# Idea of greater force as neccesary to do anything

1. Jun 9, 2006

### Jessie

hello anyone i am new here and i will admit to no credentials of any kind on any physics subject now i dont know if it was newton or einstein but whoeveres law it is that states that for every action there is an equal and opposite reaction seems? to have something very wrong with it.to me it seems that if i use any amount of kinetic energy to say make my leg move under said physical law shouldnt there be some indivisible force welling up in front of my knee to stop it ?shouldnt some force or some absorbing force?happen to prevent the crankshaft in my snowmachine from turning period when i pull on the starting rope to go for a ride with my friends into the mountains or to prevent my finger from pulling the trigger on my rifle when i hunt moose. it goes on and on and on any and all answers to this would be greatly appreciated but probably not well understood aparently?

2. Jun 10, 2006

### teilchen

hold a sandbag/other equally random object in your hands while standing on some bathroom scales. now yank the object upwards as fast as possible, and observe the scales (this'll be tricky, pulling and observing simulatenously). put simply, there is your "opposing" force.

3. Jun 10, 2006

### Danger

As for walking, the opposing actions are that you are pushing the Earth away from you (not to any measureable degree, of course) as well as forcing air out of the space in front of your leg. There are also internal ones, what with the relationship between bones and tendons and muscles. The effect doesn't have to be noticeable to you for it to be there.

4. Jun 10, 2006

### Parlyne

The trick to understanding Newton's third law is to recognize that a force is actually an interaction between two objects. When you pull the trigger on your rifle, your finger and the trigger are interacting.

The action and reaction refer to the effects of the interaction on the two interacting objects. So, for example, when your finger exerts a force on the trigger, the trigger must exert and equal force back on your finger.

To put this more mathematically, we consider two interacting objects, A and B. Newton's third law states that:
$$\vec F_{A \rightarrow B} = - \vec F_{B \rightarrow A}.$$

This mean that the force A exerts on B is equal in strength and opposite in direction to the force B exerts on A.

Since these forces are acting on different objects, in no way do they prevent motion, since the only forces that can affect any object's motion are those acting directly on it.

5. Jun 10, 2006

### Hootenanny

Staff Emeritus
Parlyne hit the nail on the head, but I'd like to emphisise it again: Although the forces are equal in magnitude and opposite in direction they act on different objects