# Figuring forces (need some help)

1. Oct 12, 2004

### benji

Alright so I'm in AP Physics B, senior in HS. We've just started on forces and I was gone from a couple class periods and I'm just not getting how to work these problems. They seem easy enough when I read them, but then when I try to put it on paper I just get stumped. So I have a couple problems on my homework that if anyone would be able to tell me how to set it up and what not--I'd really appreciate it.

• 1
A student presses a book between his hands. The forces that he exerts on the front and back covers of the book are perpendicular to the book and are horizontal. The book weighs 31 N. The coefficient of static friction between his hands and the book is 0.40. To keep the book from falling, what is the magnitude of the minimum pressing force that each hand must exert?

2
The speed of a bobsled is increasing, because it has an acceleration of 2.4 m/s/s. At a given instant in time, the forces resisting the motion, including kinetic friction and air resistance, total 450 N. The mass of the bobsled and its riders is 270 kg. (a) What is the magnitude of the force propelling the bobsled forward? (b) What is the magnitude of the net force that acts on the bobsled?

Alright so those are two of the many problems I've been assigned to do, but the others are a lot like these. If you could just tell me how I would go about solving it or setting it up--just pointing me in the right direction, I'd really appreciate it. If you want to solve it go ahead, an example wouldn't hurt ;) --but by no means am I asking you to do this for me.

Thanks.

2. Oct 12, 2004

### arildno

Now, let's focus at 1.
Assume that the book is at rest.
1. Which forces must balance in the horizontal direction?
2.Which forces must balance in the vertical direction?

3. Oct 12, 2004

### benji

1. The force of friction and the applied force?
2. The force of gravity and the applied force?

...that may be wrong... :-/

4. Oct 12, 2004

### arildno

If you push horizontally on the book with your two hands, which direction will the forces they generate have?

5. Oct 12, 2004

### benji

Well they would be pushing against eachother so it would look like this:

-->[]<--

...however, my physics teacher likes the arrows heading away like this:

<--[]-->

...but it really shouldn't matter either way, should it?

I have to leave here in like 10 minutes to class :-/

6. Oct 12, 2004

### arildno

Well, so what you've just found out is that the forces exerted by your hands on the book are horizontal, and oppositely directed (right?)
In order to have balance in the horizontal direction, they must be equal in magnitude.
The balance in the vertical direction is cause by the two frictional forces and the force of gravity.
You should be able to figure out the trest on your own

7. Oct 12, 2004

### benji

So would this look like it's set up correctly to you:

x Fnet = P1 + P2 + Ff = 0
...where P1 is the force of hand 1, P2 is that of hand 2, and Ff is the force of friction. See this is where I get messed up, I can't solve anything from that equation. I know most of the logistics as we just disucssed, but putting it all into an equation and solving for the forces is the tough part for me. I'm sure that equation isn't even set up correctly. Here's my attempt at the Fnet in the y direction:

y Fnet = Fg + (P1 + P2 + Ff) = 0
...where Fg is the force of gravity and you already know the other variables.

8. Oct 12, 2004

### arildno

THE FORCES OF FRICTION IS VERTICAL; NOT HORIZONTAL!!!!!!!!!!!!!!!!!
SO: P1+P2=0 (horizontal)
f1+f2-mg=0
f1,f2 (upwards forces of friction between respective hands and book)

9. Oct 12, 2004

### benji

Alright, well thanks for your help arildno--really appreciate it. I have to go to class now, though.

I need to start posting sooner than 45 minutes before class!

10. Oct 12, 2004

### arildno

That's true; to have time at your disposal is necessary in order to learn physics