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## Activity: Friction

##### 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
Frictional resistance to the relative motion of two solid objects is proportional to the force which presses the surfaces together as well as the roughness of the surfaces. The force that presses two surfaces together is the normal force: Fn. It is called ‘normal’ because that is an old word for perpendicular, not b/c it is normally there. The frictional resistance force may be written:

Ffr = μFn where μ = coefficient of friction
μk = coefficient of kinetic friction
μs = coefficient of static friction
(FYI: μ is the Greek letter mu.) Static Friction Kinetic Friction
There are two kinds of friction: static and kinetic.

Static friction is the force that you have to overcome to get something to start moving. This forces follows Newton’s Third Law and only matches the force on an object up to a threshold (see graphic).

Kinetic friction is the force that opposes motion while something is moving. It is by nature less than static friction (see graphic). If it were greater then it would be harder to keep something moving than to get it started!

One important point: friction is independent of the surface area of contact. The force of friction depends only on the normal force!

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Solve the following problems using what you know about Newton’s Laws and the four powerful equations. All problems in this activity involve friction. Be careful, some of the problems have multiple parts! Do your work neatly on a separate piece of paper. Keep this page in your binder for future reference.

Hint: Use meters, kilograms, and seconds or units derived from these for all measurements! Draw pictures and use force arrows.
1. The coefficient of static friction between a brick and a concrete floor is 0.6. If the mass of the brick is 1.6 kg, what force is required to get the brick to slide across the concrete? What is the coefficient of kinetic friction if it takes 3.2 N to keep the brick moving at a steady speed?
2. If the coefficient of kinetic friction between a 12.0-kg crate and the floor is 0.30, what horizontal force is required to move the crate at a steady speed across the floor? What horizontal force is required if μk is 0?
3. A force of 25.0 N is required to start a 6.0-kg box moving across a horizontal concrete floor.
1. What is the coefficient of static friction (μs) between the box and the floor?
2. If the force continues, it accelerates at 0.50 m/s2. What is the coefficient of kinetic friction (μk)?
4. Suppose that you are standing on a train accelerating at 0.20g. What minimum coefficient of static friction must exist between your feet and the floor if you are not to slide?
5. A 1,000 kg car skids to a halt in 3.4 s. If the car was moving at 30 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.
6. Consider a wooden block that weighs 500 g with a mass of 2 kg resting on top of it. The surface area of the bottom of the block is 200 cm2. What is μs if it takes 7.0 N to start the block moving forward? What is μk if it takes 4.0 N to keep it moving at a steady speed?
7. The block in the previous problem is now turned onto its side so that the surface area in contact is just 100 cm2. What are μk and μs? What force is required to start the block moving? What force is required to make the block accelerate at 0.2 m/s2 once the static friction has been overcome?
Images courtesy of Hyperphysics
Last updated: Nov 16, 2006 Home