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## Homework: Friction

Solve the following problems using what you know about Newton’s Laws and the four powerful equations. Be careful, some of the problems have multiple parts!

##### Kinematics Equations
1.) d = ½(v + v0)t
2.) d = v0t + ½at2
3.) v2 = v02 + 2ad
4.) v = v0 + at

##### Newton’s Laws
You know these! Remember: forces come in pairs,
no force is acting on a body in a constant state of motion,
and Fnet = ma
1 N = 1 kg · m/s2
##### Friction
Ffr = μFn where μ = coefficient of friction
μk = coefficient of kinetic friction
μs = coefficient of static friction

Hint: Use meters, kilograms, and seconds or units derived from these for all measurements! And answer questions with complete sentences.
1. What is the definition of kinetic friction?
2. What is the definition of static friction?
3. What force does the amount of friction depend on exclusively?
4. Which is greater: static or kinetic friction? Justify your answer using an example from your own experience.
5. The coefficient of static friction between a brick and a concrete floor is 0.5. If the mass of the brick is 2.1 kg, what force is required to get the brick to slide across the concrete?
6. What is the coefficient of kinetic friction if it takes 3.2 N to keep the 2.1-kg brick moving at a steady speed?
7. A force of 30.0 N is required to start a box moving across a horizontal concrete floor. If the coefficient of static friction (μs) is 0.4, what is the mass of the box?

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1. A 1,200 kg car skids to a halt in 6.9 s. If the car was moving at 37 m/s before skidding, calculate:
1. the acceleration of the car
2. the frictional force that stopped the car
3. the coefficient of friction between the car and the road.
Is the coefficient you calculated in part c. μk or μs?
2. The coefficient of kinetic friction between a wooden block with a mass of 0.5 kg and a table-top is 0.4. The coefficient of static friction is 0.5.
1. How much force is required to get the block to start sliding across the table?
2. How much force is requied to keep it moving at a steady speed?
3. How much force is required to accelerate it at 1 m/s2?
4. A 2.0-kg piece of steel is placed on top of the wooden block. Repeat the calculations you made above but now for the wooden block with the piece of steel on top of it.
3. The coefficient of kinetic friction on an air table is 0.000 001. How much force does it take to keep a 20-g puck moving at a steady speed? How much force is required to accelerate the puck at 1 m/s2?
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Last updated: Nov 14, 2006 Home