Impulse graphs
The force on the squash ball in the previous question is an average force and often the force changes during the collision. For this example the force鈥搕ime graph could look like this.
Notice the peak force is greater than the average force calculated.
The area under a force time graph is equal to the impulse.
For any collision with a fixed change in momentum, if the time of contact can be increased, the peak force is reduced:
For example if the squash ball was replaced with a softer version of same mass the collision graph would look like this:
If the squash ball was replaced with a harder version of same mass the collision graph would look like this:
In each case the change in momentum is the same, so the shaded area is equal but the peak force depends on the time of contact.
Question
Modern cars are designed to crumple on impact in a collision.
How does this help to protect the occupants from harm?
The change in momentum (area under the force time graph) can't be changed at the time of the accident (mass is fixed and it is too late for the driver to slow down!)
Crumpling on impact increases the time of collision. This means the peak force is less and hopefully lets the occupants come to less harm as a result.