Yet another Physics question...?

Question

A 90.0 kg box of books slides from rest down a frictionless incline from a height of 5.0 meters. A constant frictional force introduced at point A, then brings the box to rest at point B which is located 19.0 meters to the right of point A. (Assume sliding from A to B is along a horizontal surface).

a) What is the speed of the box just before it reaches point A?
b) What is the coefficient of kinetic friction, µ sub k, of the surface from A to B?

Answer 1

I will assume that the point A is while the box is sliding horizontally on a level surface.


Using conservation of energy
.5*m*v^2=m*g*h

v is the speed
I will use g=9.81

v=sqrt(2*g*h)
v=sqrt(2*9.81*5)
=9.90 m/s

for part b, the frictional energy is the frictional force times displacement
This will be equal to the energy gained sliding down the incline
the frictional force is
m*g*µ

so
m*g*µ*d=m*g*h

=h/d

=5/19
=0.263

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