# Homework Help: A small question about action-reaction pair

1. Sep 21, 2010

### issacnewton

Hi

I have a small question in my mind about laws of forces. consider that we are hitting
a ball with a bat. now again lets say we are in outer space.. so there is no earth around...
before bat hits the ball, bat exerts its gravitational force on the ball and as a reaction
to this, ball exerts a gravitational force on the bat... now as bat hits the ball, bat exerts
a force on the ball and as a reaction to this , ball also exerts equal force on the bat....
and here I am little confused.. since gravitational forces between bat and the ball are still there, do we have to add the force of gravity to the force of impact while considering action-reaction pair at the time of impact ? how would you think about this

thanks
newton

2. Sep 21, 2010

### Lok

The gravitational force between bat & ball is very small, and is usually ignored. You can take it into account, but it will only change the "10th" decimal of the force that usually appears at a human powered ball bat collision.

3. Sep 21, 2010

### issacnewton

hi lok
my question was hypothetical. i know that the gravitational force will be very small. i am just trying to see if my argument is correct. to answer the action-reaction pair question, we will need to add the impact force + the gravitational force.........is it right ?

newton

4. Sep 21, 2010

### Lok

The two bodies bat & ball will behave from an gravitational POV like a freefall of one towards the other so any energy gained by them from bringing them closer will be lost by separating them. So basically only the collision will act upon the final velocity of the two. The Force is the addition of the 2 ( elastic collision & gravity).

5. Sep 21, 2010

### issacnewton

so it means the action is gravitational force exerted by the ball on the bat + the impact force exerted by the ball on the bat and the reaction is the gravitational force exerted by the bat on the ball + the impact force exerted by the bat on the ball ...........right ?
i want to understand this preferably without bringing in energy since usually in physics textbooks newton's third law and concept of forces is taught before introducing anything about "energy"

thanks

6. Sep 21, 2010

### Lok

Yes. Remember that there are 2 separate action reactions, gravity and collision. Each event happens simultaneous for both objects involved.

7. Sep 21, 2010

### issacnewton

so only talking about the impact between ball and the bat (forget gravity for a second), if we want to consider the magnitude of the impact forces, then we will need to consider the change in momentum when they collide............right ?

this hypothetical case is so interesting. when the ball and bat are still away from each other, the action-reaction pair involves only gravity but when they collide, there are two action-reaction pairs at the same time...