Static friction
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Static Friction. Holding in Place. Objects on an incline will often stay put. There must be a force that holds the object in place. Static friction is from the contact of resting objects. Force holds up to a certain point Force is based on the type of contact (rough, smooth)

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Holding in place
Holding in Place

  • Objects on an incline will often stay put.

  • There must be a force that holds the object in place.

  • Static friction is from the contact of resting objects.

    • Force holds up to a certain point

    • Force is based on the type of contact (rough, smooth)

    • Maximum force is proportional to the pressing force of the object (normal force)


Inequality
Inequality

  • The approximate formula for static friction is:

    ms is the coefficient of static friction

  • This is an inequality.

    • The force of static friction is generally less than the coefficient times the normal force


Starting motion

m

q

Starting Motion

  • If Ffr < msFN = msmg cosq, then the block will hold.

  • At equality the block just begins to move.


Measuring friction pulley

Use a pulley and vary the weight to see when movement begins.

Forces balance on mass 1

Tension is equal on the rope

Forces balance on mass 2

m1

m2

Measuring Friction: Pulley

FT

Frope

FT

Frope


Coefficient of friction pulley

The pulley determines the coefficient of friction through the ratio of masses.

m1

m2

Coefficient of Friction: Pulley


Measuring friction incline

Use an incline and vary the angle to see when movement begins.

One component of gravity balances the normal force

The other component of gravity balances friction

m

q

Measuring Friction: Incline


Coefficient of friction incline

The incline determines the coefficient of friction through the tangent of the angle.

m

q

Coefficient of Friction: Incline


Normal force and friction
Normal Force and Friction the tangent of the angle.

  • Static friction depends on both the normal force and on the coefficient of friction.

  • To reduce friction requires reducing one of those factors.

    • Reduce normal force by lightening the load

    • Reduce normal force by adding additional upward force

    • Add a lubricant to reduce the coefficient of friction

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