# Collision at constant velocity

1. Jan 14, 2010

### R Power

According to Sir Newton, F = ma
i.e if a body needs to be accelerated it requires some force basically.
or in reverse can we say that if a body needs to exert some force, it must have some acceleration.????????????????

then does it means a body moving with constant vel can't exert force on other body.??????

2. Jan 14, 2010

### Staff: Mentor

Newton's 2nd law says: In order to accelerate, a body must have a net force on it.
No, that doesn't follow. Hold a book in your hand. Is your hand exerting a force? Is your hand accelerating?
When the moving body collides with something, the force of the collision will change its velocity.

Last edited: Jan 14, 2010
3. Jan 14, 2010

### R Power

BTW the first and second law are all same, f=ma incporporates both laws, i never understood why two laws were made

4. Jan 14, 2010

### R Power

Yes Sir
Book is applying force on my hand and it is accelerating downwards at 10 m/s2 and my hand is applying force to book and accelerating at -10 , so as book appears to be at rest on my hand.
When i hold the book i found that:
Actually hand accelerates relative to book and book accelerates relative to hand simultaneously but for an observer outside of hand-book system like a man standing near me, both are at rest.

Last edited: Jan 14, 2010
5. Jan 14, 2010

### Staff: Mentor

You can think of the first law as defining an inertial frame of reference.

Nope. If the book is at rest in your hand, it's not accelerating. (Unless you are in free fall.) Just because the 'acceleration due to gravity' is 10 m/s2 doesn't mean that everything is accelerating at that rate. That would only be the case if gravity were the only force acting on the book.

6. Jan 14, 2010

### R Power

No sir,
U don't see book not accelerating doesn't mean it's not accelerating, u are outside of book-hand system.
Consider you are a book. You are freely falling, so you will experience acceleration(say relative to something), now suddenly you see a book down in your way which is accelerating exactly at negative rate of yours. When you hit it u transfer force on it because you had acceleration and it also exerted opposite force on you because it had negative acceleration. So that relative to some external observer, you both are at rest.

7. Jan 14, 2010

### Staff: Mentor

You're saying that you see the book accelerating, but I standing next to you will not? Come on.
So now you're talking about a book in free fall, not one held in your hand?
Are you viewing things from the accelerating frame of a falling book???
This is not making much sense.

If a dropped book collides with a table they will exert forces on each other, but not because they have 'acceleration'.

8. Jan 14, 2010

### R Power

I didn't saw the book accelerating but I imagined.