# Homework Help: Newtons 3rd law with a truck hitting a person

1. Feb 20, 2013

### Painguy

I having a little trouble grasping the 3rd law because of this idea in my head about a situation where a large truck going very very fast hits a person standing still.

1)Lets say the truck is going fast enough where the person's body will slightly rupture due to the impact. How can that tiny person exert such a large force back to the truck?

2)If during the collision the two bodies exert an equal magnitude of force on each other how is it that one of the bodies doesn't stop?

3)If the truck hit the person and continued moving in the same direction without hesitation then would the person start moving with the same force of the truck since its initial force was 0N or would it go flying ahead?

My guess for the last question is that person would move with the truck(if not on it). I figured that the scenario where the person goes flying is an exaggeration and that the person would seem to go flying ahead if the truck stopped after the collision. Thanks in advance. Sorry for all the questions I'm just genuinely curious. This isn't really a homework related question.

2. Feb 20, 2013

### Dick

The truck exerts the same force on the person as the person exerts on the truck. Just face it. Using F=ma, then if the truck is much more massive than the person then the effect on the truck is much less than the effect on the unfortunate person. That's really the answer to all your questions.

3. Feb 20, 2013

### Painguy

I appreciate the feedback, and I suppose your post answers my 1st question, but isn't there a more technical answer involving momentum and some other things for the other 3?

4. Feb 21, 2013

### Dick

Sure there's a technical answer. Suppose the mass of the truck is M, mass of the person is m and initial velocity of the truck is v. On one extreme the collision could be completely inelastic where the person splats onto the front of the truck and sticks there. You can easily work out the final velocity of the truck using conservation of momentum and get Mv/(M+m), so if m is small compared with M the truck will continue with slightly smaller velocity. On the other extreme the collision could be completely elastic (the bouncy kind). Then you can use conservation of momentum and energy to find the final velocity of the truck is v(M-m)/(M+m) and the final velocity of the person is 2vM/(M+m). So again the truck has slightly diminished velocity but the person get bounced ahead at almost twice the speed the truck was moving. Make up some numbers and put them in and see.