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### Newton’sLaws

Unit 3

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

3.1 Force and mass

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

3.2 Newton’s First Law of Motion

2nd Law

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

Simulation

3.3 Newton’s Second Law of Motion

N

W

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

3.3 Newton’s Second Law of Motion

N

T

W

Free body diagrams

If a rope pulls the object toward the right, then

T = Tension

Practice Free Body

3.3 Newton’s Second Law of Motion

Free body diagrams

- Steps in problems solving
- Sketch the forces
- Isolate the Object
- Choose a Coordinate System
- Resolve the Forces into Components
- Apply Newton’s Second Law of Motion

3.3 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?

3.3 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

3.3 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

N

T

3.3 Newton’s Second Law of Motion

W

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

Axis

N

T

3.3 Newton’s Second Law of Motion

W

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

Axis

Ty

N

T

3.3 Newton’s Second Law of Motion

Tx

W

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

Axis

Equation

Ty

N

3.3 Newton’s Second Law of Motion

Tx

W

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.

3.4 Newton’s Third Law of Motion

What are the action reaction pairs in the following?

3.4 Newton’s Third Law of Motion

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.

- What is the acceleration of
- the man?
- B. What is the acceleration of the canoe?

3.4 Newton’s Third Law of Motion

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.

Free Body Diagrams

Nc

Nm

3.4 Newton’s Third Law of Motion

P

P

Wm

Wc

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.

Equations

Nc

Nm

3.4 Newton’s Third Law of Motion

P

P

Wm

Wc

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-acceleration of man

Nc

Nm

3.4 Newton’s Third Law of Motion

P

P

Wm

Wc

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 – acceleration of canoe

Nc

Nm

3.4 Newton’s Third Law of Motion

P

P

Wm

Wc

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?

a

3.4 Newton’s Third Law of Motion

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

3.5 The Vector Nature of Forces

L

B

W

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?

3.5 The Vector Nature of Forces

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?

Components?

L

B

By

3.5 The Vector Nature of Forces

Bx

W

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?

Components?

Ly

L

By

3.5 The Vector Nature of Forces

Bx

Lx

W

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?

Equations?

Ly

By

3.5 The Vector Nature of Forces

Bx

Lx

W

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?

Values?

Ly

By

3.5 The Vector Nature of Forces

Bx

Lx

W

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?

Values?

Ly

By

3.5 The Vector Nature of Forces

Bx

Lx

W

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?

Solve?

Ly

By

3.5 The Vector Nature of Forces

Bx

Lx

W

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?

Ly

By

3.5 The Vector Nature of Forces

Bx

Lx

W

Friction – force that opposes motion

Caused by microscopic irregularities of a surface

Increases as pushing

force increases

3.6 Frictional Forces

Depends on the normal force and the type of surface

f – force of friction (N)

N – normal force

m – coefficient of friction (1 or less)

3.6 Frictional Forces

Three types of friction

- Static – object at rest
- Kinetic – object in motion
- 3.Rolling – just like it sounds

3.6 Frictional Forces

Example

3.6 Frictional Forces

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