Friction

Objective

• Calculate static and kinetic friction. • Calculate the angle of repose.

Homework

Size 2. Show your solution. •Answer no. 97 of page 142. •Answer no. 104 of page 143.– Additional Question – If the sled is on an inclined plane covered with

snow, what is the max angle of inclination that would still make the sled stay at rest?

Types of Friction

• Static Friction– This is the force exerted on one surface by

another when there is no motion between the two surfaces.

• Kinetic Friction – This is the force exerted on one surface by

another when the two surfaces rub against each other because one or both of them are moving.

Types of Kinetic Friction

• Sliding Kinetic Friction

• Rolling Kinetic Friction

Friction

Ff = µFn

Frictional Force

Coefficient of Friction

Normal Force

Coefficient of Friction

Ff = µFn

Fn

Kinetic Friction

Ffk = µkFn

Coefficient of Kinetic Friction

Static Friction

Ffsmax = µsFn

Coefficient of Kinetic Friction

The force required to start it moving. However, you need MORE than this amount to effectively

move it.

Coefficient of Friction

Surfaces µs µk

Rubber on dry concrete 0.80 0.65

Rubber on wet concrete 0.60 0.40

Wood on wood 0.50 0.20

Steel on steel (dry) 0.78 0.58

Steel on steel (with oil) 0.15 0.06

Example

a. You push a 25.0-kg wooden box across a wooden floor a constant speed of 1.0 m/s. How much force do you exert on the box?

b. If the force you exert on the 25.0-kg box is doubled, what is the resulting acceleration of the box?

Note: µk = 0.200

a. 49.0 Nb. 1.96 m/s2

Seatwork Size 4

• A 60.0-kg crate rests on a level floor at a shipping dock. The coefficients of static and kinetic friction are 0.760 and 0.410, respectively. What horizontal pushing force is required to (a) just start the crate moving and (b) to keep it moving at a constant speed.

Angle of Repose

• The angle of repose is the measure of an angle of inclination wherein a body at rest remains at rest and a body is motion will travel at a constant speed.

Angle of Repose

Θrepose = tan-1(µs)

Example

If a wooden block is at rest on an inclined plane whose surface is made of wood. the coefficient of static friction between them is 0.500. At what maximum angle of inclination would the block stay at rest?

Question

What is the angle of inclination is greater than the angle of repose?

The body moves downward at an increasing speed.

Seatwork Size 4

• A 60.0-kg crate rests on a level floor at a shipping dock. The coefficients of static and kinetic friction are 0.760 and 0.410, respectively. What horizontal pushing force is required to (a) just start the crate moving and (b) to keep it moving at a constant speed.