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AQA A-level Physics Velocity

Velocity

This page covers the following topics:

1. Terminal velocity
2. Velocity-time graphs for terminal velocity

Any object dropped near the surface of the Earth will accelerate downwards due to the force of gravity. As the speed of the object increases, the frictional forces (drag/air resistance) increase too. When the frictional forces increase enough to match the weight of the object, the resultant force on the object will be 0. Since there is no resultant force, the object will no longer be accelerating and will be travelling at a constant velocity called the terminal velocity.

In the case of a skydiver, when the parachute is opened, the air resistance suddenly increases due to the great surface area of the parachute, causing the skydiver to slow down. As the speed of the skydiver decreases, the air resistance force decreases too, until a new terminal velocity is reached when the weight of the skydiver and the air resistance become equal again.

Terminal velocity

The motion of a falling object that reaches terminal velocity can be modelled by the given velocity-time graph. The beginning of the graph shows the acceleration of the object downwards due to its weight. The slope then starts to decrease as the resistive forces start to increase with increasing speed. Eventually, the graph flattens out as the object reaches its constant terminal velocity.

Velocity-time graphs for terminal velocity

1

Use Newton's second law to explain terminal velocity.

Use Newton's second law to explain terminal velocity.

2

A pebble is dropped into a cylinder filled with water. Given that the mass of the pebble is 100 g, what is the drag force that will act on the pebble at terminal velocity? Use g = 9.8 m/sΒ².

A pebble is dropped into a cylinder filled with water. Given that the mass of the pebble is 100 g, what is the drag force that will act on the pebble at terminal velocity? Use g = 9.8 m/sΒ².

3

Explain what happens when a skydiver opens their parachute after having reached terminal velocity.

Explain what happens when a skydiver opens their parachute after having reached terminal velocity.

4

Draw the velocity-time graph for a skydiver who first reaches a terminal velocity of 60 m/s after 35 seconds of falling, opens their parachute 25 seconds after that and it reaches a second terminal velocity of 10 m/s 12 second after that, at which they travel for a further 8 seconds.

Draw the velocity-time graph for a skydiver who first reaches a terminal velocity of 60 m/s after 35 seconds of falling, opens their parachute 25 seconds after that and it reaches a second terminal velocity of 10 m/s 12 second after that, at which they travel for a further 8 seconds.

5

Define terminal velocity.

Define terminal velocity.

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