the area of contact surface.

EXPERIMENT 7.1: HOW DIFFERENT TYPES OF SURFACES AFFECT

FRICTIONAL FORCE

TYPES OF SURFACES MAGNITUDE OF FRICTIONAL FORCE (N)

Table

Sand paper

Glass rods

Beans

CONCLUSION:______________________________________________The rougher the surface, the greater the frictional force.

What are the advantages of frictional force?

7.3 APPLICATION OF FRICTIONAL FORCE

ADVANTAGES OF FRICTIONAL FORCE

1. Enable us to carry out daily activities such as walking, running, climbing, dancing etc.

7.3 APPLICATION OF FRICTIONAL FORCE

ADVANTAGES OF FRICTIONAL FORCE

2. Braking system making moving vehicles slow down or stop.

7.3 APPLICATION OF FRICTIONAL FORCE

ADVANTAGES OF FRICTIONAL FORCE

3. Enable us to hold things.

4. Enables us to move without slipping.

7.3 APPLICATION OF FRICTIONAL FORCE

ADVANTAGES OF FRICTIONAL FORCE

5. Enable things to lean on.

What are the disadvantages of frictional force?

7.3 APPLICATION OF FRICTIONAL FORCE

DISADVANTAGES OF FRICTIONAL FORCE

1. More energy is needed to move a

vehicle or object

2. Causes the surface in contact to wear

out.

3. Produces unnecessary heat –

Car engines becomes hotter because of friction.

The wooden block is just about to move when the spring balance shows 4 N. Which statements below is correct regarding the magnitude and direction of friction.

A. The magnitude of friction is twice that shows on the spring balance and acts in the same direction as the pulling force.

B. The magnitude of friction is 4N and it acts in the opposite direction to movement.

C. The magnitude of friction 2N and it acts in the opposite direction to movement

pulled

Methods of reducing friction

1. Rollers or ball bearings reduce friction.

2. Lubricants such as oils & grease are applied to moving parts to reduce friction.

Methods of reducing friction

3. A space shuttle is streamlined to reduce friction & provide very little resistance to a flow of air.

4. Hydrofoil boat The layer of air between the hull of a hydrofoil &the water allows the hydrofoil tomove at high speed.

Methods of reducing friction

5. HOVERCRAFT Vehicle that moves above water or land by gliding on a cushion of air it creates by blowing downward.

Methods of reducing friction

6. MAGLEV TRAINThis train use the basic principle of magnet. As the train floats on the track, there is no contact with the ground, so less friction is produced.

7.4 WORK & POWER

1. Work is the product of the force acting on an object & the distance moved by the object in the direction of the force.

2. S.I. unit for work is newton metre (Nm) or joule (J).3. Work is also a measure of the amount of energy used.

WORK

Work done (J) = Force (N) x Distance moved in the direction of the force (m)

FORMULA

WORKwall

Is work done in the above situation?

nO work is done in above situation.

Work is done when a force moves an object over a distance.

Try this out...!

QUESTION 1

Jamie uses 200 J of energy to lift a 10 kg box to a certain height. What is the height?

Solution

[Assume 1 kg = 10 N]

200 J = 100 N x d d = 200/ 100 d = 2 m

1. Power is the rate of work done2. The S.I unit for power is joule per second ( Js⁻ ˡ ) or

watt (W).3. Power is a measure of how fast work is done.

POWER

Power (W) = Work done (J)Time taken (s)

FORMULA

7.4 WORK & POWER

Try this out...!

QUESTION 2

A boy weighs 55 kg carries a box of 2kg when climbing a stairs of vertical height 5 metres. Calculate the work done by him.

ANSWER 2850 Nm/ 2850 J

Try this out...!

QUESTION 3

A girl weighs 45 kg carries books of 3kg. She climbs a stairs in 20 seconds. Calculate the power generated by her.

ANSWER 72 Watt

Try this out...!

QUESTION 4

A student climbs up a 6m high staircase with a 3.5 kg parcel in 40 seconds. If the power used by the student is 87 W, what is the mass of the student?

ANSWER 54.5 kg

Try this out...!

QUESTION 5

A soldier weighing 750 N carries a knapsack & a rifle weighing 100 N and 50 N respectively. He is ordered to go on a 10 km march. How much power does the soldier have if he completes the march in 5 hours?

Solution

[Assume 1 km = 1000 m]

Work done (J) = (750 + 100 + 50) N x (10 km x 1000) m = 9 000 000 Nm

Power (W) = 9 000 000 Nm / (5 hours x 60 x 60) s

= 500 W

Answer the questions

1. A man carrying a sack of rice walks for 40 m in 20 seconds. He weighs 50 kg and the sack of rice weighs 10 kg. How much power does he produce?

2. A pupil weighing 60 kg climbs 15 steps up a staircase in 40 seconds. Each step of the stairs is 20 cm high. How much power is produced as he climbs the stairs?

3. A car weighing 500 kg is being towed through a distance of 0.4 km. How much power in kW is needed to tow a car in 10 seconds?

Try this out...!

QUESTION 1

A spring of 7 cm is stretched to 10 cm when a 10 N load is placed on it. A load X is placed on it & the spring stretched to 13 cm. Find the weight of load X?

ANSWER

Weight of load X = 20 N