# Homework Help: I have no intuition. Momentum problem

1. Aug 31, 2011

### flyingpig

1. The problem statement, all variables and given/known data

Q1) Suppose the mass of object A is greater than that of object B and that it is moving towards B, which is at rest.
Will:
A. Object A exerts a larger force on object B
B. The objects exert the same size force on each other
C. Object B exerts a larger force on object A

Q2) Suppose the mass of object A is much greater than that of object B and that object B is moving in the same direction as object A but not as fast. Predict:
A. Object A exerts a larger force on object B
B. The objects exert the same size force on each other
C. Object B exerts a larger force on object A

Q3) Suppose the mass of object A is much greater than that of object B and that both objects are at rest until an explosion occurs. Predict:
A. Object A exerts a larger force on object B
B. The objects exert the same size force on each other
C. Object B exerts a larger force on object A

3. The attempt at a solution

My so called intuition tell mse all the answer is B because of action = minus reaction. i know I am wrong...

Could anyone show me the intuiton method vs math method.

2. Aug 31, 2011

### Pengwuino

You're right, Newton's 3rd Law is the key here.

3. Aug 31, 2011

### PeterO

Your intuition is no doubt calling from the back of your mind.

Luke Skywalker heard "use the force" in Star Wars

You are hearing " Use Newtons Third law ! " in Physics.

Trust the voices you hear!

4. Aug 31, 2011

### Pengwuino

And see a medical professional.

5. Aug 31, 2011

### flyingpig

But I am using the dark forces...lead me to the right way.

6. Aug 31, 2011

### flyingpig

Okay I misread the question. It's asking the force at the impact or after? If it is after, it will always be B?

If it is at the point of contact then

1) It has to be A because B will experience a change in velocity?
2) No clue still
3) I am just going to guess that at the point of exploding, it is the same as 1)?

7. Aug 31, 2011

### Pengwuino

There will be no force after impact. The impact IS where the forces are happening.

And Newton's 3rd law says the forces will be equal in magnitude. Forces ALWAYS occur in equal magnitude pairs.

8. Aug 31, 2011

### flyingpig

For a second there I confused velocity and acceleration. Anyways I htink I was right initally then.

In all cases, the force is the same because the heavier mass will probably travel a shorter distance with a slower velocity given the same force, but the light mass will travel further with a greater change in velocity. Yes?

9. Aug 31, 2011

### PeterO

NO no no, it is always B. Newtons third law.

The size of the forces is equal. The effects of the force are different [Newtons Second law explains that one]

When the multi-kg baseball bat strikes the fraction-of-a-kg ball, the bat slows slightly, while the ball accelerates at an enormous rate [and the velocity it gains in that fraction of a second is enough to see it clear the fence before is falls back to ground]
Actually it would have to be golf - Body B was at rest

Example 3 is just the example of a rifle. Once the cartridge explodes, the rifle "recoils" slightly and the bullet heads off at extremely quickly under the influence of two forces, equal in magnitude and opposite in dirction.

10. Aug 31, 2011

### PeterO

In all cases, the force is the same. because The heavier mass will have probably travel a shorter distance with a slower smaller velocity change given the same force, but the light mass will travel further withhave a greater change in velocity.

Yielding:

In all cases, the force is the same. The heavier mass will have a smaller velocity change given the same force, but the light mass will have a greater change in velocity.

11. Sep 2, 2011

### Lobezno

You're right with all Bs.

But the acceleration will be different:

ma=m'a'
a=m'a'/m

For the acceleration on the first object, be it bigger or smaller.