4 Forces in action
Exam-style questions
OCR Physics A
1
a
Figure 1a shows two masses, A and B, attached to a light rope. The rope passes
over a fixed pulley and the mass B is held at rest in contact with the ground.
The mass of A is 3.0 kg.
The mass of B is 1.8 kg.
Figure 1a
i
Calculate the tension in the rope.
tension =
ii
N (1 mark)
The mass B is now released and A accelerates towards the ground. You
may assume that air resistance is negligible, and that there is no friction
between the rope and the pulley.
Calculate:
1 the downward acceleration of A
acceleration =
m s–2 (3 marks)
2 the time taken for A to reach the ground.
time =
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s (2 marks)
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4 Forces in action
Exam-style questions
OCR Physics A
b Figure 1b shows a porter checking the weight of a suitcase with a
newtonmeter. The mass of the suitcase is 19 kg.
Figure 1b
i
State the reading, in newtons, that you would expect to see when the
porter is standing on the Earth’s surface.
weight =
ii
N (1 mark)
The porter repeats the measurements in a lift as it moves up a tall building.
Figure 1c shows the speed–time graph for the lift.
Figure 1c
Describe and explain, without further calculation, the variation in the
newtonmeter reading as the lift ascends the building.
(3 marks)
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2
4 Forces in action
Exam-style questions
OCR Physics A
2
a
Define the newton.
(1 mark)
b A car of mass 950 kg travels along a horizontal road at a speed of 30 km h–1.
At this speed the air resistance is 90 N and the total forward (driving) force
is 170 N.
Figure 2 shows the horizontal forces acting on the car.
Figure 2
i
Calculate the acceleration of the car at 30 km h–1.
acceleration =
ii
m s–2 (2 marks)
A student is told that the driving force is kept constant for 5 s and is asked
to calculate the final speed of the car. Explain why the equation v = u + a t
cannot be used to complete this task.
(3 marks)
3
a
Define the torque of a couple.
(1 mark)
b Express the unit for torque in terms of its S.I. base units.
(1 mark)
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4 Forces in action
Exam-style questions
OCR Physics A
c
Figure 3a shows a metal plate of cross-sectional area 0.11 m², and constant
thickness 5.0 mm. There are three small holes, labelled A, B, and C, drilled in
the plate.
Figure 3a
Describe a simple experiment to determine the position of the centre of
mass, G, of the plate.
(3 marks)
d The plate is now pivoted at A and held in the position shown in Figure 3b by
a horizontal string passing through the hole C. By taking moments about A,
determine the tension, T, in the string.
The density of the metal plate is 8850 kg m–3.
Figure 3b
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4 Forces in action
Exam-style questions
OCR Physics A
T=
4
a
N (3 marks)
A heavy metal beam of weight 12 kN is suspended at rest from the hook of a
crane. The beam is held at an angle to the horizontal by two ropes AB and
BC. The rope AB is at 55° to the vertical, and rope BC is at 20° to the
vertical.
Figure 4a shows this arrangement and the free-body diagram for the hook.
Figure 4a
i
ii
Mark on Figure 4a the position of the centre of gravity of the beam and
label the point G.
Determine the tension forces T1 and T2.
(1 mark)
T1 =
N
T2 =
N (4 marks)
b Figure 4b shows a rectangular steel block of height 7.0 cm, width 5.0 cm, and
thickness 3.0 cm, hanging from the point A on a beam. The beam is pivoted
at the point P, and has mass 2.0 kg acting through its midpoint. The beam is
held horizontal by the counterweights suspended from C. The length CP is
10 cm, and the length AP is 36 cm.
Figure 4b
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4 Forces in action
Exam-style questions
OCR Physics A
i
Calculate the mass of the steel block.
Density of steel = 7.9 × 10³ kg m–3.
mass =
ii
kg (2 marks)
Calculate the mass of the counterweights required to maintain the beam
in equilibrium.
mass =
kg (2 marks)
iii State and explain how you would expect your answer to part ii to change
if the steel block was fully immersed in oil.
(2 marks)
5
A sphere of mass 12 g is allowed to fall from rest in a tank containing a liquid of
density 1600 kg m–3. Figure 5 shows the graph of velocity, v, against time, t, for
the falling sphere.
Figure 5
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4 Forces in action
Exam-style questions
OCR Physics A
a
Using the graph, calculate the initial acceleration of the sphere.
initial acceleration =
m s–2 (2 marks)
b Explain, with reference to the forces acting on the sphere, why the answer to
part a is not 9.81 m s–2.
(2 marks)
c
i
Determine the upthrust on the sphere.
upthrust =
ii
N (1 mark)
State Archimedes’ principle.
(1 mark)
iii Determine the volume of the sphere.
volume =
m3 (2 marks)
d Explain, with reference to the forces acting on the sphere, the shape of the
graph between t = 0 s and t = 6 s.
(3 marks)
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4 Forces in action
Exam-style questions
OCR Physics A
6
A sphere of radius, R, and mass, m, falling in a thick liquid experiences a drag
force determined by Stoke’s law. When the sphere has reached terminal
velocity, v, Stoke’s law suggests that m g = k R v, where k is a constant
dependent upon the liquid used.
a Describe how you would measure the radius and the terminal velocity for a
particular sphere. You should state the equipment required to complete the
measurements.
(4 marks)
b The measurements in part a were repeated for several spheres of identical
mass but different radii. Explain how you would use the data obtained to
verify that Stoke’s law may be applied to this situation.
(2 marks)
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8