Everyday Forces

• Weight

• Normal Force

• Friction Force

Weight is Force of gravity acting on mass.

Mass is amount of matter.Weight = Fg = mg.

g = -9.81 m/s2 on Earth.

The direction of weight is straight toward the center of Earth .

Inertial and Gravitational Mass

What is the difference between the two?

Nothing!!

Inertial mass - defined by F = ma.

When a force F is applied to an object, it accelerates proportionally, the constant of proportion is the mass of that object.

a = F/m.

Gravitational mass has to do with force of gravity pulling on an object.

Gravitational mass is measured by comparing the Fg of an unknown mass to the Fg of a known mass. This is done with a balance scale.

Physically, no difference between gravitational and inertial mass.

Einstein used the fact that gravitational and inertial mass were equal to begin his Theory of General Relativity in which he postulated that gravitational mass was the same as inertial mass.

Normal Force Fn.

Perpendicular to surfaces in contact.

Fn

How does Normal Force Arise?

How does a wall know to push back harder when I push with increasing force?

Objects on a surface are repelled by electromagnetism.

Outer electrons in the object are electrically repelled by the electrons that at the surface.

The electrons offer a stronger and stronger repulsive force the closer and closer the object is moved to the surface.

The more massive, the greater gravity pulls them onto a surface, & the greater the surface repels the object.

The object will be at rest only if the surface can exert an equal & opposite force to sustain it, otherwise the object crashes through the surface.

Friction Between Surfaces

Friction Force Ff

Acts in direction opposite motion (or attempted motion). Which way is the box below moving?

Left

Friction depends on the nature of Friction depends on the nature of the materials in contact and the the materials in contact and the smoothness of their surfaces.smoothness of their surfaces.

A closer look at friction

At the microscopic level even two smooth surfaces look bumpy this is what produces friction

Magnified view of surfaces

. Friction is independent of surface area.. Friction is independent of surface area.

. F. Fff is directly proportional the normal is directly proportional the normal

force.force. Ff = Fn. Where is a constant.

The ratio of The ratio of FFff,,

to to FFnn, is, is

the the coefficient of frictioncoefficient of friction, , ..

=FFff

FFnn

hashasno units.no units.

Look at reference table

File Cabinet Demo

• depends on two surfaces in contact

• the type of sliding motion – starting or moving.

• Does not depend on any other factor.

• It is a constant.

Types of Friction

• Static Friction use st – Present for objects at rest. Need to overcome to start objects moving.

• Kinetic Friction or sliding use k – Present for objects in motion. Need to overcome to keep objects in motion. Always less than static friction.

Static friction

If we push on an object and it doesn’t move thenthe force we exert is less than the max Ff.

push, P friction, f

This is the static friction force at work.

If I push a little harder, the block may still not move the Ff can have any value upto some maximum.

Kinetic friction

• Keep increasing the Fap, at some point the block moves this occurs when the push P exceeds the maximum static friction force.

• When the block is moving it experiences a smaller friction force called the kinetic Ff .

• It takes more force to get something moving than to keep it moving.

• for kinetic always less than static.

1. A 10-kg rubber box is at rest on a dry horizontal concrete surface. Find:

.• a. Sketch the free body diagram.

• b. the weight of the box.

• c. the normal force on the box.

• d. the force needed to start the box

moving.

• e. the force to keep the box moving a

constant velocity.

• a.

• b. 98.1-N.

• c. 98.1-N

• d. 88-N.

• e. 66.7-N.

2: A 60-kg sled is being dragged on a horizontal snow surface by a force of 20-N at a constant velocity.

•Sketch the free body diagram.

•What is the friction force on the sled?

3. A 1-kg plastic toy chest is dragged across a horizontal wooden floor at constant velocity by an applied force of 5-N.

• What is , the coefficient of friction?• Would that be static or kinetic friction?

Fri

ctio

nal

For

ce

Res

isti

ng

Mot

ion

Force Causing the Object to Move

Kinetic RegionStatic Region

sF

kFMax

kFF s

kFF s

Constant Velocity with applied force.

F apF f

On a flat surface, as Fg increases Ff must increase.

Push down on a book as you slide it across a table – Fn increases so Ff increases too.

The coefficient of friction, cannot change, it is a constant.

More weight (Fg) = Higher Fn.

Ff = Fn increases.

Friction Problems 1 wksht

• Hwk Txt Read 141 – 145

• Do pg 145 #2,3 & pg 149 #4, 6

on separate sheet.

Friction Problems Inclines.

Inclined Planes

Fn is perpendicular to surface. Where is friction force if box is at

rest?

Friction opposes motion or attempted motion.

Friction

What is causing box to want to slide??

What holds the box in place?Friction

For a box at rest or constant velocity motion, Ff = Wll.