# Inertial Frames Example

1. Feb 28, 2014

### jdawg

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

So I'm having some trouble understanding this paragraph from my text book. I was hoping that maybe someone could explain it to me.

For example, we can assume that the ground is an inertial frame provided we can neglect Earth's astronomical motions(such as its rotation). That assumption works well if, say, a puck is sent sliding along a short strip of frictionless ice-we would find that the puck's motion obeys Newton's laws. However, suppose the puck is sent sliding along a long ice strip extending from the north pole. If we view the puck from a stationary frame in space, the puck moves south along a simple straight line because Earth's rotation around the north pole merely slides the ice beneath the puck. However, if we view the puck from a point on the ground so that we rotate with Earth, the puck's path is not a simple straight line. Because the eastward speed of the ground beneath the puck is greater the farther south the puck slides from our ground-based view the puck appears to be deflected westward. However, this apparent deflection is caused not by a force as required by Newton's laws but by the fact that we see the puck from a rotating frame. In this situation, the ground is a noninertial frame, and trying to explain the deflection in terms of a force would lead us to a fictitious force, A more common example of inventing such a nonexistent force can occur in a car that is rapidly increasing in speed. You might claim that a force to the rear shoves you hard into the seat back.

2. Feb 28, 2014

### Staff: Mentor

Which part is unclear?

3. Feb 28, 2014

### jdawg

All of it! I'm beginning to get a grasp on the idea of inertial frames, but that specific example is throwing me off for some reason. I like to be able to put it into my own words when I'm outlining the chapter and I'm having difficulty visualizing what its trying to say.

4. Feb 28, 2014

### tiny-tim

hi jdawg!

the ice (in theory) has no friction, so the puck is not affected by whether the ice is moving

perhaps this is clearer on a rotating turntable made of ice …

if you slide a puck along the ice, it will move in the same straight line (relative to the ground) as if the turntable was stationary

relative to the rotating turntable, however, that straight line will not be straight

5. Mar 1, 2014

### jdawg

Ohh thank you so much! That makes so much more sense :)