Episode 215: Forces on vehicles

This brief episode considers the forces on vehicles when accelerating/decelerating. It considers the effect of the distance over which the force acts rather than the time of action of the force. The latter requires an understanding of momentum.


  • Discussion: Why wear seatbelts? (5 minutes)
  • Demonstration: An analogy for seatbelts (15 minutes)
  • Student questions: For discussion or homework (15 minutes)
Car crash

Discussion: Why wear seatbelts?
Why wear seatbelts? Students will happily suggest answers to this simple question. Let them. Then turn the discussion towards an answer framed in the physics of the situation. Be clear that they are, for the moment, considering the distance over which the force is acting. Many students will want to give answers in terms of the time over which the deceleration takes place. This is entirely reasonable and, of course, an equivalent argument. Nonetheless, it is a good discipline to limit the variables to force and distance.

It is unlikely that you will not, at some point in your career, teach a student who has been involved in a serious car crash or has had a relative that has suffered injury in this way. Be aware of this possibility when discussing this area and don’t treat it too lightly.

Students should be able to argue that work must be done to stop a person during a crash. Seatbelts stop the body over a greater distance and therefore reduce the average force on the body.

Ask: Can they think of other safety features which function similarly? Crumple zones and crash barriers do the same thing, as do egg boxes.

Demonstration: Why wear seatbelts?
An analogy for seatbelts:

Force experienced by an egg in car crash

A diverting demonstration that highlights the issues raised in the discussion. It is worthwhile putting in a few numbers to get a feel for the force on the egg as it breaks. A scheme for such a calculation is suggested in the experimental description. This can be compared to the force required to crack an egg by adding newton weights to it. The force the egg experienced in the crash will have been many times the weight of the egg. Similarly, the force on a crash victim can be much higher than body weight.

Episode 215-1: Why wear seatbelts? (Word, 43 KB)

Student questions: For discussion or homework
These questions cover similar ground to those in the previous episode but focus on the application of W = Fs to car safety.

Episode 215-2: Forces and car safety (Word, 26 KB)

Download this episode
Episode 215: Forces on vehicles (Word, 83 KB)