101 PHYS

CH3Part 2

Section (12)

Objectives

• understand Friction Force.• Draw free-body diagrams for objects at

rest and in motion with Friction Force.• Apply your understanding of kinetic

and static friction to solving problems.

Friction Force

• Friction forces are parallel to the surfaces in contact and opposite the motion or opposite the net force .

• Friction is a force that resists motion when two objects are in contact.

FFfriction

Why friction?

If you look at the surfaces of all objects, there are tiny bumps and ridges. Those microscopic peaks and valleys catch on one another when two objects are moving past each other.

Type of Frictional Forces

• 1- Static frictional force: is the frictional force that prevents two surfaces from sliding past each other.

.2- kinetic frictional force: when object is sliding.

• Static frictional forces always greater than kinetic frictional force.

Law of Frictional Forces

Ffs

FN

Where:fs : static friction force. ms : Coefficient of static friction force .

FN : normal force.

2 N

Friction and the Normal Force

4 N

The force required to overcome static or kinetic friction is proportional to the

normal force, FN.

FN

12 N

6 N

FN8 N

4 N

FN

Coefficient of Friction

Material on Material s = static friction k = kinetic friction

steel / steel 0.6 0.4

Synovial Joint in Humans

0.01 0.003

metal / ice 0.022 0.02

brake lining / iron 0.4 0.3

tire / dry pavement 0.9 0.8

tire / wet pavement 0.8 0.7

Law of Maximum Frictional Force

• When an attempt is made to move an object on a surface, static friction slowly increases to a Maximum value.

Where:fs : static friction force. ms : Coefficient of static friction force .FN : normal force.

In this module, when we use the following equation, we refer only to the maximum value of static friction and simply write:

Fs max = ms FN

kinetic friction

When F is greater than the maximum fs the object move.

fk = mk FN

Ffk

fk : Kinetic friction force. mk : Coefficient of kinetic friction force.FN : normal force.

Coefficient of Friction

Material on Material s = static friction k = kinetic friction

steel / steel 0.6 0.4

Synovial Joint in Humans

0.01 0.003

metal / ice 0.022 0.02

brake lining / iron 0.4 0.3

tire / dry pavement 0.9 0.8

tire / wet pavement 0.8 0.7

Friction forces and area

If the total mass pulled is constant, the frictional force is independent of the contact area.

4 N 4 N

Friction forces and speed

2 N

5 m/s2 N

20 m/s

The force of kinetic friction is the same at 5 m/s as it is for 20 m/s. So:kinetic friction forces are independent of speed.

EXAMPLE

If ms = 0.5, what horizontal pull F is required to just start a 250 N block moving?

F

For this case: F – fs = 0

fs max =

msNFN = ?

SFy = 0 FN – Fg = 0

Fg = 250 N FN = 250 N

For this case: F– fs max = 0

Next we find fs max from:

fs max = ms FN = 0.5 (250 N)

F = fs max = 0.5 (250 N)

F = 125 NF = 125 N

EXAMPLE If the horizontal force F = 75 N pull the 250 N block is required to move with constant velocity. What is

a) The Kinetic friction force? b) Coefficient of kinetic friction force mk?

F

SFy = may = 0

FN- Fg= 0

FN= Fg=250N

b) fk = mkFN

mk = fk / FN

) a SFx= 0; F - fk = 0

fk = F=75N

mk = fk / FN =75/250=0.3

mk = fk / FN =75/250=0.3

Example

A force F = 67 N applied on a 300 N block by a rope at an angle of 400 above the horizontal surface. If the block moves with constant velocity and mk = 0.2.

a) Find the normal force FN exerted on the block by the surface?

b) What the frictional force?

F = 67 N

SFy= may

FN+ 67 sin 40 – 300 = 0

FN = 300 –44=256 N

a) FN =?

fk - F cos 400 = m (0)

fk - 67 cos 400 = 0

fk = 67 cos 400

= 51 N

SFx= max

b) fk = ?

Home Work

• Q: 75-79 Page 72