Transcript P2.3.1_-_PE+KE_answers 1439KB Jun 06 2014 10:44:17 AM

Work, potential and kinetic energy
1. What equation links work, force and distance? Work(J) =
force(N) X distance(m)
2. A force of 10N is applied to a box to move it 2m along the
floor. What is the work done on the box? 2x10 = 20J
3. If 50J of work is done on a box using 15N of force, how far
was the box moved? 50/15 = 3.3m
4. What equation is used to calculate increases in gravitational
potential energy? PE = mass x gravity x height OR PE =
weight x height
5. A crate of mass 100kg was lifted to a height of 75m. How
much work was done on the crate. Is this the same as the
gravitational potential energy that the crate now has?
Assuming gravity is 9.8m/s2. 100x9.8x75 = 73500 J. Yes
6. A book of weight 2N is lifted 1.5m onto a shelf. How much
potential energy has the book gained? 2x1.5 = 3J
7. What is the equation that links kinetic energy with mass and
velocity? KE = ½mv2. (½ x mass x velocity2)
8. If an object of mass 15kg is travelling at 5m/s, how much
kinetic energy does it have? ½ x 15 x 25 = 187.5J
9. A car is travelling at 30m/s and has kinetic energy of 450kJ,
what is its mass? m=2xKE/v2=2x450000/900=1000kg
10. A tennis ball has a mass of 0.06kg and has kinetic energy of
2.94kJ when served, what is its speed? v=(2KE/m)
=(5.88/0.6)=3.13m/s
A forklift truck was used to stack boxes on a trailer.
It lifted a box weighing 1900N through 4.5m.
(a) Calculate the work done on the box. Show your working.
…work=force x distance = 1900X4.5 = 8550
………………………………………Work done = …8550…..J
(3 marks)
(b) The efficiency of the forklift truck is about 80%.
Explain what happens to the wasted energy?
……the motor, due to friction, has heated up and it is also noisy
this energy has been lost to the surroundings…
(1 mark)
The diagram shows how far a dummy in a car crash test moves
before it is stopped by the seat belt. The graph shows the force
exerted by the seat belt on the dummy during the crash.
(i) Write down the equation that links distance moved, force
applied and work done.
.........work = force x distance......................
(1 mark)
(ii) Calculate the work done to stop the dummy.
Show clearly how you work out your answer.
.....................work = 7000 x .03 =
.........................................................................................................
Work done = .........2100............... joules
(2 marks)
(iii) How much energy is transferred from the dummy during the
crash?
..............2100 J........................................................................
(1 mark)
The diagram shows a tower used for bungee jumping.
(a) A lift takes a person of mass 55 kg, to a platform 64 metres
above the ground.
(i) Write down the equation that links change in gravitational
potential energy, change in vertical height and weight.
..................PE = weight x change in vertical height ....... (1 mark)
(ii) Calculate the increase in the gravitational potential energy of
the person when going from the ground to the platform.
Show clearly how you work out your final answer.
(Gravitational field strength = 10 N/kg)
..................PE = 55 x 10 x 64 =
........................................................... Increase in gravitational
potential energy ....35200.. joules
(2 marks)
(b) The falling bungee jumper reaches a maximum speed when
15 840 joules of gravitational potential energy have been
transferred into kinetic energy.
(i) Write down the equation that links kinetic energy, mass and
speed.
...............KE = ½ mv2......................... (1 mark)
(ii) Calculate the maximum speed of the falling bungee jumper.
Show clearly how you work out your final answer.
.....................v = (2KE/m) = (2x15840/55) = (576)
....................................................................................
Speed = ........24.................. m/s
(3 marks)
(c) As the rope stretches the bungee jumper slows down.
Describe fully the energy transfer that happens as the bungee
jumper continues to fall towards the ground.
To gain full marks in this question you should write your ideas in
good English. Put them into a sensible order and use the correct
scientific words.
.....From maximum KE the bungee starts to stretch and the KE in
transferred into elastic potential energy... (3 marks)
A pile driver is used to push steel supports into the ground. A pile
driver does this because it has a lot of kinetic energy as it hits the
steel support.
(a) Give two ways in which the kinetic energy of the pile driver as
it hits the steel support could be increased.
1 increase the mass of steel
2 increase the height from which it was dropped
(2 marks)
(b) The mass of the pile driver is 500 kg.
The kinetic energy of the pile driver as it collides with the steel
support is found to be 36 000 J.
Calculate the speed of the pile driver just before it hits the steel
support.
Show clearly how you work out your answer and give the unit.
KE = ½ mv2
m=500
KE = 36000 sqrt(2x36000/500) = sqrt(144)
........................................................................................................
........................................................................................................
Speed of the pile driver = ....12m/s................
(4 marks)
A car which is moving has kinetic energy.
The faster a car goes, the more kinetic energy it has. The kinetic
energy of this car was 472 500 J when travelling at 30 m/s.
Calculate the total mass of the car.
Show clearly how you work out your answer and give the unit.
.........................................................................................................
KE = ½ mv2
.........................................................................................................
m = 2KE/v2
M = 2 x 472500/302 ............................................................
.........................................................................................................
.........................................................................................................
Mass of the car = ..1050 kg...............
(5 marks) (4 marks for getting to the correct answer, 1 mark for
the unit)