Question

A 20 kg block rests completely flat on a rough table. The coefficients of static and kinetic friction between the block and table are 0.650 and 0.420. How much force is required to get the box moving?

Answer 1

To get it moving, you only need to consider the static friction.

Answer 2

Friction force = weight × coefficient.

Answer 3

ma = fpull - ff?

Answer 4

ffriction*

Answer 5

Yes.

Answer 6

if that is the case how do I find acceleration? Or how do I solve the problem with an unknown acceleration

Answer 7

What is the force of the pull?

Answer 8

I don't know. Does force pull have a specific equation?

Answer 9

Hmm, I think I'm getting what your actual problem is.

You have the mass of an object and you want to know how fast it'll accelerate after overcoming the static friction?

Answer 10

yes :)

Answer 11

Ah. Got it! It's a two-part question then.

Finding the pull force = static friction force is first, then you use that in ma=pull - kinetic friction to find the acceleration.

Answer 12

Use the two friction coefficients to find max static friction and max kinetic friction.
The max static friction is equal to the pull force, then subtract the max kinetic friction from the pull force to find ma.

\(\large F_{pull}=F_{static}=mg\mu_s\)

\(\large F_{kinetic}=mg\mu_k\)

\(\large ma = F_{pull} - F_{kinetic}=mg(\mu_s - \mu_k).\)

Answer 13

oops i did something wrong, hold on a second

Answer 14

so the answer is 45.08 N?

Answer 15

That's the answer to something. What's the question?

Answer 16

what is the force required to get the box moving

Answer 17

No, the force required to get the box moving is only the static friction to overcome.

Once the box has started moving, less force is required since kinetic friction is less than static friction.

Answer 18

If you want to know what the final net force will be, then yes, it is about 45.1N. You can divide that by the mass to get acceleration.