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Physics 101 Thursday 9/29/11 Class 11"

Chapter 7.3 – 8.2"Work with variable force"Power"Conservative and non-conservative forces"Potential Energy"

Worked example on the web!

I promised a banked question….here is a hard one: (like example 6-9). A car, mass 900 kg, is going around a curve with radius R. The coefficient of static friction is 0.50, the corner is banked by 10 degrees. How fast can the car go around the corner without slipping? Answer in lecture notes on the web……try to do the question first by looking at example 6-9..then look at what I did……

Work-­‐  Energy  

a) 0 - 30 mph

b) 30 - 60 mph

c) both the same

 A  car  starts  from  rest  and  accelerates  to  30  mph.    Later,  it  gets  on  a  highway  and  accelerates  to  60  mph.      Which  takes  more  energy,  the  0  -­‐  30  mph,  or  the  30  -­‐  60  mph?    

Work-­‐  Energy  

a) 20 m

b) 30 m

c) 40 m

d) 60 m

e) 80 m

 If  a  car  traveling  60  km/hr  can  brake  to  a  stop  within  20  m,  what  is  its  stopping  distance  if  it  is  traveling  120  km/hr?      Assume  that  the  braking  force  is  the  same  in  both  cases.  

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A child on a

skateboard is moving

at a speed of 2 m/s.

After a force acts on

the child, her speed is

3 m/s. What can you

say about the work

done by the external

force on the child?

a) positive work was done

b) negative work was done

c) zero work was done

Work  and  KE   Work Energy Theorem!

Review! Work from a graph!

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Work from a graph! Work from a graph!

Work with springs!

When you do work on a spring….. A) the work is always positive B) the work is positive when compressing it, negative when pulling on it C) the work is negative when compressing it, positive when pulling on it

Work with springs!

A 2 kg block slides at 2 m/s then hits a spring and compresses it 2 m, what is the spring constant of the spring? A) 1 N/m B) 2 N/m C) 4 N/m D) 8 N/m E) 16 N/m

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Time  for  Work  I  

a) Mike

b) Joe

c) both did the same work

 Mike  applied  10  N  of  

force  over  3  m  in  10  

seconds.    Joe  applied  the  

same  force  over  the  

same  distance  in  1  

minute.    Who  did  more  

work?  

Mike performed 5 J

of work in 10 secs.

Joe did 3 J of work in

5 secs. Who

produced the greater

power?

a) Mike produced more

power

b) Joe produced more

power

c) both produced the

same amount of

power

Time  for  Work  II  

Engine #1 produces

twice the power of engine

#2. Can we conclude

that engine #1 does twice

as much work as engine

#2?

a) yes

b) no

Power  

a) energy

b) power

c) current

d) voltage

e) none of the

above

Electric  Bill  

 When  you  pay  the  electric  

company  by  the  kilowaI-­‐

hour,  what  are  you  

actually  paying  for?  

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Energy  Consump>on  

 Which  contributes  

more  to  the  cost  of  

your  electric  bill  each  

month,  a  1500-­‐WaI  

hair  dryer  or  a  600-­‐

WaI  microwave  

oven?  

a) hair dryer

b) microwave oven

c) both contribute equally

d) depends upon what you cook in the oven

e) depends upon how long each one is on

1500 W

600 W

Power and work!

A sports car accelerates from zero to 30mph in 2 seconds. How long does it take to accelerate from zero to 60mph, assuming that the power output of the engine is independent of velocity and neglecting friction and air resistance? A: 4sec B: 6 sec C: 8 sec D: 16 sec E: None of these.

Worked example!

The empire state building is approximately 90 flights of stairs, 1600 steps, and the record is climbing this in 9.5 minutes. If the height of each step is 0.2 m, and a runner had a mass of 70 kg, what is the average power output during the climb? Give answer in both watts and horsepower. How many joules of energy are there in a kilowatt-hour?

Conservative forces!

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Conservative forces! Conservative forces!

Suppose you want to ride your mountain bike up a steep hill. Two paths lead from the base to the top, one twice as long as the other. (Neglect friction i.e. consider only your "fight against gravity") Compared to the average force you would exert if you took the short path, the average force you exert along the longer path is A: four times smaller. B: half as small. C: twice as big. D: the same. E: undetermined—depends on time taken.

Conservative forces!

Suppose you want to ride your mountain bike up a steep hill. Two paths lead from the base to the top, one twice as long as the other. (Neglect friction i.e. consider only your "fight against gravity") Compared to the work you would exert if you took the short path, the work along the longer path is A: four times smaller. B: half as small. C: twice as big. D: the same. E: undetermined—depends on time taken.

Using work and conservative forces!

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Pulleyʼs and work!

Is it possible for

the gravitational

potential energy

of an object to be

negative?

a) yes

b) no

Sign  of  the  Energy  II  

You and your friend both solve a problem involving a skier going down a slope, starting from rest. The two of you have chosen different levels for y = 0 in this problem. Which of the following quantities will you and your friend agree on?

a) only B

b) only C

c) A, B, and C

d) only A and C

e)only B and C

KE  and  PE  

A) skier’s PE B) skier’s change in PE C) skier’s final KE

Reading Assignment!

Tuesday 10/4 8.3 – 8.5 Thursday 10/6 9.1-9.4