P2 Revision Booklet

Energy, Work and Power

Medium (Grade C)

Q1.           A student uses an electric motor to lift a load.

In the motor, the electrical energy is transferred into other types of energy. Some of this energy is useful and the rest of the energy is wasted. 
(a)     (i)     Name the useful energy output from the electric motor.
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(1)

(ii)      What eventually happens to the wasted energy?
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(1)

(b)     The graph shows the input energy the motor needs to lift different loads by one metre.

What can you conclude from the graph about the relationship between the load lifted and the input energy needed?
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(2)

 
(c)	A shop uses escalators to lift customers to different floor levels. The escalators use electric motors. When the shop is not busy some escalators are turned off. A sign tells the customers that the escalators are turned off to save energy.		

(i)      Each escalator has one motor with an average power of 4000 W. The motor is turned on for an average of 8 hours each day, 6 days each week. Electricity costs 15 pence per kilowatt-hour.
Calculate the cost of the electricity used in an average week to run one escalator.
Use the correct equation from the Physics Equations Sheet.
Show clearly how you work out your answer.
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                                           Cost = ............................................. pence
(3)

(ii)     Give one environmental advantage to turning off electrical appliances when they are not being used.
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(1)
(Total 8 marks)
 Q2.          (a)     The diagram shows a builder using a plank to help load rubble into a skip.

The builder uses a force of 220 N to push the wheelbarrow up the plank.
Use information from the diagram and the equation in the box to calculate the work done to push the wheelbarrow up the plank to the skip.
 
work done    =    force applied    ×    distance moved in the direction of force	

Show clearly how you work out your answer.
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                          Work done = ............................................................ J
(2)

(b)     A student investigated how the force needed to pull a brick up a slope, at a steady speed, depends on the angle of the slope.
The apparatus used by the student is shown in the diagram.

The student used the results from the investigation to plot the points for a graph of force used against the angle of the slope.

(i)      Draw a line of best fit for these points.
(1)

(ii)     How does the force used to pull the brick up the slope change as the angle of the slope increases?
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(1)

(iii)    Consider the results from this experiment.
Should the student recommend that the builder use a long plank or a short plank to help load the skip?
Draw a ring around your answer.
 
long plank	short plank	

Explain the reason for your answer.
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(2)
(Total 6 marks)

 

Q3.          (a)     The diagram shows a cable car used to take skiers to the top of a mountain.

(i)      The total mass of the cable car and skiers is 7500 kg.
Use the equation in the box to calculate the weight of the cable car and skiers.
 
weight    =    mass    ×    gravitational field strength	

gravitational field strength = 10 N/kg
Show clearly how you work out your answer and give the unit.
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      Weight = .........................................................................................
(3)

(ii)     The cable car moves at a constant speed. It lifts skiers through a vertical height of 800 metres in 7 minutes.
Use the following equation to calculate the work done to lift the cable car and skiers.
 
work done    =    force applied    ×    distance moved in the direction of force	

Show clearly how you work out your answer.
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                                       Work done = ............................................... J
(2)

(b)     The diagram shows a skier who is accelerating down a steep ski slope.

(i)      Draw an arrow on the diagram to show the direction of the resultant force acting on the skier.
(1)

(ii)     How and why does the kinetic energy of the skier change?
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(2)

(c)     Last year, 18 000 skiers suffered a head injury. It is thought that nearly 8000 of these injuries could have been avoided if the skier had been wearing a helmet.
However, at present, there are no laws to make skiers wear helmets.
Suggest why skiers should be made aware of the benefits of wearing a helmet.
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(1)
(Total 9 marks)
 Q4.          (a)     A chair lift carries two skiers, Greg and Jill, to the top of a ski slope. Greg weighs 700 N and Jill weighs 500 N.

(i)      Write down the equation that links distance moved, force applied and work done.
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(1)

(ii)     Calculate the work done to lift Greg and Jill through a vertical height of 200 m. Show clearly how you work out your answer and give the unit.
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work done = ....................................................................
(3)

(b)     The chair takes 5 minutes to move from the bottom to the top of the ski slope.
          Use the following equation to calculate the power required to lift Greg and Jill to the top of the ski slope. Show clearly how you work out your answer.
power = 
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power = .................................................................. watts
(2)

(c)     The chair lift is driven by an electric motor.
(i)      Why would the power output of the electric motor need to be larger than your answer to part (b)?
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(1)

(ii)     Complete the following sentence.
When the ski lift is working ........................................ energy supplied to the motor
is usefully transferred as gravitational ........................................ energy.
(1)
(Total 8 marks)
 Q5.          The diagram below shows one way of lifting a bucket of bricks.

(a)     When the free end of the rope is pulled down, the load is lifted.
          Complete the following sentence.
          The work done in pulling the rope down is used to increase the .............................
          energy of the ..................................... and bricks.
(2)

(b)     The weight of the bricks is 100 N and they are lifted 3 m.
          Calculate the work done on the bricks.
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Answer ............................................ J
(2)
(Total 4 marks)
 Q6.          A crane on a barge lifts a girder and then carries it along the river.

