7.3. Practice Sets: Force of Friction and Normal Force; Free Body Diagrams

Practice

1. A force of 355 N is needed to keep a 50 kg block sliding on the floor with a constant speed. What is the coefficient of kinetic friction
   Answer:

   0.72
2. Rubber tires and wet blacktop have a coefficient of kinetic friction of 0.550. A pickup truck with mass 999 kg travelling 35.0 m/s skids to a stop.
   a) Find the normal force of the truck.
   b) What is the frictional force that the road exerts on the truck?
   c) Find the acceleration of the truck.
   d) How far would the truck travel before coming to a rest?

   Answer:

   a) 9790.2 N
   b) 5384.6 N;
   c) -5.39 m/s²
3. A 10.0 kg block is pushed across a rough horizontal surface with a coefficient of kinetic friction of 0.480 by an applied force of 140 N. Find:
   a) Normal force acting on the block
   b) Force of friction
   c) Acceleration of the block

   Answer:

   a) 98 N
   b) 47.04 N;
   c) 9.3 m/s²
4. A 100 kg wooden crate rests on a wooden ramp with a 10° angle of inclination.
   Determine:
   a) the components of the crate’s weight that are parallel and perpendicular to the ramp
   b) the normal force between the crate and the ramp
   c) the static friction between the crate and the ramp

   Answer:

   a) 170 N, 9655 N;
   b) 965 N;
   c) 170 N
5. The coefficients of static and kinetic friction between an 8 kg concrete block and a level surface are 0.80 and 0.50 respectively. A horizontal force applied to the block just sets it in motion.
   a) Find the applied force.
   b) If this force is maintained while the block keeps moving, what is the acceleration of the block?

   Answer:

   a) 62.72 N;
   b) 2.94 m/s²)