Warm-UpOctober 26, 2015

“Super Sliders” and other similar products claim to allow you to “effortlessly slide heavy furniture.” How do they work?

Free Body Diagram Answers

Force Situations Worksheet

FrictionSection 5.2

Two Types of FrictionKinetic Friction: When an

object is movingStatic Friction: When an object

is not moving (stationary)

(There are other types of friction that we will not discuss in this course.)

Kinetic FrictionKinetic friction is always in the

opposite direction of the object’s motion.

Depends on the Normal Force () and the types of surfaces that are sliding against each other

The Coefficient of Kinetic Friction () is a numerical property of the surfaces.

= Greek letter “mu”

Kinetic Friction Force

Nkkf FF ,

Example 1You are pushing a 20.0 kg box

horizontally at a constant speed of 2.0 m/s with . How much force do you exert on the box?

You-Try #1You are pushing a 25.0 kg box

horizontally with a force of 125 N. The coefficient of kinetic friction is 0.35.

a) What is the force of friction?b) What is the resulting

acceleration?

Warm-Up: October 27, 2015

A 30.0 kg box is being pushed at a constant speed on a horizontal surface with

Draw a free body diagramCalculate the force of

gravityCalculate the normal forceCalculate the force of

frictionCalculate the push force

Static FrictionStatic friction is always in the

direction that makes .Depends on the Normal Force (),

the types of surfaces that are in contact, and the magnitude of the applied force(s).

The Coefficient of Static Friction () is a numerical property of the surfaces.

= Greek letter “mu”

Static Friction Force

NSsf FF ,If there is a force trying to cause

motion, the static friction force will increase up to a maximum value before it is overcome and motion starts.

Example 2A 30.0 kg box is at rest on the

floor. If the coefficient of static friction is 0.55, what is the maximum push force you could apply to the box before it starts moving?

You-Try #2A 15.0 kg box is at rest on

the floor. If you have to push with a force of 95 N before the box starts to move, what is the coefficient of static friction?

Friction CoefficientsFor a given surface combination,

the coefficient of static friction will always be larger than the coefficient of kinetic friction.

It’s harder to get something to start moving than to keep it moving.

kS

Assignments:Read Section 5.2 of your

textbook (pages 126-130)Page 128 #17-20Page 130 #27, 28, 30Braddock House Clean-Up

Worksheet◦On website

Worksheet“Braddock House Clean-Up”

Warm-Up: October 28, 2015

A box is pushed along a horizontal floor giving the box a speed of 3.2 m/s. 1.7 seconds after the end of the push, the box stops. What is the coefficient of kinetic friction between the box and the floor?

Force Situations

October 28, 2015Read Section 5.2 of your

textbook (pages 126-130)Page 128 #17-20Page 130 #27, 28, 30Braddock House Clean-Up

Worksheet◦On website

Read Section 5.3 of your textbook