Download Force of Friction When an object moves or attempts to move along

Force of Friction
When an object moves or attempts to move along another surface, there
is a force that opposes this movement called friction. A surface may
seem smooth to us when we touch or look at them but as we look with
microscopes even the most smooth surface is not truly smooth. As an
object slides over another surface the ridges of the rough surface will
contact and we believe this is what creates friction.
There are 2 types of friction that we will speak about.
1. Static Friction
Static Friction is friction between two objects that are not in motion. An
example of this is when you push a desk and it does not move, the desk
legs are feeling a static friction force that is equal to the force you are
applying to the desk.
In fact static friction has a maximum value for different objects and the
surfaces they are in contact with. That’s why as you push harder on an
object you get to a certain force and it suddenly starts moving. Through
experimentation it was found that this maximum static friction value
only has to do with the material of the two surfaces in contact (rubber
on ice, steel on wood) and the magnitude of the normal force.
Example: A sled is resting on a horizontal patch of snow, and s= 0.35. If
the sled and rider have a total mass of 38 kg, What is the magnitude of
the horizontal force needed to just start moving the sled?
2. Kinetic Friction
Kinetic Friction is a force between a moving object and the surface that
it is sliding upon. Again this force is found to be only reliant upon the
normal force and the coefficient of kinetic friction.
Example:
The sled is now in motion because you applied more force than the
static force of friction. Once in motion k= 0.05 and you are pulling with
a force of 100N.
a) Draw a free body diagram
b) What is FNet
c) What is the acceleration of the sled
d) If the sled gets up to 4.0 m/s and you stop pulling, how far will it
slide?
Friction Questions
1. A block with a weight of 45.0 N rests on a horizontal table. You apply a
horizontal force of 36.0 N. The coefficient of static and kinetic friction is
0.650 and 0.420, respectively.
a. Draw a free body diagram.
b. Will the block move and if it does what will be it’s acceleration?
[3.7 m/s2]
2. A drink is sitting on the table in an airplane. The plane all of a sudden starts
accelerating horizontally. If the coefficient of static friction is 0.30, what is
the maximum acceleration before the drink spills?
[2.94 m/s2]
3. A 95 kg baseball player slides into home. The coefficient of kinetic friction is
0.61.
a. What is the force normal?
[931 N up]
b. What is the force of friction?
[568 N back]
c. If the player comes to rest in 1.4 s, what was his initial velocity?
[ 8.37 m/s]
4. A skater with an initial speed of 7.60 m/s is gliding across the ice. Between
ice and steel k = 0.10.
a. Find the deceleration of the skater
[0.98 m/s2]
b. How far will she travel before coming to rest?
[29.4 m]
5. Cars have antilock brakes to shorten the distance required to stop. Antilock
brakes allow you to stop using static friction by prevent the tires from
sliding. A 1500 kg car is moving at 17 m/s on a snow packed road and slams
on the brakes. The two coefficient of frictions are k = 0.614 and s = 0.764.
a. What is the Normal Force?
[14700 N]
b. What is the friction force with no antilock brakes?
[9026 N]
c. What is the friction force with antilock brakes?
[11230 N]
d. How much less stopping distance is there with antilock brakes?
[4.7 m]
6. A 6.00 kg box is sliding across a horizontal floor of an elevator. The
coefficient of friction between the box and the floor is 0.360. Determine the
kinetic friction force acting on the box when the elevator is:
a. moving with a constant velocity of 10 m/s down
[21.2 N]
b. accelerating upward at 1.30 m/s2
[24.0 N]
2
c. accelerating downward at 1.30 m/s
[18.4 N]