          The girder has a weight of 1 000 000 N and is lifted to a height of 1500 cm.

(a)     Complete the sentence.
The weight of the girder is caused by the Earth’s gravitational field strength acting on
 its ............................................................. .
(1)

(b)     Calculate the work done in lifting the girder.
          Write the equation you are going to use.
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(1)
          Show clearly how you work out your answer and give the unit.
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Work done = ..............................................................
(3)

(c)     The velocity–time graph represents the motion of the barge after the girder had been lifted.

          To gain full marks in this question you should write your ideas in good English. Put them in a sensible order and use the correct scientific words.
          Describe the motion of the barge over this period of seven hours. You must refer to the points A, B, C, D, E and F in your description.
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(5)
(Total 10 marks)

 

Q7.          When you transfer energy to a shopping trolley, the amount of work done depends on the force used and the distance moved.

          Complete the table by using the correct units from the box.
 
joule (J)          metre (m)       newton (N)	

          The first one has been done for you.
 
Quantity	Unit	
energy (transferred)	joule	
force	 	
distance (moved)	 	
work done	 	

(Total 2 marks)
 Q8.          A forklift truck was used to stack boxes on to a trailer.
It lifted a box weighing 1900 N through 4.5 m.

          Calculate the work done on the box. Show your working.
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Work done = .............................. J
(Total 3 marks)
Q9.          A book weighs 6 newtons.
          A librarian picks up the book from one shelf and puts it on a shelf 2 metres higher.

(a)     Calculate the work done on the book. [Show your working].
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 (3)

(b)     The next person to take the book from the shelf accidentally drops it.
The book accelerates at 9.8m/s².
Use this information to calculate the mass of the book. [Show your working].
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Answer ................................. kg.
(3)

(c)     If the book was dropped from an aeroplane high in the sky, it would accelerate to begin with. Eventually it would fall at a steady speed.
Explain, in as much detail as you can, why this happens.
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(3)
(Total 9 marks)
 Q10.          A man’s car will not start, so two friends help him by pushing it.

          By pushing as hard as they can for 12 seconds they make the car reach a speed of
3 metres per second.

(a)     Calculate the acceleration they give to the car.
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.............................................................................................. Answer ................ m/s2
(2)

(b)     Whilst pushing the car the two friends together do a total of 2400 joules of work. Calculate their total power.
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............................................................................................ Answer ................ watts
(2)

(c)     Another motorist has the same problem.  The two friends push his car along the same stretch of road with the same force as before.
          It takes them 18 seconds to get the second car up to a speed of 3 metres per second.
          What does this tell you about the mass of the second car?
(You can ignore forces of friction.)
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(2)

(d)     On a flat stretch of a motorway a lorry driver changes into top gear.  He then makes the lorry go as fast as he can.
          The graph shows what happens to the speed of the lorry.

          Explain why the speed of the lorry increases at first but then levels out.
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 (3)
(Total 9 marks)
 Energy, work and Power-Mark scheme
M1.         (a)    (i)        kinetic (energy)
allow gravitational potential (energy) / gpe
movement is insufficient
1

(ii)     dissipates into the surroundings
allow warms up the surroundings / air / motor
accept lost to the surroundings
accept lost as heat
ignore reference to sound
it is lost is insufficient
1

(b)     energy (required) increases with load
accept positive correlation
do not accept (directly) proportional
1

further amplification eg increases slowly at first (or up to 4 / 5 N), then increases rapidly
simply quoting figures is insufficient
an answer that only describes the shape of the line gains no marks
1

(c)     (i)      E = P × t
2880
accept £28.80 for all 3 marks
an answer £2880 gains 2 marks
allow 1 mark for obtaining 48 h or converting to kW
allow 2 marks for correct substitution
ie 4 × 48 × 15
note: this substitution may be shown as two steps
an answer 2 880 000 gains 2 marks
an answer £4.80 / 480 gains 2 marks
an answer of 192 (ie calculation of energy without subsequent calculation of cost) gains 1 mark)
3

(ii)     any sensible suggestion eg
conserves fossil fuels
less (fossil) fuels burned
less pollutant gas (produced)
accept a named pollutant gas
less greenhouse gas (produced)
saves energy is insufficient
1
[8]

M2.          (a)     572
allow 1 mark for correct substitution,
ie 220 × 2.6
allow 1 mark for
220 × 260 = 57 200
or
220 × 2600 = 572 000
but to score this mark the entire calculation must be shown
2

(b)     (i)      smooth curve drawn 
accept a line that is extrapolated back to 0 degrees, but not through the origin
accept a straight line of best fit (point at 40 degrees can be treated as anomalous and line may stop at 30 degrees)
do not accept straight lines drawn ‘dot to dot’ or directly from first to last point or a line going through the origin 
1

