# Homework Help: Static and kinetic friction of a crate

1. Mar 10, 2009

### dcangulo

a dockworker loading crates on a ship finds that a 20kg. crate, intially at rest on a horizontal surface, requires a 75-N horizontal force to set it in motion. However, after the crate is in motion, a horizontal force of 60-N is required to keep it moving with a constant speed. Find the coefficients of static and kinetic friction between crate and floor.

how do i find the coefficients? there is a table in the book with coefficients but it doesnt have one with wood on floor.

2. Mar 10, 2009

### tnutty

K + U = K_o + U_o

what is k,u,k_o,u_o ?

3. Mar 10, 2009

### Staff: Mentor

You're supposed to solve for the coefficients using the information given in the problem, not look them up in some table.

Hint: How does static and kinetic friction relate to the normal force?

4. Mar 10, 2009

### dcangulo

$$\mu$$=f/N

where N is the normal force and f would be the 20kg?

Last edited: Mar 10, 2009
5. Mar 10, 2009

### Staff: Mentor

Good. But realize there's a difference between static and kinetic friction.

N is the normal force. What does the normal force equal? Hint: It's related to the 20 kg mass.

f is the friction force. Hint: The static and kinetic friction forces are given in the problem statement.

6. Mar 10, 2009

### Oscar Wilde

You need to solve for the two values of friction. Static friction being the frictional force that one must "overcome" to put the object into motion, and kinetic friction being the frictional force that must be "overcome" to sustain the objects motion.

7. Mar 11, 2009

### dcangulo

ok so what i did was find the normal force of the static friction which i believe was N=mg which is N=20(9.8) and then i did the little $$\mu$$=f/N with $$\mu$$=75/196

and that found me, what i believe to be the static friction.

Now how do i find kinetic friction coefficient??

8. Mar 12, 2009

### Staff: Mentor

Good! The normal force is just the weight of the crate. The maximum static friction force was given as 75 N. You combined those facts to find the static friction coefficient.

Exactly the same way. What's the kinetic friction force? Does the normal force change?

9. Mar 12, 2009

### dcangulo

i think kinetic friction force is 60... or do i subtract 75-60 and that'll give me the kinetic friction??

this is what i did

75-f$$_{}s$$=0
f$$_{}s$$=75

this is the only thing i can think of

10. Mar 12, 2009

### Staff: Mentor

Right.
No. Kinetic and static friction are two different things, so don't combine them.

But you calculate the coefficient of friction in the exact same manner for both:
μ = F/N, where F is the friction force and N is the normal force.