# B Acceleration and Forces

#### Kaneki123

This example is written in several books, that is if you strike a ball against a wall it applies an action force on the wall and the reaction force from the wall is the reason for the ball bouncing back..Now i have a question here that when you throw the ball, you accelerate it upto some velocity..When the reaction force from wall acts on the ball, there is a deceleration in the ball...As a result, the velocity should become zero or a positive one... Then why does the ball bounce back???

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#### jbriggs444

Homework Helper
Newton's third law does not tell you whether the ball will bounce back from the wall. If you make the ball out of lump of damp clay, it won't bounce back. If you make it out of a lump of rubber, it will. Both still obey Newton's third law: The force of the ball on the wall is, at all times, equal to the force of the wall on the ball.

#### A.T.

Then why does the ball bounce back???
Because the force keeps acting after the velocity became zero, due to deformation..

#### Kaneki123

Because the force keeps acting after the velocity became zero, due to deformation..
So, if we consider an ideal situation in which there is no deformation in the body,and the ball strikes with exactly the same force as the force we use to throw it, the velocity of ball will become zero...right???...

#### jbriggs444

Homework Helper
So, if we consider an ideal situation in which there is no deformation in the body,and the ball strikes with exactly the same force as the force we use to throw it, the velocity of ball will become zero...right???...
That's the opposite of an ideal situation. It's an indeterminate case. An infinite force exerted over an infinitesimal distance with an infinitesimal duration. Unless you fill in the missing details about elasticity, the result is undefined.

#### FactChecker

Gold Member
2018 Award
You have to say that the wall supplies the force that moves the center of gravity of the ball. Without a wall, a compressed ball that is released will just expand back to round without moving. It is the wall on one side, not allowing the expanding ball to expand in that direction, that makes the ball rebound away from the wall.

#### CWatters

Homework Helper
Gold Member
So, if we consider an ideal situation in which there is no deformation in the body,and the ball strikes with exactly the same force as the force we use to throw it, the velocity of ball will become zero...right???...
No. Force = mass * acceleration. So same force produces the same acceleration not the same velocity.

To work out the velocity you need to know how long the force was applied or the distance over which it was applied.

#### Kaneki123

No. Force = mass * acceleration. So same force produces the same acceleration not the same velocity.

To work out the velocity you need to know how long the force was applied or the distance over which it was applied.
The wall produces a negative acceleration in the ball...Let's say we throw the ball at the wall...And we are accelerating the ball from rest or zero velocity with some force...The ball moves with constant velocity until it strikes the wall...Let's say that the wall produces the same reaction force as the force we threw the ball with...Now this force will produce a negative acceleration in the ball until its velocity becomes zero...When the velocity becomes zero, no more force will be acting on the ball as the ball would no longer be striking the walll...It would be like deja vu...Please correct me if anything in above example is wrong...

#### jbriggs444

Homework Helper
A ball which is squished against the wall because of the positive velocity with which it initially impacted will still be squished against the wall when it comes to a stop.

#### nasu

The force during the collision is not constant. So unless the force you use to accelerate the ball has the same profile (F versus t) it does not make sense to say that the reaction force is the same as the force that accelerates the ball. And anyway, there is no relationship between the two processes. The collision force depends on the velocity of the ball. The same velocity of the ball can be reached by a practically infinite variety of forces accelerating the ball in thr initial stage of the process.

And second, during collision, the zero velocity is reached when both wall and ball are distorted form initial shape. Unless thy are both perfectly "plastic", the force wont be zero. Think about compressing a spring by trowing a ball at it. When the ball has zero velocity the spring has maximum compression. So the force exerted by the spring (or wall) on the ball has maximum value.

"Acceleration and Forces"

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