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# Physics/newtons law of motion

Question
I can't understand newton's 3rd law of motion.as example.....
1.the gravitational force acts on a freely falling body downward the centre of earth.imagine....it is an action force.so where the reaction force?
2.every action has an equal and opposite reaction.but when we push a car it moves.Why?

many students can't understand 3rd law.i'm also.please explain this clearly....

Hello sabbir,

This law causes problems for many. The key is to avoid misunderstanding what it says. This is out of my 1960's Physics book:
"To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts." Newton said it in Latin, but this is a common English translation.

Now let me go on to your questions. After discussing them, I'll try to say simply what Isaac was saying in today's language.

1. The action force is the gravitational force, which is the Earth attracting the body. The reaction force is the body attracting the Earth. You may think the Earth does not feel that attraction. Seems true, but is false. The Earth is so massive that the acceleration of the Earth due to its attraction to that body is so small that it is imperceptible. (Besides, I suspect someone on the opposite side of the Earth dropped something at the same time. lol)

Remember from Newton's 2nd that acceleration = force/mass. The Earth's mass is large, so this acceleration is minuscule. Consider the Earth and the Moon. Both the Earth and the Moon orbit around the system's center of gravity (in addition to orbiting around the sun). That center of gravity is a point within the Earth's surface. Reference:
http://en.wikipedia.org/wiki/Center_of_mass#Astronomy

2. Newton's 3rd does not say that the car should not move. The car accelerates in response to the sum of the forces acting on it. The reaction force in this scenario acts not on the car, it acts -- ON YOU. The reaction force is not another force acting on the car, it acts on you.

So, the car is pushed by you -- so it accelerates. Probably. There are forces on the car that act in the direction opposed to your push. They are mostly friction. Perhaps you are trying to push it uphill. That would provide another force that opposes your push. But if you push hard enough, it will move in the direction you are pushing it.

What is the reaction force doing then? It is pushing back on your hands, good thing too. Otherwise they might pass through into the car's body. Examine the forces on you. Your feet are trying to push backward on the ground, so the ground provides grip (friction) to push you forward. That's one reaction force on you. You try to push the car forward so the car pushes back on you. That's a second reaction force on you. Those are horizontal forces, I'm ignoring vertical forces on you for this discussion.

So, the ground is pushing you forward and the car is pushing you back. The result is that you slowly go forward (assuming your force on the car exceeds the forces of friction trying to hold the car in place).

Now I'll try to simply reword what Newton said.
If body A exerts a force on body B, then body B will exert an equal and opposite force on body A.
Notice that in this scenario, body B has only one force on acting it and body A has only one force acting on it.

I hope this helps,
Steve

Physics

Volunteer

#### Steve Johnson

##### Expertise

I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

##### Experience

I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

Education/Credentials
BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University