# A few questions regarding Newton's Laws

1. Apr 24, 2008

### Cursed

Okay, I have a few questions. Feel free to answer any that you can. (Even if it's only one.) :tongue:

1. What is an inertial reference frame? (Provide examples please.)

2. The mass of an object is a quantitative measure of inertia. What does this mean?

3. Is a net force being applied to an object when the object is moving downward with a constant velocity of 9.80 m/s?

4. A father and his seven-year-old daughter are facing each other on ice skates. With their hands, they push off against one another. (A) Compare the magnitudes of the pushing forces that they experience. (B) Which one, if either, experiences the larger acceleration?

2. Apr 24, 2008

### Nick89

1. A frame of reference is basically a set of coordinates + a time scale.
An inertial frame of reference is (I believe) defined by the fact that Newton's first law is valid in the frame of reference.
Basically, if the frame of reference is accelerating (also counting rotation) then it's NOT an inertial frame of reference.

Consider yourself looking at two cars. One car is moving with constant speed and the other is accelerating. You and the car moving at constant speed are examples of an inertial frame of reference. The accelerating car however is NOT.

2. Not sure how to explain this in words, but consider Newton's first law which will be very familiar to you.
If you are in a car moving at constant speed, you will not feel any force (you wont get pushed in your seat or something). Now, the car suddenly brakes. You are pulled away from your chair.
Why is this? Because you were in motion before and are now stopping. Any body with mass 'does not like to stop moving' if it's moving. A body with mass will always want to keep moving at the same speed it is moving at.
I know I didn't explain this properly lol... Can't think of some better way to explain.

3. Consider Newton's second law: $$F_{\text{total}} = ma$$. If the object is movign at constant speed, a = 0, so from this it follows that F = 0.

4. A, newton's third law comes into play here.
If you apply a force to some object, the object applies a force back on you with equal magnitude but opposite direction.
This might sound weird at first but it's actually very logical. Consider yourself pushing down on the table. If the table is not pushing back against your hand, what is stopping your hand from moving right through the table??

4B. You know the forces that act on each body from 4A, now apply newton's second law again to find the answer to 4B.

3. Apr 24, 2008

### jandl

To help Nick here on his response to number 2, not that he gave a bad explanation.

Think of mass as a body's resistance to acceleration. Therefore if something is very massive and you push on it with a force it won't accelerate very much compared to if you pushed on a very low mass object with the same force. To call a mass a measure of inertia is equivalent. When we discuss linear accelerations we speak of a mass, when we discuss rotation we speak of a moment of inertia, it is important to note that this rotational moment of inertia is not the same as the moment of inertia to which you referred to originally, and this may be part of the reason we call it mass - to avoid confusion.

4. Apr 24, 2008

### Cursed

@Nick: I really appreciate your help. I understand everything much better now.

Thank you for clearing that up.