crate on truck=150kg,static friction=.70,kinetic friction=.35,g=10,acceleration of truck=2?

Question

what is the magnitude of the force?
a) is it static or kinetic friction? why?
b) what is the max acceleration the truck can have before the crate starts slipping?

Answer 1

ID all the forces on the crate.

There is the force due to acceleration, its pushing backward at A = ma = 150*2; where m = 150 kg crate mass and a = 2 m/sec^2 crate/truck acceleration.

There is the static friction force pushing forward, a F = kN = kmg; where k is the static coefficient of friction = .70.

The net force f = F - A = 0 because the crate is not moving. Thus, F = kmg = ma = A; so that a = kg. This means as long as a <= kg the crate will not slide.

a) As a = 2 <= .7*10 = kg, this is static friction.
b) At a > kg or a > 7 m/sec^2 the crate will begin to slide.

Answer 2

a. F = 9.8*150 = 1470 N

b. a = Fnet/m= 1470(0.70 - 0.35)/150 = 3.43 m/s