AQA GCSE Velocity time graphs and acceleration

AQA GCSE Velocity time graphs and acceleration(Physics)

Velocity time graphs and acceleration

Acceleration is defined as the rate of change of velocity.

A velocity time graph will show us how the velocity of an object changes over time.

The gradient (steepness of the line) will indicate the acceleration.

Straight line velocity time graphs

In the velocity time graph above, the gradient is 0 as its a straight horizontal line, so the acceleration will be 0m/s².

Is the object moving?

Yes, it is because it has a velocity of 10m/s. However, it is not accelerating as its velocity is not changing.

If the horizontal line were on the 0m/s then the object would be stationary and the object would still not be accelerating.

In the velocity time graph above, the line is straight, so gradient is constant and therefore acceleration is constant or uniform.

As the gradient is positive and the velocity is increasing with time, then the object is accelerating. However, as line is straight the acceleration is constant.

Finally, as the velocity has positive numbers, we will state that the object is moving in the forward direction. This means if the velocity had negative numbers the object would be moving backwards.

In the velocity time graph above, the line is straight, so there is a constant gradient, so constant acceleration or uniform acceleration.

Gradient is negative, velocity is decreasing, but always a positive value. So, the object has deceleration, or negative acceleration.

In the velocity time graph above, the gradient is constant so acceleration is constant or uniform.

Velocity is increasing in negative direction, so the object is accelerating and moving backwards.

The Train Journey

Lets use a velocity time graph to show a train journey.

Sections of the graph

a) Train moving forward, constant acceleration

b)Train moving forward at 10m/s, zero acceleration

c)Train moving forward, constant deceleration

d)Train moving backwards, constant acceleration

e)Train moving backwards at -10m/s, zero acceleration

f)Train moving backwards, constant deceleration

As a summary:

Velocity positive-move forward

Velocity negative -move backwards

horizontal line – constant velocity, zero acceleration

straight diagonal line constant acceleration or deceleration

Practice Questions

1.What information will the gradient of a velocity time graph give us?

2. How can you tell if the acceleration is uniform from a velocity time graph?

3. How could you identify if an object is not accelerating by examining the velocity time graph?

4. A ball is thrown upwards in the air, where it reaches a maximum height and then falls back to its original position. Use the velocity time graph below to answer the following questions.