Impulse on a Force-Time Graph (AQA A Level Physics): Revision Note

Impulse on a force-time graph

• In real life, forces are often not constant and will vary over time

• If the force is plotted against time, the impulse is equal to the area under the force-time graph

• This is because

$\mathrm{Impulse}=\mathrm{Force}\times \mathrm{Change}\mathrm{in}\mathrm{time}$

• The impulse is therefore equal whether there is a small force over a long period of time or a large force over a small period of time

• Since change in momentum is equal to impulse, the same change in momentum over a longer period of time will produce less force (and vice versa)

• The force-time graph may be a curve or a straight line

  • If the graph is a curve, the area can be found by counting the squares underneath

  • If the graph is made up of straight lines, split the graph into sections

    • The total area is the sum of the areas of each section

Impact forces

• Impact forces are reduced by increasing the contact time

• This fact is used in everyday life to lower the risk of injury

• Some examples of where reducing impact force is important:

  • in sport

  • in packaging

In sports

• For example, in cricket, when a fielder relaxes their hands and pulls them back when catching a ball

  • A cricket ball travels at very high speeds and therefore has a high momentum

  • When a fielder catches the ball, it exerts a force onto their hands

  • Stopping a ball with high momentum instantly will cause a large force on their hands

  • This is because a change in momentum (impulse) acts over a short period of time which creates a large force on the fielder's hands and could cause serious injury

  • A fielder moves their hands back when they catch the ball, which increases the time for its change in momentum to reduce

  • This means there will be less force exerted on the fielder's hands and, therefore, less chance of injury

• In football:

  • Increasing the contact time is sometimes used to advantage, as the longer the contact time, the larger change in momentum

  • When kicking a football, after a strong kick the motion is followed through

  • The momentum from the foot is transferred to the ball

  • This creates a large impulse and the ball then has a higher velocity

In packaging

• Packaging, especially for fragile items, uses bubble wrap or polyester packaging to reduce the impact forces that items experience in transit

• These help cushion the items by increasing the time over which they experience a force, which reduces the risk of damage