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Forces

Introduction
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Why do objects speed up or slow down?
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Forces: push, pull, friction, etc.
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Sir Isaac Newton (1642-1727)
• Laws of motion, law of gravitation, telescopes,
properties of light, calculus, etc.
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Forces

Force
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Contact (push) and field (gravity)
• A vector quantity
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Net force = sum of all the force vectors
• This causes an acceleration or maintains
equilibrium
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Forces

Newton’s First Law
An object remains at rest or moves at a
constant velocity unless acted upon by a net
external force.
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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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Forces
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Mass (m)
The property of an object to resist changes in
its velocity.
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• Inertia
• Unit: kg
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Mass ≠ Weight
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Forces

Gravitational Force or weight
A force directed toward the center of the
Earth (or other object)
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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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Fnet = m a
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Forces
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Newton’s Third Law
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Action and reaction
• Forces come in pairs and involve two objects
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You and the Earth
• Are the forces equal in magnitude?
accelerations equal in magnitude?
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Are the
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Forces
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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
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Friction
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A force that resists sliding motion
• Depends on the materials in contact and the normal
force between the materials.
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Reduction of friction – lubricants (ie. engine oil)
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When is friction a good thing?
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