# Homework Help: Inertial frame of reference

1. Nov 16, 2008

### alpha372

1. The problem statement, all variables and given/known data
What is an inertial frame of reference?

2. Relevant equations
-A particle at rest or moving at a constant velocity in an inertial frame of reference implies that the sum of the forces acting on the particle is zero
-The tendency of a body to keep moving once it is set in motion results from a property called inertia.

3. The attempt at a solution
My best guess is that it is a location in space where internal or external forces are not acting on it.

2. Nov 16, 2008

### HallsofIvy

Why guess? That's just asking for a definition. You can probably look it up in the index of your textbook.

3. Nov 16, 2008

### alpha372

My text just says, "A frame of reference in which Newton's first law is valid is called an inertial frame of reference." But, it doesn't give specific if then conditionals on the the phrase "in which Newton's first law is valid." For instance, it doesn't say, newton's first law is valid if.... or, "newton's first law is not valid if..."

Although, with a bit more reading, the text seems to hint that a frame which is accelerating is not an inertial frame of reference. I would like some verification for this, because the book doesn't just come right out and say it; I tend to have a preference for outright definitions.

4. Nov 16, 2008

### The 'Hoff

A simple phrasing of Newton's first law is "Objects will not change velocity unless acted upon by a force". Velocity is relative to your frame of reference.

Suppose you have a brick just floating there in space, light years from the nearest gravity well, for all intents and purposes unaffected by outside forces. Further suppose you arbitrarily accelerate your frame of reference to 200 miles per hour in some random direction. The brick has now changed velocity in your frame, despite not having been acted upon by a force.

So, no, accelerating frames are not inertial. As for definitions, Newton defined inertial frames as frames not accelerating relative to "the fixed stars", but that formulation is a bit out of date.