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Forces
Short-structured Questions
1. The diagram below shows two horizontal forces of magnitude 15 N and 20 N,
acting in perpendicular directions on a small body.
(a)
(b)
Using a scale of 2 cm to 5 N, determine the magnitude and direction of
the resultant force acting on the body.
[3]
The body rests on a horizontal ice surface and friction between the body and the ice can be neglected. The
body accelerates at 4.0 m s"1. Calculate the mass of the body.
[2]
[2]
(a) Calculate the power of the engine, assuming 100% efficiency.
1 David drives a bike at a constant speed of 25 m s'1 as shown in the figure below. The two forces resisting motion are
a wind resistance of 30 N and a frictional force of 80 N.
Forces
5. A car of mass 500 kg is being towed along a straight road so that its velocity changes uniformly from 10 m s'1 to 20
m s'1 in a time of 8 seconds. During this time, the frictional resistance is constant and equals to 500 N.
(a)
Calculate the acceleration of the car.
[2]
(b)
What is the resultant force on the car during the 8 seconds?
[2]
(c)
What is the force exerted on the car by the towing vehicle?
[2]
(d)
h
e
n
(ii) What is the towing force?
t
h
e speed of 20 m s"1 is reached, the towing force is reduced so that the car now moves with constant
velocity.
(i) What is the resultant force on the car now?
[1]
[1]
W
Forces
The car has a mass of 1000 kg. At one point: in its motion, when the combined forces of air resistance and friction
acting backward are 500 N, its acceleration is 1.6 m s-2.
(a)
Calculate the forward driving force required to accelerate the car.
[3]
(b)
With the engine working at constant full power, the car’s acceleration decreases as it goes faster. Explain why
this is so.
[2]
2
The figure on the right above shows the directions of the forces acting on a person of weight 0.70 kN when
being towed horizontally at a constant speed of 7.5 m s-1. The 1200 N force is the tension in the tow-rope
and the force labelled F is the drag force.
(a) State why the resultant force on the person must be zero.
[1]
2 In the sport called parascending, a person attached to a parachute is towed over the sea by a rope attached to a
motorboat, as shown in the figure on the left below.
Forces
9. In a performance test, a car of mass 800 kg is first accelerated to 20 m s'1 on a dry straight road. After
travelling at this speed for a while, the brakes are applied. The figure below shows the results,Speed ( m s 1 )
(a) What is the resultant force acting on the car in the time from (i) t =
0 to 5.0 s, and,
(ii) t = 5.0 s to 10.0 s?
(b) Find the total distance travelled by the car.
[2]
[1]
[3]
(c) Now the car is loaded so that its total mass is 1600 kg and is accelerated to 20 m s"1 from rest. If the net force
acting on the car is unchanged, find the time taken for it to accelerate to 20 m s'1.
[2]
Forces
10. A 5.0 kg mass is suspended by two identical strings as shown in the figure below. The strings are connected to two
other rubber pads which can be moved along a horizontal rod.
(a) What is the weight of the 5.0 kg mass?
[1 ]
11
.
A block of mass 4.0 kg was placed on a rough, horizontal surface where the force of friction wa.s 2.0 N. A man pulled the
block with a force of 14 N for 2.0 s.
14 N
(a)
On the diagram above, draw an arrow to indicate the force of friction.
[1]
What was the resultant, or effective, force causing the block to accelerate?
[1]
*
(b)
(c) What was the acceleration of the block while the man was pulling?
[2]
(e) What happened after 2.0 s?
[1]
[2]
Forces
13. The diagram below shows a racing car.
(a) Mark on the diagram the approximate position of the centre of mass of the car.
[1]
Label this as point C.
[2]
[2]
(b)
(c)
Mark and label external forces acting on the car.
State two features of the racing car that make it more stable than a normal passenger car.
14. A lorry of mass 5000 kg, which has been stuck in muddy ground, is being pulled by two jeeps. Each jeep exerts a
(a)
force of 3000 N at an angle of 20 to the direction in which the lorry is supposed to move.
pulled by jeep
(b)
Draw a suitable scale diagram, stating the scale used, to determine the magnitude of the resultant force
pulling the lorry forward.
[3]
Calculate the initial acceleration of the lorry, if the maximum friction between the mud and the lorry is
1500 N.
roi
(b) Two suggestions were proposed for the diagram on the right. One was to shorten the string in order to conceal
it behind the picture so that the point of suspension, is behind the picture itself. Another suggestion was
proposed to halve the 1.00 m separation to 0.500 m in the diagram on the right.
What are the differences between these two suggestions?
(c)
[2]
Using scale drawing, determine the tension of the string in the diagram on the right. Use 1 cm to represent 5.0
N.
[4]
16.
The figure below shows a truck of mass 2500 kg moving along a straight level road. The engines enables a forward
(a) Calculate the acceleration of the truck.
(b)
[2]
As the truck goes further with the same forward force, its acceleration decreases. Eventually, the truck
travels at a constant speed. Explain why this happens.
[3]
17. A car of mass 1500 kg is moving along the road at constant velocity. The forward driving force is 500 N and the
frictional force is f
force of 12 000 N to act on the truck. There are also counter forces totalling 3200 N against the truck.
(a) What is the magnitude of the frictional forcey?
(b) If the speed of the car is 25 m s'1, calculate the work done by the forward driving force in 20 s. [2]
[1]
18.
Position
Just after throwing
Force on the stone
5 N down
Acceleration
Speed
At the highest point
Just before being caught
10 ms"2 down
8 m s"1
A stone of mass 0.5 kg was thrown upwards and caught at the same position. Neglecting air resistance, complete the
following table.
[3]
19. In the diagram below, XY represents a flat kite of weight 4.0 N. At a certain instant, XY is inclined at 30 degrees to
the horizontal and the wind exerts a steady force of 6.0 N at right angles to XY so that the kite flies freely.
'
In the space below, draw a scale diagram to find the magnitude of the resultant force acting on the kite and the
angle which the force acts.
[3]
(ii)
How does the velocity change during the next 5 s?
(iii)
Does the velocity of the mass change during the last 10 s?
(iv)
Plot a velocity-time graph and calculate the distance travelled by the object in the 30 s.
[3]
"
[2]
[3]