Chapter 4

Dynamics: Newton’s Laws of Motion

Objectives

• Apply the Law of Inertia to explain physical phenomena.

• Compare and contrast weight and mass.

• Use mass and weight units.

Units of Chapter 4

• Force

• Newton’s First Law of Motion

• Mass

• Newton’s Second Law of Motion

• Newton’s Third Law of Motion

• Weight – the Force of Gravity; and the Normal Force

• Solving Problems with Newton’s Laws: Free-Body Diagrams

• Applications Involving Friction, Inclines

• Problem Solving – A General Approach

Units of Chapter 4

Newton’s Contributions• Calculus• Light is composed of

rainbow colors• Reflecting Telescope• Laws of Motion• Theory of Gravitation

4-1 Force

A force is a push or pull. An object at rest needs a force to get it moving; a moving object needs a force to change its velocity.

The magnitude of a force can be measured using a spring scale.

4-2 Newton’s First Law of Motion

Newton’s first law is often called the law of inertia.

Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no

net force acts on it.

Demo – 1st law

Period 1 stopped here

Forces may be balanced or unbalanced

• Balanced forces – all forces acting on an object are equal– There is NO MOTION

• Unbalanced forces – one or more forces acting on an object are stronger than others– There is MOTION

• A NET FORCE

Balanced Force

Equal forces in opposite directions produce no motion

Unbalanced Forces

Unequal opposing forces produce an unbalanced force

causing motion

If objects in motion tend to stay in motion, why don’t moving objects keep moving

forever?

Things don’t keep moving forever because there’s almost always an unbalanced force

acting upon them.

A book sliding across a table slows down and stops because of the force of friction.

If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

4-2 Newton’s First Law of Motion

Inertial reference frames:

An inertial reference frame is one in which Newton’s first law is valid.

This excludes rotating and accelerating frames.

15

Inertial frames of reference

• Inertial reference frames are reference frames in which Newton’s laws hold

• From Newton’s 1st law:– If an object experiences no net force, the

object either remains at rest or continues with constant speed in a straight line

• The Earth is not an inertial frame (it rotates) but is close enough to be considered an inertial frame

16

Inertial frames of reference

• Inertia is a property of matter• It is that property of matter which opposes

changes in velocity • So, one must centre one's physics reasoning on

these thoughts: – An object's velocity will not change all on its own.– Pushes, or pulls, are necessary to change an object's

velocity.

17

Inertial frames of reference

• There are several ways to describe an inertial frame. Here are a few descriptions:– An inertial frame of reference is a frame of reference

with constant velocity. – An inertial frame of reference is a non-accelerating

frame of reference. – An inertial frame of reference is a frame of reference

in which the law of inertia holds. – An inertial frame of reference is a frame of reference

in which Newton's laws of motion hold. – In an inertial frame of reference no fictitious forces

arise.

First LawFirst Law

NewtonNewton’’s first law says that with no net force there is no s first law says that with no net force there is no acceleration.acceleration.• Objects at restObjects at rest• Objects at constant velocityObjects at constant velocity

If there is no observed acceleration on an object with no If there is no observed acceleration on an object with no net force, the observer is in an inertial reference frame.net force, the observer is in an inertial reference frame.• NewtonNewton’’s laws of mechanics apply equallys laws of mechanics apply equally• No absolute motionNo absolute motion

Inertial Frame Inertial Frame An observer on the table sees An observer on the table sees

two ball fall.two ball fall.• First straight downFirst straight down

• Second in a parabolaSecond in a parabola

An observer with speed An observer with speed vvxx00

sees the reverse.sees the reverse.• Second straight downSecond straight down

• First in a backwards parabolaFirst in a backwards parabola

Both frames are inertial.Both frames are inertial.• Motion consistent with NewtonMotion consistent with Newton

Accelerated Frame Accelerated Frame A rotating observer throws a A rotating observer throws a

ball across a merry-go-round.ball across a merry-go-round.• Ball veers to the sideBall veers to the side

• No external forceNo external force

This is a non-inertial frame.This is a non-inertial frame.• Observed motion inconsistent Observed motion inconsistent

with Newtonwith Newton’’s lawss laws

• Fictitious forcesFictitious forces

He believed that there existed an absolute (not accelerating) reference frame, and an absolute time.

The laws of physics are always the same in any inertial reference frame.

His laws applied only when measurements were made in this reference frame…..

Newton clarified the mechanics of motion in the “real world”.

…or in any other reference frame that was at rest or moving at a constant velocity relative to this absolute frame.

Inertial

reference

frame

Inertial

reference

frame

Inertial reference

frame

4-3 Mass

Mass is the measure of inertia of an object. It is the measure of the amount of matter in an object. In the SI system, mass is measured in kilograms.

Mass is not weight:

Mass is a property of an object. Weight is the force exerted on that object by gravity.

If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same.

Mass: A measure of Inertia

• Kick the can. • Kick the same can full of soda. • Kick the same can full of concrete.

• Mass is not volume!

• Mass is not weight!

• The more mass an object has, the greater its inertia and the more force it takes to change its state of motion.

Objects at Rest

• Objects at rest tend to stay at rest unless acted upon by a force. [push or pull]

• Newton described this tendency as inertia.

• Inertia can be described as the tendency of an object to keep doing whatever’s it’s doing.

Mass & Inertia

• Mass is the amount of matter in an object.

• The more MASS an object has, the more INERTIA the object has.

• Bigger objects are harder to start & stopWhich vehicle has

more inertia?

Slide from www.science-class.net

Newton’s 1st Law(also known as the law of

inertia)• A moving object moves in a straight line

with constant speed unless a force acts on it.

• The tendency of an object at rest to remain at rest and an object in motion to remain in motion unless acted upon by an unbalanced force.

• Objects do not change their motion unless a force acts on them

The truck is in motion. What is the force that causes it to stop?

The push of the stopped car.

The car is at rest. What is the force that causes it to move?

The push of the truck.

Slide from www.science-class.net

Newton’s First Law (law of inertia)

• INERTIA is a property of an object that describes how ______________________ the motion of the object

• more _____ means more ____

much it will resist change to

mass inertia

1st Law

• Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

• There are four main types of friction:– Sliding friction: ice skating– Rolling friction: bowling– Fluid friction (air or liquid): air or water resistance– Static friction: initial friction when moving an object

What is this unbalanced force that What is this unbalanced force that acts on an object in motion?acts on an object in motion?

1st Law • Once airborne,

unless acted on by an unbalanced force (gravity and air – fluid friction) it would never stop!

Inertia

Newton’s First Law

When the motorcycle stops, the rider continues his motion.

Terminal Velocity

Exploration

• Mass and Weight Lab (No lab write up.)

Homework

• Newton’s 1st law hand-out

• Questions p. 97 1, 2

Closure

• When the pellet fired into the spiral tube emerges, which path will it follow? (neglect gravity)

Answer

• While in the tube, it is forced to curve, but when it gets outside, no force is exerted on the pellet and (law of inertia) it follows a straight – line path…B!

Elaboration Computer Activity for Newton’s First Law

• http://www.physicsclassroom.com/Class/newtlaws/u2l1a.cfm

Inertia Evaluation

• Carpenters use different hammers for different jobs. A tack hammer is a light hammer used for small nails in delicate situations. A regular hammer is heavier and is used for larger nails in building walls. Compare the two hammers using Newton’s 1st law in terms of the hammers’ function, advantages and disadvantages.