4. A force of 48 N is required to start a 7.8 kg box moving across a horizontal concrete floor.

Question

(a) What is the coefficient of static friction between the box and the floor?
=
(b) Does the coefficent of friction have a unit?

A. No, it is a ratio
B. No, the units are not real
C. Yes, it is measured in kilograms
D. No, it is just a simple number
E. Yes, it is measured in radians
F. Yes, it is measured in meters
G. No, there are no units of measure
H. Yes, it is measured in Newtons
(c) If the 48 N force continues, the box accelerates at 8.9 m/s^2. What is the coefficient of kinetic friction?
=

Answer 1

a. Wt.of box=m*g=7.8kg * 9.8N/kg=76.44 N
=Fn=Normal force or force perpendicular
to the floor.

Fs = u*Fn = u*76.44 N.=Force of static friction.

Fap-Fs = m*a
48 - u*76.44 = m*0 = 0
u*76.44 = 48
u=0.628 = Coefficient of static friction

b. A. No, it is a ratio. u = Fs/Fn

c. Fap-Fk = m*a
48 - Fk = 7.8*0.89 = 6.942
Fk = 48-6.942 = 41.06 N, = Force of kinetic friction.

u = Fk/Fn = 41.06/76.44 = 0.537