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PHYSICS 001

PROJECT 2

FRICTION

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Friction:

Definition

� Friction is the force that opposes the motion of 

an object.

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Ff = Fn

� This is the equation for the model of Coulomb

Friction, which was named after Charles-Augustin

de Coulomb.

� The equation relates the frictional force to the

normal force and allows a numerical value to be

assigned to the frictional force.

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� This is the coefficient of friction.

� It is a constant, but the number varies for different

materials.� For instance, the coefficient for rubber on concrete

is different than the coefficient for paper on glass.

� Also, the coefficient of static friction is different

than the coefficient of kinetic friction.

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Static Fr iction

� Static friction is the frictional force acting on astationary object.

� The coefficient of static friction is greater than thecoefficient of kinetic friction with similar surfaces.

� When using the equation of form Ff = Fn withstatic friction, the equals sign is misleading. Thefrictional force can actually be less than or equal tothe frictional constant times the normal force. Thenumber computed for Ff  by the equation is the

maximum possible frictional force. If a force of greater magnitude than the computed Ff acts onthe object in the opposite direction, then staticfriction will cease because the object will begin to

move.

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Kinetic Fr iction� Kinetic friction is the frictional force being exerted on

a moving object.

� The coefficient of kinetic friction is less than the

coefficient of static friction meaning that the frictional

force on a moving object can not be as large as the

frictional force on the same object while it isstationary and in otherwise similar conditions.

� Unlike the static frictional force, the kinetic frictional

force is always the same when the touching

surfaces are the same. The speed of the object isalso irrelevant when looking at the kinetic frictional

force. As the opposing force changes, the net force

and velocity of the object also change. However, the

kinetic frictional force remains unchanged.

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 A car is driving on a flat road at 20 meters per second and the driver sees a

deer and needs to come to a stop as soon as possible.If it does not skid, what is the shortest amount of time that the car could take to

come to a stop?

The car and passengers combined have a mass of 2,000 kilograms, and

therefore weigh about 20,000 newtons if the gravitational constant is

estimated as ten meters per seconds^2.

The coefficient of static friction between rubber and concrete is estimated at 1.

If the brakes are applied in the most efficient manner using static friction the car 

will stop in about 2 seconds.

Example: Static

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Example: Kinetic

Now we can see how long it would take the car to come to a

stop using kinetic friction.

This time all other aspects of the problem are the same except

the car will skid to a stop.

The coefficient of kinetic friction between rubber and concreteis estimated at 0.8.

If the car skids to a stop, using kinetic friction, it will stop in

about 2.5 seconds.

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New Insight

� From the examples we have gained insight on how

these frictional concepts apply to the world around

us.

� We learned that static friction stops a car, or anyother object, more quickly than kinetic friction does.

� This shows us that hitting the breaks harder will not

always stop a car more quickly. When skidding, a

car actually takes longer to stop.� It also shows us that when pushing a couch, it is

harder to get it started than it is to keep it moving.

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Conclusion

� This information helps us to understandfriction on a deeper level.

� We now see the difference between thefrictional force on a moving object and thefrictional force on a stationary object.

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Citation Page

� "Applications of Newton's Laws: Friction." 2 Dec.

2008

<http://gardner.byu.edu/105w1/index_files/class

%207.pdf>.� "Friction." Wikipedia. 2 Dec. 2008

<http://en.wikipedia.org/wiki/friction>.

� 2 Dec. 2008<http://images.google.com/imgres?imgurl=http://

www.mut.ac.th/~physics/physicsmagic/pic%252

0magic/nfw1.gif&imgrefurl>.