Download PGS Physics Department IB Work, Energy amd Power Worksheet

PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
1.
Name:
Electric motor
(a)
In an experiment to measure the efficiency of a small dc electric motor, the motor is
clamped to the edge of a bench. The motor is used to raise a small weight that is attached
to a pulley wheel by cotton thread. The pulley wheel is rotated by the motor. The thread
wraps around the pulley wheel, so raising the weight.
axel
motor
pulley wheel
cotton thread
weight
Side view
End-on-view
The time taken for the motor to raise the weight through a certain height is measured. It is
assumed that the weight accelerates uniformly whilst being raised. The weight of the
cotton thread is negligible.
(i)
Draw a labelled free-body force diagram of the forces acting on the accelerating
weight.
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(3)
(ii)
The weight has a mass of 15 g and it takes 2.2 s to raise it from rest through a
height of 0.84 m. Calculate the tension in the thread as the weight is being raised.
(Acceleration of free fall g = 10 m s−2.)
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(4)
1
PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
(b)
Name:
In a second experiment, the current is adjusted so that the weight of mass 15 g is raised at
constant speed. The motor is connected to a 6.0 V supply and it now takes the motor 3.4 s
to raise the weight through 0.84 m.
(i)
Suggest how it might be determined that the weight is being raised at constant
speed.
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(2)
(ii)
Determine the power delivered to the weight by the motor. (Acceleration of free
fall g = 10 m s−2.)
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(2)
(iii)
The current in the motor is 45 mA. Estimate the efficiency of the motor.
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(2)
(Total 13 marks)
2
PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
2.
Name:
Motion of a ball
A ball of mass 0.25 kg is projected vertically upwards from the ground with an initial velocity
of 30 m s–1. The acceleration of free fall is 10 m s–2, but air resistance cannot be neglected.
The graph below shows the variation with time t of the velocity v of this ball for the upward part
of the motion.
v / ms–1
30.0
25.0
20.0
15.0
10.0
5.0
0.0
0.0
(a)
0.5
1.0
1.5
2.0
2.5
3.0
t/s
State what the area under the graph represents.
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(1)
3
PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
(b)
Name:
Estimate the maximum height reached by the ball.
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(1)
(c)
Determine, for the ball at t = 1.0 s,
(i)
the acceleration;
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(3)
(ii)
the magnitude of the force of air resistance.
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(2)
(d)
Use the graph to explain, without any further calculations, that the force of air resistance
is decreasing in magnitude as the ball moves upward.
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(2)
4
PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
(e)
Name:
The diagram below is a sketch graph of the upward motion of the ball.
Draw a line to indicate the downward motion of the ball. The line should indicate the
motion from the maximum height of the ball until just before it hits the ground.
v / ms–1 30
20
10
0.0
0.0
2.0
4.0
t/s
–10
–20
–30
(2)
(f)
State and explain, by reference to energy transformations, whether the speed with which
the ball hits the ground is equal to 30 m s–1.
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(2)
(g)
Use your answer in (f) to state and explain whether the ball takes 2.0 s to move from its
maximum height to the ground.
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(2)
(Total 15 marks)
5
PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
3.
Name:
Block on an inclined plane
A block is held stationary on a frictionless inclined plane by means of a string as shown below.
string
block
inclined plane
(a)
(i)
On the diagram draw arrows to represent the three forces acting on the block.
(3)
(ii)
The angle  of inclination of the plane is 25. The block has mass 2.6 kg. Calculate
the force in the string. You may assume that g = 9.8 m s–2.
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(2)
(b)
The string is pulled so that the block is now moving at a constant speed of 0.85 m s–1 up
the inclined plane.
(i)
Explain why the magnitude of the force in the string is the same as that found in
(a)(ii).
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(2)
6
PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
(ii)
Name:
Calculate the power required to move the block at this speed.
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(2)
(iii)
State the rate of change of the gravitational potential energy of the block. Explain
your answer.
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(2)
(Total 11 marks)
4.
Kinematics
(a)
State the principle of conservation of energy.
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(1)
(b)
An aircraft accelerates from rest along a horizontal straight runway and then takes-off.
Discuss how the principle of conservation of energy applies to the energy changes that
take place while the aircraft is accelerating along the runway.
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(3)
7
PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
(c)
Name:
The mass of the aircraft is 8.0  103 kg.
(i)
The average resultant force on the aircraft while travelling along the runway is 70
kN. The speed of the aircraft just as it lifts off is 75 m s–1. Estimate the distance
travelled along the runway.
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(3)
(ii)
The aircraft climbs to a height of 1250 m. Calculate the potential energy gained
during the climb.
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(1)
(Total 8 marks)
5.
This question is about estimating the energy changes for an escalator (moving staircase).
The diagram below represents an escalator. People step on to it at point A and step off at
point B.
B
30 m
A
40°
8
PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
(a)
Name:
The escalator is 30 m long and makes an angle of 40° with the horizontal. At full
capacity, 48 people step on at point A and step off at point B every minute.
(i)
Calculate the potential energy gained by a person of weight 700 N in moving from
A to B.
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(2)
(ii)
Estimate the energy supplied by the escalator motor to the people every minute
when the escalator is working at full capacity.
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(1)
(iii)
State one assumption that you have made to obtain your answer to (ii).
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(1)
The escalator is driven by an electric motor that has an efficiency of 70%.
(b)
(i)
Using your answer to (a)(ii), calculate the minimum input power required by the
motor to drive the escalator.
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(3)
9
PGS Physics Department
IB Work, Energy amd Power Worksheet (Half Term work)
(ii)
Name:
Explain why it is not necessary to take into account the weight of the escalator
when calculating the input power.
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(1)
(c)
Explain why in practice, the power of the motor will need to be greater than that
calculated in (b)(i).
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(1)
(Total 9 marks)
10