N e w t o n’ s L a w s

Newton’s

LawsUnit 3

3.1 Force and Mass

3.1 Force and mass

Force – push or pull; required to change an object’s motion.

Vector – so magnitude and direction

Example of Contact Forces

Friction

Tension

Applied

Spring

Examples of Field Forces

Gravitational

Electric

Magnetic

3.1 Force and mass

Mass – measurement of how difficult it is to change the objects velocity

Inertia – resistance to change in velocity

So mass is a measurement of an object’s inertia

3.2 Newton’s First Law of Motion

1st Law

An object at rest remains at rest as long as no net force acts on it.

An object moving with constant velocity continues to move with the same speed and in the same direction as long as no net force acts on it.

3.2 Newton’s First Law of Motion

Sometimes called the Law of Inertia

3.3 Newton’s Second Law of Motion

2nd Law

A net force causes an acceleration in the direction of the net force.

Simulation

Free body diagrams

Show all the forces acting on an object

For example an object sitting on a table

W – weight = mg

N – Normal Force (perpendicular) to the surface

Free body diagrams

If a rope pulls the object toward the right, then

T = Tension

Practice Free Body

Free body diagrams

Steps in problems solving

1.Sketch the forces

2.Isolate the Object

3.Choose a Coordinate System

4.Resolve the Forces into Components

5.Apply Newton’s Second Law of Motion

A 50 kg gopher has a string tied around his neck and pulled with a force of 80 N at an angle of 30o to the horizontal. What is his acceleration?

Free Body

diagram

Free Body

diagram

Free Body

diagram

Free Body

diagram

Free Body

diagram

Equation

maWNTF

5030cos80

3.4 Newton’s Third Law of Motion

For every force that acts on an object, there is a reaction force acting on a different object that is equal in magnitude and opposite in direction.

If object 1 exerts a force F on object 2, then object 2 exerts a force –F on object 1.

What are the action reaction pairs in the following?

A 60 kg man walks off a 3 m long canoe by walking from one end to the other. He applies a force of 20 N to the canoe, which has a mass of 150 kg.

A.What is the acceleration of

the man?

B. What is the acceleration of the canoe?

Free Body Diagrams

Equations

A-acceleration of man

amPF 233.0

A – acceleration of canoe

Two boxes are tied together with a rope, and the first one is pulled by a second rope. Both boxes accelerate at 2.0 m/s2. If the front box has a mass of 25 kg, and the second a mass of 50 kg, what is the tension on each rope?

Free body diagrams

Equations

Solve (add)

ammT bf

2)5025(

2)50( 22

3.5 The Vector Nature of Forces

Forces are vectors, so they can be treated using vectors rules

Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 7N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force?

Free Body Diagram?

Components?

Equations?

maWBLF

Values?

maWBLF

Values?

Solve?

Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 12.3N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force?

Solve?

)8.9)(3.1(28sin11

28cos11

o387.9

6.7tan 1

3.6 Frictional Forces

Friction – force that opposes motion

Caused by microscopic irregularities of a surface

Increases as pushing

force increases

Depends on the normal force and the type of surface

f – force of friction (N)

N – normal force

– coefficient of friction (1 or less)

Three types of friction

1.Static – object at rest

2.Kinetic – object in motion

3.Rolling – just like it sounds

Surfaces µ (static) µ (kinetic)

Steel on steel 0.74 0.57

Glass on glass 0.94 0.4

Metal on Metal (lubricated) 0.15 0.06

Ice on ice 0.1 0.03

Teflon on Teflon 0.04 0.04

Tire on concrete 1 0.8

Tire on wet road 0.6 0.4

Tire on snow 0.3 0.2

Example

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