# Homework Help: Physics question involving vectors and force (from Calc II)

1. Feb 27, 2013

### Sooneroo

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

A ball rests on a straight track aligned in the direction <x,y,z> . The ball will remain stationary unless it experiences a force of at least 120 N along the direction of the track. If a wind is blowing in the direction <a,b,c> , what is the magnitude of the wind's force required to move the ball, and in which direction will the ball move?

2. Relevant equations

What is the magnitude of the wind's force required to move the ball, and in which direction will the ball move?

3. The attempt at a solution

I'm not really sure how I should approach this, I managed to get the angle between these two vectors (using dot product) and try to use trig functions to get the x, y, z components of the force and equate that to 120 N. I'm not too sure though.

Any hints to get me started?

[I took off the points and replaced them with variables deliberately]

2. Feb 27, 2013

### Whovian

First, this really ought to be in the physics subsection of the homework and coursework, not the calculus subsection.

Second, note that $\left|\mathrm{proj}_{\vec{u}}\left(\vec{v}\right)\right|=\dfrac{\vec{u}\cdot\vec{v}}{\left|\vec{u}\right|}$.

3. Feb 27, 2013

### Sooneroo

Yep, probably, but since we're here...

What exactly does projection mean? I know thats the equation for it, but how does it relate to the question?

Thanks.