Let's take a look at some question on forces and motion from IGCSE Physics 😁

Example 1

The graph shows part of a journey by a truck. The truck accelerates and then travels at constant velocity.

(a) Calculate the acceleration of the lorry during the first 6 seconds. 

acceleration = gradient of velocity-time graph 

= (14.8 - 6) / (6 - 0) 

= 1.5 m/s² 

(b) Calculate the distance traveled during the 8 seconds shown on the graph.

distance = area under velocity-time graph 

= (area of trapezoid from 0 to 6 s) + (area of rectangle from 6 to 8 s)

= (1/2) x (6 + 14.8) x 6 + 14.8 x (8 - 6) 

= 92 m

Example 2

A car travels along a very busy road. The graph shows how the distance traveled by the car changes during a six minute period.

(a) Calculate the total amount of time the car is stationary during this period.

• The car is stationary during the horizontal sections of the distance-time graph. 
• The car is stationary during stages B and D. 
• The car is stationary for 3.5 minutes.

(b) Explain which stage of the graph, A, B, C, D or E, shows the car moving at the slowest speed.

• speed = magnitude of gradient of distance-time graph
• During B and D, the car is not moving.
• If you compare A, C and D, A has the least steep line.
• The car moves at the slowest speed at stage A.

(c) Calculate the speed of the car at stage C.

speed = distance moved / time taken = 200m / 60s = 3.3 m/s

Example 3

The diagrams show the forces acting on four balls falling in air. Which diagram shows a ball decelerating as it falls?

• For the ball to decelerate as it falls, the acceleration should be upwards.
• The net force acting on the ball should be upwards.
• B has a net force of 3 N acting upwards.

Example 4

The graph represents the motion of a cyclist at the start of an Olympic race.

(a) Calculate the initial acceleration.

acceleration = gradient of velocity-time graph = (15 - 0) / (2.4 - 0) = 6.25 m/s² 

(b) Explain why the cyclist has to keep pedaling to maintain constant velocity. 

• Constant velocity means the acceleration of the cyclist is zero.
• From Newton's second law, F = ma, zero acceleration means the net force acting on the cyclist is zero.
• Air resistance, drag force, or friction acts backwards on the cyclist.
• To make the net force zero, there should be a forward force on the cyclist.
• The pedaling provides a forward force on the cyclist.

(c) The photographs show cyclists and the winning times for the same event in two different Olympic Games. The designs of the cyclists’ clothing and their bicycles have changed. These changes have helped the modern cyclist to improve the winning time from 5 min 14 s to 3 min 51 s. Describe these changes and use scientific principles to explain how this change has helped cyclists improve the winning time.

• The bicycle has less mass; when the same force is applied on the bicycle, there will be greater acceleration.
• Clothing of the cyclist has become more streamlined; less air resistance, so with the same driving force, there will be greater acceleration.
• Tires reduce frictional force.

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