# Homework Help: Basic normal force problems

1. Oct 11, 2007

### mattystew

starting the problem and then direction from there requested

A 38 kg crate rests on a horizontal floor, and a 63 kg person is standing on the crate.
(a) Determine the magnitude of the normal force that the floor exerts on the crate.
(b) Determine the magnitude of the normal force that the crate exerts on the person.

A woman stands on a scale in a moving elevator. Her mass is 58.0 kg, and the combined mass of the elevator and scale is an additional 815 kg. Starting from rest, the elevator accelerates upward. During the acceleration, the hoisting cable applies a force of 9430 N. What does the scale read during the acceleration?

A 19.5 kg sled is being pulled across a horizontal surface at a constant velocity. The pulling force has a magnitude of 77.0 N and is directed at an angle of 30.0° above the horizontal. Determine the coefficient of kinetic friction.

The drawing shows a large cube (mass = 46 kg) being accelerated across a horizontal frictionless surface by a horizontal force P. A small cube (mass = 3.6 kg) is in contact with the front surface of the large cube and will slide downward unless P is sufficiently large. The coefficient of static friction between the cubes is 0.71. What is the smallest magnitude that P can have in order to keep the small cube from sliding downward?

drawing: http://www.webassign.net/CJ/04_47.gif

Last edited: Oct 11, 2007
2. Oct 11, 2007

### learningphysics

can you show some work? what did you try? where are you getting stuck...

3. Oct 11, 2007

### mattystew

everywhere, our teacher didn't teach us this yet so I have no idea where to even start.

4. Oct 11, 2007

### learningphysics

Do you know what the normal force is? For the first problem... part a) take the system of the crate and the person together... what are the forces acting on this system?

5. Oct 11, 2007

### mattystew

gravity, and the floor pushing back on both

6. Oct 11, 2007

### learningphysics

Yes, so what do these forces add to?

So taking the system of the crate and the person together... what is the gravitational force... what is the normal force?

7. Oct 11, 2007

### mattystew

gravity would be 9.8 and normal would be 91

8. Oct 11, 2007

### mattystew

oh wait, 9.8x91=891.8

9. Oct 11, 2007

### learningphysics

63+38 = 101 not 91...

10. Oct 11, 2007

### mattystew

oh whoops. 989.8 then
it's been a long day :P

11. Oct 11, 2007

### learningphysics

lol. no prob. But I think you know more than you're letting on. ;) you solved part a) of the first problem with no troubles...

12. Oct 11, 2007

### mattystew

for part b, I'm not sure if I should use the mass of the crate, or use the force from part a, or add them together?

13. Oct 11, 2007

### learningphysics

For part a), the trick was to analyze the crate+person together as one system.

In part b), you want to analyze the person alone... what are the forces acting on the person?

14. Oct 14, 2007

### mattystew

I figured out the first two problems, but I'm having trouble with the friction problem.

15. Oct 14, 2007

### pooface

Coefficient of Kinetic friction = Kinetic friction force / Normal force.
Kinetic friction force is the force which opposes the force pulling or pushing an object on a surface.

if Kinetic friction force < acting force = object will be sliding/moving.
if Kinetic friction force > acting force = object will not be sliding/moving.

Change masses to their weights. Then see which formulas will give you what you need.

More than welcome to comeback if you are stuck or have questions. Show us what work you have done on the problem and we will see if we can guide you.

16. Mar 3, 2010

### toakley7

For the part B, the forces acting on the person are the weight of the person and the normal force of the crate. Would you set them equal to one another?