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Transcript
Forces

Introduction

Why do objects speed up or slow down?

Forces: push, pull, friction, etc.

Sir Isaac Newton (1642-1727)
• Laws of motion, law of gravitation, telescopes,
properties of light, calculus, etc.
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APHY101
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Forces

Force

Contact (push) and field (gravity)
• A vector quantity

Net force = sum of all the force vectors
• This causes an acceleration or maintains
equilibrium
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2
Forces

Newton’s First Law
An object remains at rest or moves at a
constant velocity unless acted upon by a net
external force.

This applies in a non-accelerating reference
frame such as this room. Newton’s laws don’t
apply in your car as it accelerates.

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APHY101
3
Forces

Mass (m)
The property of an object to resist changes in
its velocity.

• Inertia
• Unit: kg

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Mass ≠ Weight
APHY101
4
Forces

Gravitational Force or weight
A force directed toward the center of the
Earth (or other object)


Weight = m g
• Unit: Newtons (N) or Pounds (lbs)
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Forces

Newton’s Second Law
Acceleration is proportional to the net force
applied and inversely proportional to the mass.

or
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APHY101
Fnet = m a
6
Forces

Newton’s Third Law

Action and reaction
• Forces come in pairs and involve two objects

You and the Earth
• Are the forces equal in magnitude?
accelerations equal in magnitude?
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APHY101
Are the
7
Forces

Normal Force
Perpendicular to the surface between two
objects.

• A cup sitting on a table
• Leaning your back against a wall
• A car parked on a hill
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Forces

Friction

A force that resists sliding motion
• Depends on the materials in contact and the normal
force between the materials.

Reduction of friction – lubricants (ie. engine oil)

When is friction a good thing?
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APHY101
9