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Chapter 4

Dynamics: Newton’s Lawsof Motion

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 theNormal Force

4-1 ForceA force is a push or pull. Anobject at rest needs a forceto get it moving; a movingobject needs a force tochange its velocity.

The magnitude of aforce can be measuredusing a spring scale.

4-2 Newton’s First Law of MotionNewton’s first law is often called the law ofinertia.

An object in motion stays in motion and anobject at rest stays at rest, unless acted uponby a force.

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Newton’s First Law

Every object continues in its state of rest, or ofuniform velocity in a straight line, as long asno net force acts on it.

4-2 Newton’s First Law of Motion

Inertial reference frames:

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

This excludes rotating and accelerating frames.

4-3 Mass

Mass is the measure of inertia of an object. Inthe SI system, mass is measured in kilograms.

Mass is not weight:

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

If you go to the moon, whose gravitationalacceleration is about 1/6 g, you will weighmuch less. Your mass, however, will be thesame.

4-4 Newton’s Second Law of Motion

Newton’s second law is the relation betweenacceleration and force. Acceleration isproportional to force and inversely proportionalto mass.

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4-4 Newton’s Second Law of Motion

Force is a vector, so is true alongeach coordinate axis.

The unit of force in the SIsystem is the newton (N).

Note that the pound is aunit of force, not ofmass, and can thereforebe equated to newtonsbut not to kilograms.

4-5 Newton’s Third Law of MotionAny time a force is exerted on an object, thatforce is caused by another object.

Newton’s third law:

Whenever one object exerts a force on asecond object, the second exerts an equalforce in the opposite direction on the first.

More traditionally

• For every action, there is an equal andopposite reaction.

4-5 Newton’s Third Law of Motion

A key to the correctapplication of the thirdlaw is that the forcesare exerted on differentobjects. Make sure youdon’t use them as ifthey were acting on thesame object.

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4-5 Newton’s Third Law of MotionRocket propulsion can also be explained usingNewton’s third law: hot gases from combustionspew out of the tail of the rocket at high speeds.The reaction force is what propels the rocket.

Note that therocket does notneed anything to“push” against.

4-5 Newton’s Third Law of MotionHelpful notation: the first subscript is the objectthat the force is being exerted on; the second isthe source.This need not bedone indefinitely,but is a good ideauntil you get used todealing with theseforces.

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