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Chapter 3 Newton’s Laws

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Chapter 3 Newton’s Laws. Every object possesses inertia. Inertia is the tendency of a body at rest to remain at rest, and of a body in motion to continue moving with unchanged velocity. or Inertia is the sluggishness of an object to changes in its state of motion.

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the mass
Every object possesses inertia.

Inertia is the tendency of a body at rest to remain at rest, and of a body in motion to continue moving with unchanged velocity.

or

Inertia is the sluggishness of an object to changes in its state of motion.

Mass - a measure of the inertia of an object

THE MASS
slide4
Demo - Inertia rocks

Demo – Hoop and chalk

the standard kilogram
slug.

A slug weighs 32.2 lb.

The standard kilogram is an object whose mass is defined to be one kilogram.

Abbreviation is kg.

There is an English unit of mass. The

THE STANDARD KILOGRAM
force
In general force is the agency of change.

In mechanics forces cause accelerations.

It is a vector.

An external force is one whose source lies outside of the system being considered.

FORCE
slide8
The combination of forces that act on an object is the net force.

(Only the net force is shown in the figures on this slide.)

m

m

m

This symbol means

proportional to

F

a

F

a

F

a

The acceleration of an object is directly proportional to the net force.

slide9

m

m

m

m

m

m

a

F

Consider the same net force applied to different mass objects.

F

a

The acceleration is inversely proportional to the mass of the object.

F

a

slide10
The acceleration is proportional to the force and inversely proportional to the mass of the object.
the newton
The Newton (N) is the SI unit of force.

A resultant 1 N force will give a 1 kg mass an

acceleration of 1 m/s2.

The pound (lb) is the English (US customary) unit of force.

A resultant 1 lb force will give a 1 sl mass an

acceleration of 1 ft/s2.

THE NEWTON
newton s first law
An object at rest will remain at rest;

an object in motion will continue in motion with constant velocity, except insofar as it is acted upon by an external force.

“If you leave an object alone, it has constant velocity.”

NEWTON’S FIRST LAW
newton s second law
Newton stated it in terms of momentum.

A less rigorous form of the second law will be used here.

If the net external force acting on an object is not zero, then the acceleration of the object is directly proportional to the net external force and inversely proportional to the mass of the object.

NEWTON’S SECOND LAW
slide14

or

The force in each of these equations represents the vector sum of all of the forces acting on the object of mass m.

newton s third law
Forces represent interactions between one piece of matter and another.

Therefore, forces come in pairs.

For each force exerted on one body, there is an equal, but oppositely directed, force on some other body interacting with it.

“You cannot touch without being touched.” Paul Hewitt

This is often called the Law of Action and Reaction.

NEWTON’S THIRD LAW
slide20

4 units

3 units

2 units

1 unit

Let’s look at the inverse-square nature for the force of gravity.

the weight
The weight of an object is the gravitational force acting downward on the object.

Because the Earth is not a perfect uniform sphere, and because it is spinning, the weight measured by a scale (often called the effective weight) will be very slightly different from that defined here.

THE WEIGHT
relationship between mass and weight
Look at the force of gravity on a freely falling object.

We call that the weight of the object.

A 1 kg object would weigh 9.81 N

or 2.20 lb.

RELATIONSHIP BETWEEN MASS AND WEIGHT
the friction force
It is a tangential force acting on an object that opposes the sliding of that object on an adjacent surface with which it is in contact.

The friction force is parallel to the surface and opposite to the direction of motion or of impending motion.

Only when the applied force exceeds the maximum static friction force will an object begin to slide.

THE FRICTION FORCE
the normal force
The normal force is a part of the contact force between surfaces in contact.

The normal force is perpendicular to the surfaces in contact.

The frictional force is parallel to the surfaces in contact.

THE NORMAL FORCE
the coefficient of kinetic friction
For surfaces in contact that are sliding with respect to each other, the coefficient of kinetic friction is the ratio of the friction force to the normal force.THE COEFFICIENT OF KINETIC FRICTION
the coefficient of static friction
For surfaces in contact on the verge of sliding with respect to each other the coefficient of static friction is the ratio of the maximum static friction force to the normal force.THE COEFFICIENT OF STATIC FRICTION
friction
FRICTION

Friction opposes the motion between surfaces in contact with one another.

When there is a tendency for movement between two surfaces and yet there is no motion, the friction is static friction.

Static friction has an upper limit.

When there is motion between the two surfaces, the friction is kinetic (sliding) friction.

slide29

FA

FA

FA

F

FA

F

F

F

FA

On the verge of slipping

Sliding

Maximum Static Friction

Friction, F

Kinetic (sliding) Friction

Applied Force, FA

dimensional analysis
DIMENSIONAL ANALYSIS:

Fundamental Dimensions

Length - L

Mass - m

Time - t

Derived dimensions are combinations of the fundamental dimensions.

For example: length/time2

Remember that you can only add or subtract things that are alike.

For example velocity plus velocity

mathematical operations with units
MATHEMATICAL OPERATIONS WITH UNITS:

When numbers are placed into equations, their units must appear with them.

Units undergo the same mathematical operation as the numbers do.

Conversion from one form of the unit to another may be necessary.

For example meters to centimeters.

See examples in text, page 30.

free body diagrams fbd
Free Body Diagrams (FBD)

This is a diagram showing some object and the forces applied to it.

It contains only forces and coordinate information, nothing else.

There are only two kinds of forces to be considered in mechanics:

Force of gravity

Contact forces

example fbd

y

x

Example FBD

FBD

A car of mass m rests on a 300 incline.

N

F

q

q

mg

newton s second law nsl

y

x

Newton’s Second LawNSL

FBD

A car of mass m rests on a 300 incline.

NSL

N

F

q

q

mg

What if friction is smaller?

newton s second law nsl1

y

x

Newton’s Second LawNSL

FBD

A car of mass m rests on a 300 incline.

NSL

N

F

q

q

oops

mg