Student Question

Factors Affecting Friction Experiment

I have to design an experiment to determine how certain factors (weight, surface area, and speed) affect the force of friction.

I have taken a tissue box weighing 122g and attached an elastic with a spring constant of 14.9N/m. The surface area is 688cm^2, but I'm pretty certain this has no effect, and neither would velocity.

Basically, I need some help understanding these friction equations. I need to figure out the force of friction, but I'm not sure which equations to use or how to use them. If someone could explain and/or offer some tips, I would be extremely grateful.

Expert Answers

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Simple friction is caused by the motion of one surface sliding against another.  The amount of frictional force is proportional to the force pressing two surfaces together.  For a book resting on the surface of the table the force is the weight of the book.  If we press the book into the surface the force increases by the amount of press.  If we lift up on the book the force is reduced by the amount of lift.  The force that best represents the amount that is pressing the two surfaces together is what we call the Normal force.

Normal force = Weight + Press into Surface - Lift from Surface

The friction is proportional to the Normal force.  The proportion deprends upon what the surfaces are, whether lubrication is present, the surface area and the speed.

For example, friction for wood sliding on wood is 30%-40%.  So if the Normal force is 100 N then the friction is 30 N -40 N.  The proportionality factor is called the co-efficient of friction.  (The coefficient of wood on wood is .3 - .4) There are tables of approximate values of the co-efficient of friction.

Friction force = coefficient of friction x Normal force

There are two forces of friction: static friction and sliding or kinetic friction.  Static friction is the force you need to overcome to get an object starting to slide from rest.  Kinetic friction is the friction you need to overcome to keep it going.  In general static friction is more than kinetice friction.

To measure friction have the box at rest.  Pull slowly on the spring until it just starts to move watching all the time.  You will see the force increase until it just starts to move and then ease up a bit.  The maximum force you see on the spring at the moment it starts to move is the static friction.  Once it gets going you need to pull just hard enough so that the box slides at a steady speed.  At steady speed the spring force will be equal to the kinetic friction.

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