coefficients of static and kinetic friction?

Question

A 52.0 kg crate rests on a level floor at a shipping dock. The coefficients of static and kinetic friction are 0.700 and 0.440, respectively.

(a) What horizontal pushing force is required to just start the crate moving?
_ N

(b) What horizontal pushing force is required to slide the crate across the dock at a constant speed?
_N

Answer 1

a) Force required = normal force * coefficient of static friction
= mass*acceleration due to gravity*coefficient of static friction
=52*9.8*0.7= 356.72N (hint: since its not moving u use static friction)
b) force required= normal force*coefficient of kinetic friction
= 52*9.8*0.44=224.224N

Answer 2

first you draw your free body diagram

then you sum the forces in the x- and y-direction.

Fx = Force - friction = 0 (since the box is on the verge of moving, but isn't actually moving)

Fy = Normal Force - Weight = 0 (since the box is not moving up or down)

We know that friction is = friction coefficient * Normal Force. The friction coefficient depends on whether the box is still (static) or moving (kinetic). In part a the box is on the verge of moving, so we use the static coefficient.

First from our sum of forces in they y-direction

Normal Force = Weight = m*g (g = 9.8 m/s^2)

From the x-direction:

Force = friction = static coefficient * Weight

a) Force = (0.7) * (52) * 9.8 = 356.72 N

For part b the summation would still be:


Fx = Force - friction = ma = m * 0 = 0(since the box is moving, but isn't actually speeding up or slowing down, constant speed)

Fy = Normal Force - Weight = 0 (since the box is not moving up or down)

In this case the difference would be the friction coefficient, since the box is moving.

Force = friction = kinetic coefficient * Weight

b) Force = (0.44) * (52) * 9.8 = 224.224 N

Answer 3

I'm guessing that your physics text book will tell you how to calculate the answer. You should be able to find the formula you need, it will involve the normal force (the downward force due to gravity) and the coefficient. A hint, with the lower coefficient, it takes less force to keep it moving at a constant speed than to get it moving in the first place.

Answer 4

I wish I had done physics sorry lol,
but the secret is always to divide/multiply something