Net ForceNet Force

Normal ForceNormal Force

Frictional ForceFrictional Force

Free Body DiagramsFree Body Diagrams

Forces ReviewForces Review

Yesterday you learned Newton’s three laws:Yesterday you learned Newton’s three laws:1.1. An object with zero or constant velocity will An object with zero or constant velocity will

remain at zero or with a constant velocity unless remain at zero or with a constant velocity unless acted upon by an outside NET force.acted upon by an outside NET force.

2.2. An object experiencing an unbalanced net force An object experiencing an unbalanced net force experiences an acceleration in the direction of experiences an acceleration in the direction of the net force. (F = ma)the net force. (F = ma)

3.3. For every force there is an equal and opposing For every force there is an equal and opposing force.force.

These laws are the basis for the study of These laws are the basis for the study of dynamicsdynamics

Zero VelocityZero Velocity For example a book sitting For example a book sitting

on a table, is not moving and on a table, is not moving and does not move unless acted does not move unless acted upon by an unbalanced force.upon by an unbalanced force.

When it isn’t moving there When it isn’t moving there are two forces: gravity and are two forces: gravity and the table.the table.

The table’s force on the book The table’s force on the book is known as a NORMAL is known as a NORMAL FORCEFORCE

Since the two forces are Since the two forces are balanced then the book isn’t balanced then the book isn’t moving. The NET FORCE is moving. The NET FORCE is zerozero

If you remove one of the If you remove one of the forces and the net force forces and the net force becomes unbalanced then becomes unbalanced then the book accelerates.the book accelerates.

0NETF

NETF

Constant VelocityConstant Velocity Constant velocity can be Constant velocity can be

a little more difficult.a little more difficult. When a car is travelling When a car is travelling

down the highway at a down the highway at a constant speed the constant speed the forces balance and you forces balance and you have a zero net force.have a zero net force.

If you want to accelerate If you want to accelerate then you need to then you need to increase the force increase the force applied by the engine of applied by the engine of the car which creates an the car which creates an unbalanced NET FORCE.unbalanced NET FORCE.

0NETFNETF

Frictional ForceFrictional Force Frictional force Frictional force

depends entirely on depends entirely on the normal force the normal force applied to an object.applied to an object.

Depending on how Depending on how the two types of the two types of materials interact materials interact decides what the decides what the force of friction is.force of friction is.

Force of friction can Force of friction can be calculated using be calculated using the formula:the formula:

Materials in Contact

Coefficient of Static Friction* S

Coefficient of Kinetic Friction * K

Wood on wood 0.5 0.3

Waxed ski on snow 0.1 0.05

Ice on ice 0.1 0.03

Rubber on concrete (dry)

1.0 0.8

Rubber on concrete (wet)

0.7 0.5

Glass on glass 0.94 0.4

Steel on aluminium 0.61 0.47

Steel on steel (dry) 0.7 0.6

Steel on steel (lubricated)

0.12 0.07

Teflon on steel 0.04 0.04

Teflon on Teflon 0.04 0.04

Synovial joints (in humans)

0.01 0.01

* These values are approximate and intended only for comparison.

Nkf FF