# Homework Help: 3 problems

1. Jul 8, 2004

### 1moretry

i'm trying to finish up my online hw before the due date tonite at midnite. i have the rest perfect, just these three got me stumped.

7. [CJ6 4.P.038.] A cup of coffee is sitting on a table in an airplane that is flying at a constant altitude and a constant velocity. The coefficient of static friction between the cup and the table is 0.20. Suddenly, the plane accelerates, its altitude remaining constant. What is the maximum acceleration that the plane can have without the cup sliding backward on the table?

11. [CJ6 4.P.088.] A 75 kg water skier is being pulled by a horizontal force of 525 N and has an acceleration of 2.5 m/s2. Assuming that the total resistive force exerted on the skier by the water and the wind is constant, what force is needed to pull the skier at a constant velocity?

12. [CJ6 4.P.089.] A student presses a book between his hands, as the drawing indicates. 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 29 N. The coefficient of static friction between his hands and the book is 0.31. To keep the book from falling, what is the magnitude of the minimum pressing force that each hand must exert?

2. Jul 8, 2004

### NateTG

General Ettiquette

It's a good idea to post questions seperately. It's also generally a good idea to show some work. Otherwise people will not help you because they're not interested in doing your homework for you, and it doesn't look like you're trying.

That said, what do you know about static friction?

Can you list the forces in each situation, or write equations for them?

3. Jul 8, 2004

### 1moretry

whoops, my bad. i'll keep that in mind next time. thanks for not blowing up on me. this is what i got so far.

7. i really don't know where to go with this one. all i got is...

static frictional force = (coefficient of static friction) x (normal force)

fs = (.20)(Fn)

i'm trying to find the acceleration, so i'd have to use F=ma somewhere, but i don't have a mass, and the only acceleration i know is gravity.

11. the skier is 75kg, so the force of weight would be 735N down, and so the normal force would be 735N up, since there are no other up/down forces and that he's being pulled horizontally.

the question is what force is needed to pull the skier at a constant velocity, so that would mean zero acceleration, right? or no? i don't see where i can plug 0 into an equation like F=ma.

12. i understand that it is the static friction that is keeping the book from falling between the hands. so that would mean that the force of static friction is equal to the force of the weight of the book.

static frictional force = (coefficient of static friction) x (normal force).

fs = (.31)(29N) = 8.99

what can i do with this to find out the horizontal forces? they should be equal in this case, too, right?

Last edited: Jul 8, 2004
4. Jul 8, 2004

### Gokul43201

Staff Emeritus
7. What is the normal force equal to ?

11. You are given the force that the skier is being pulled with (525N). His mass is 75 kg. If this were the only horizontal force on the skier, his acceleration would be a = F/m = 525/75 = 7 m/s^2. But his real acceleration is less than this. Why ?

12. The normal force is not the weight of the book. The normal force is the force between the surfaces that are touching each other, ie : the force between the hand and the book cover.

For all these problems, the first thing to do is draw a free-body (force) diagram. Surely your text shows you how to do this.