Download 5.3 Kinetic Friction and Accelerating

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Physics
5.3 Kinetic Friction and Accelerating
When considering an object being pulled along a horizontal surface with friction, if the applied force is
greater than the kinetic frictional force, the object will accelerate in the direction of the applied force (in
other words, the object will speed up).
In the illustration below, a force (Fa) is applied to an object. This force is resisted by kinetic friction (Ffk), also a force.
Note that the normal force, (FN), includes all forces pressing the moving surfaces together. Also, note that the Force of
Kinetic Friction is greater than the applied force in this case, so the block accelerates because the forces are unbalanced
and not in equilibrium.
Example: A cinder block being pulled by an applied force along a
sidewalk weighs 90 Newtons (9.2 kg). The coefficient of kinetic
friction is 0.35. How much force is required to accelerate the block
at a rate of 2 m/s2?
First find the Force of Kinetic Friction:
Ffk = μk FN
Ffk = (0.35)(90 N)
Ffk = 31.5 N
Then find what the Net Force on the block should be to accelerate
it at a rate of 2 m/s2:
Fnet = ma
Fnet = (9.2 kg)(2 m⁄s 2 )
Fnet = 18.4 N
So if we want the Net Force to be 18.4 N, and the Frictional Force is 31.5 N, here’s how we find the Applied Force:
Fnet = Fa + Ffk
18.4 N = Fa + (−31.5 N)
Note: The Frictional Force was made negative based on the direction in the diagram above
Fa = 49.9 N
To slide the block with an acceleration of 2 m/s2, Fa must be 49.9 N.
Directions: Answer each question completely. Where appropriate, show all work and substitute with units.
1. As more force is applied to a steel box sliding on a steel surface, the coefficient of kinetic friction will
a. Decrease
b. Increase
c. Remain the same
2. The table below lists the coefficients of kinetic friction for four materials sliding over steel.
Coefficient of
Material
Kinetic Friction
Aluminum
0.47
Brass
0.44
Copper
0.36
Steel
0.57
A 10-kilogram block of each of these materials is pulled horizontally across a steel floor at constant velocity.
Which block requires the largest applied force to keep it moving at constant velocity? ________________
3. A 75 kg copper block is being pushed across a steel surface by a force of 300 N. The block’s initial velocity is 5
m/s, and it is pulled until it reaches a velocity of 20 m/s.
a. Draw a free body diagram of the block.
b. What is the weight of the block?
c. What is the normal force acting on the block?
d. What is the frictional force acting on the block?
e. What is the net force acting on the block? (Hint: Use your free body diagram)
f.
What is the acceleration of the block? (Hint: Newton’s 2nd Law)
g. For how many seconds is the block accelerating? (Hint: Kinematic equations)
4. The diagram to the right shows a 2-kilogram object accelerating at 5 m/s2 on a rough horizontal surface.
a. What is the Frictional Force acting on the object?
b. What is the coefficient of friction between the object and the surface?