(ii)     increases
accept a positive correlation
do not accept proportional
1

(iii)    long plank
no mark for this, the marks are for the explanation
makes the angle small(er) (than a short plank)
accept increases the distance
accept small(er) slope
1

a small(er) force is neededorshort plank
no mark for this, the marks are for the explanation
a large(r) force is used over a short(er) distance (1)
less work done (1)
accept less energy transfer
1
[6]
 
M3.         (a)      (i)     75 000
accept correct substitution for 1 mark
ie 7500 × 10
2

newtons / N
do not accept n
full credit for using g = 9.8 or 9.81
1

(ii)     60 000 000
accept for both marks
their (a)(i) × 800 correctly calculated
accept correct substitution for 1 mark
ie their (a)(i) × 800
2

(b)     (i)      arrow drawn parallel (to) and down (the) slope
accept arrow drawn anywhere on the diagram
1

(ii)     increases
1

GPE transformed to KEor
speed increasing
accept is accelerating
however ‘speed increasing’ only scores if correctly linked to increasing kinetic energy 
1

(c)     so more likely to wear one
or
they know wearing a helmet is likely to / will reduce (risk) head injury
or
so can make an (informed) choice (about wearing one)
1
[9]

M4.          (a)     (i)      work (done) = force (applied) × distance (moved)
accept W = F × s or W = F × d
accept provided subsequent method is correct
1
(ii)     240 000
allow 1 mark for correct substitution or correct use of 1200 (N)
2
         joules
accept J
do not accept j / Nm
1

(b)     800 (watts)
accept 0.8 kW
accept their (a)(ii) ÷ 300 correctly evaluated for 2 marks
allow 1 mark for correct substitution
(a)(ii) ÷ 5 correctly evaluated for 1 mark
2

(c)     (i)      any one from:
•        needs to raise the chair / lift
•        lifting more than one chair
allow lifting more than 2 people
implication of a heavier weight
•        energy transfer to the surroundings
correctly qualified
accept loss for transfer
do not accept motor inefficient
do not accept motor gets hot
do not accept friction unless the location is specified as external to the motor
1

(ii)     electrical
accept electric
         potential
both answers required for the mark
1
[8]
 
M5.          (a)     potential; bucket/pulley
for 1 mark each
2

(b)     300
gains 2 marks
          else working
gains 1 mark
2
[4]
 
M6.          (a)     mass
1

(b)     work (done) = force (applied) × distance (moved in the direction of the force)
do not accept correctly substituted figures for this equation mark
accept W = Fs or W = Fd or W = Fh (well done) = force × height)
mark formula independently
1

          1 000 000 × 15
allow 1 000 000 × 
1
          = 15 000 000
= 15 000
1
          J / joules
KJ / kilojoules
1

allow 1 000 000 × 1500
= 15 00 000 000 for 1 mark
only – no unit mark
allow 3 marks for correct answer if no working / correct working is shown

(c)     Quality of written communication
The answer to this question requires ideas in good English, in a sensible order with correct use of scientific terms. Quality of written communication should be considered in crediting points in the mark scheme
Max.4 if ideas not well expressed
          A – B not moving
accept stationary or at rest
1

          B - C acceleration or C – D
acceleration
accept increases speed / velocity accept gets faster
1
          comparison made that the acceleration
B – C is less than C – D
accept comparison made that the
acceleration C-D is greater than B-C
1

          D – E constant velocity
accept steady speed or at 0.4 m/s
1
          E – F deceleration
accept decreases speed / velocity
accept gets slower
1
[10]
 M7.          newton or N
          metre or m
          joules or J
all three correct 2 marks
two or one correct 1 mark
[2]
 
M8.          8550
correct answer with no working = 3
if incorrect, allow 1 mark for work = force / weight × distance, 2 marks for = 1900 × 4.5
N.B. correct answer from the incorrectly recalled relationship mass × distance = 2 marks
[3]

 

M9.          (a)     WD = force × distance or 6 × 2
gains 1 mark
          but 12 gains 2 marks
units J/joules [credit Nn]
for 1 mark
3

(b)     0.6 (i.e. using the recalled 10N/kg) 
gains 1 mark
          but evidence of force = mass × acceleration
or of correct substitution e.g. 6/9.8
gains 2 marks
          but 0.61 (2...)
gains 3 marks
3

(c)     any reference to
initial acceleration due to gravity
(force due to) friction/air resistance
each for 1 mark
          ideas that
this increases as speed increases
forces eventually balance
each for 1 further mark
9
[9]
 M10.          (a)     evidence of or 
 
gains 1 mark
          but 0.25 or ¼
gains 2 marks
2

(b)     evidence of or 
 
gains 1 mark
          but 200
gains 2 marks
2

(c)     idea that
          second car has a bigger mass
(allow bigger weight/heavier)
gains 1 mark
          but
second car has 1.5 times bigger mass
or
second car has mass of 1200 kg
gains 2 marks
2

(d)     friction/resistance increases with speed
gains 1 marks
          but
friction with/resistance of air increases with speed
gains 2 marks
•        increase in speed because driving force greater than friction
•        steady speed when friction = driving force
          or
increases in speed until friction = driving force
each for 1 further mark to maximum of 3
3
[9]