P2 Revision Booklet

Energy, Work and Power

Easy (Grade D/E)

Q1.          A powerlifter lifts a 180 kg bar from the floor to above his head.

(a)     Use the equation in the box to calculate the weight of the bar.
 
weight    =    mass    ×    gravitational field strength	

gravitational field strength = 10 N/kg
Show clearly how you work out your answer.
........................................................................................................................
........................................................................................................................
                                               Weight = ............................................. N
(2)

(b)     The powerlifter uses a constant force to lift the bar a distance of 2.1 m.
Use the equation in the box to calculate the work done by the powerlifter.
 
work done    =    force applied    ×    distance moved in direction of force	

Show clearly how you work out your answer and give the unit.
Choose the unit from the list below.
 
joule	newton	watt	

........................................................................................................................
........................................................................................................................
                                           Work done = .............................................
(3)

(c)     At the end of the lift, the powerlifter holds the bar stationary, above his head, for two seconds.
How much work does the powerlifter do on the bar during these two seconds?
Draw a ring around your answer.
 
0	90	360	900	

Give a reason for your answer.
........................................................................................................................
........................................................................................................................
(2)
(Total 7 marks)
Q2.          (a)     The diagram shows a cricketer bowling a ball.

(i)      The cricketer bowls the ball at 20 m/s.
How could the kinetic energy of this ball have been increased?
...............................................................................................................
(1)

(ii)     The ball has a mass of 0.16 kg.
Use the equation in the box to calculate the momentum of the ball when it is bowled at 20 m/s.
 
momentum    =    mass    ×    velocity	

Show clearly how you work out your answer and give the unit.
Choose the unit from the list below.
 
kg m/s	m/s2	Nm	

...............................................................................................................
...............................................................................................................
                                           Momentum = .............................................
(3)

(b)     The batsman misses the ball and the ball hits the cricket stumps.

As the ball hits the stumps, the ball loses both kinetic energy and momentum.
(i)      What happens to the kinetic energy lost by the ball?
...............................................................................................................
...............................................................................................................
(1)

(ii)     Even though the ball loses momentum, the total momentum of the ball and stumps just before the ball hits the stumps is the same as the total momentum of the ball and stumps just after the collision.
Explain how this is possible.
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(Total 7 marks)

 Q3.          (a)     The weightlifter in the picture has lifted a weight of 2250 newtons above his head. The weight is held still.

(i)      In the box are the names of three forms of energy.
gravitational potential          kinetic          sound	

Which one of these forms of energy does the weight have?
..........................................................................................................................
(1)

(ii)     What force is used by the weightlifter to hold the weight still?
Size of force = .................................................... N
Give a reason for your answer ........................................................................
..........................................................................................................................
..........................................................................................................................
(2)

(b)     To lift the weight, the weightlifter does 4500 joules of work in 3.0 seconds.
          Use the following equation to calculate the power developed by the weightlifter. Show clearly how you work out your answer.

.....................................................................................................................................
.....................................................................................................................................
Power = ................................................. watts
(2)
(Total 5 marks)
 Energy, work and Power-Mark scheme
M1.          (a)     1800 (N)
allow 1 mark for correct substitution ie 180 × 10 provided no further steps shown
2

(b)     3780or
their (a) × 2.1 correctly calculated
allow 1 mark for correct substitution
ie 1800 or their (a) × 2.1 provided no further steps shown
2

joule
accept J
accept any clear indication of correct answer
1

(c)     0
reason does not score if 0 not chosen
1

work is only done when a force makes an object move
accept distance moved is zero
accept no energy transfer (to the bar)
accept the bar is not moving/is stationary
‘it’ refers to the bar/weights
1
[7]
 M2.         (a)      (i)     bowl the ball faster
accept increase its speed
accept a stated speed above 20 m/s
increase momentum is insufficient
bowl ball with greater power/force is insufficient
bowl ball harder is insufficient
do not accept increase mass of ball
1

(ii)     3.2
allow 1 mark for correct substitution i.e. 0.16 × 20 provided no further steps shown
2

kg m/s
accept any clear indication of correct answer
1

(b)     (i)      work done by ball to move stumps
accept transformed into heat / sound
accept transferred to surroundings
accept transferred to the stumps
do not accept absorbed by the stumps
1

(ii)     the stumps gain momentum
1
equal to momentum lost by ball
accept momentum is conserved
1
[7]
 M3.          (a)     (i)      gravitational potential
accept gravitational
accept potential
1

(ii)     2250 (N)
1
         forces must be balanced
or
forces are equal and opposite
do not accept because it is not moving
do not accept ‘equilibrium’ by itself
do not accept ‘it is not balanced’
do not accept ‘forces are equal’
do not accept ‘forces are the same’
1

(b)     1500
1 mark for correct substitution
2
[5]