1. Aim: More friction Do Now:
Why do objects at rest have more friction than objects in motion?

2. Draw a free-body diagram labeling all the forces
A piece of steel whose mass is 100 kg is placed on a steel beam as a force is applied to it causing it to move with a constant velocity.
Draw a free-body diagram labeling all the forces FN FF F Fg

3. What is the normal force? FN = Fg FN = 980 N What is the net force?
What is the weight?
Fg = mg
Fg = (100 kg)(9.8 m/s2)
Fg = 980 N
What is the normal force?
FN = Fg
FN = 980 N
What is the net force?
0 N (velocity is constant) FN
= 980 N FF F Fg
= 980 N

4. A new force of 700 N is applied. Does steel move?
Solve for the maximum static force of friction
FF = µsFN
FF = (0.74)(980N)
FF = N
The steel does not move.
To get it to move, you must have a force greater than N

5. Draw and label a free-body diagram
A car whose mass is 1,000 kg is accelerating at a rate of 5 m/s2 with a force of 10,684 N on a surface containing friction.
Draw and label a free-body diagram FN FF
F = 10,684 N Fg

6. What is the normal force? FN = Fg FN = 9,800 N What is the net force?
What is the weight?
Fg = mg
Fg = (1,000 kg)(9.8 m/s2)
Fg = 9,800 N
What is the normal force?
FN = Fg
FN = 9,800 N
What is the net force?
FNet = ma
FNet = (1,000 kg)(5 m/s2)
FNet = 5,000 N FN
= 9,800 N
F = 10,684 N FF Fg
= 9,800 N

7. What is the force of friction?
FN = 9,800 N FF
= 5,684 N
F = 10,684 N
Fg = 9,800 N
FNet = F – FF
5,000 N = 10,684 N – FF
FF = 5,684 N

8. What is the coefficient of friction?
FF = µkFN
5,684 N = µk(9,800 N)
µk = 0.58
What two objects are in contact with one another?
Rubber on wet concrete

9. An 80-kilogram skier slides on waxed skis along a
horizontal surface of snow at constant velocity
while pushing with his poles. What is the horizontal component of the force pushing him
forward?
F = Ff (constant velocity)
F = μkFN
F = μkFg (FN = Fg)
F = μkmg
F = (0.05)(80 kg)(9.8 m/s2)
F = 39.2 N

10. A car’s performance is tested on various
horizontal road surfaces. The brakes are
applied, causing the rubber tires of the car to slide along the road without rolling. The tires encounter the greatest force of friction to stop the car on
dry concrete
wet concrete
dry asphalt
wet asphalt