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Chapter 4
Forces in One Dimension
4.1 Force and Motion
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Force – A push or a pull exerted on an
object. May cause a change in velocity:
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Speed up
Slow down
Any change in velocity is acceleration
Therefore a force causes acceleration
Forces are vector quantities
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Direction
Magnitude
Contact Forces
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Caused when an object in the external
world (an “agent”) touches a system
thereby exerting a force upon it
System is the object of interest:
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A puck hit by a hockey stick
A book pushed off the table by a hand
Without an agent and a system, a
force cannot exist
Free-body Diagrams
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See page 89
Sketch situation
Label agent and system
Show forces as arrows pointing in the
direction that the force is applied
Indicate which way is positive (usually
up!)
Force and Acceleration
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The greater the force, the greater the
resulting acceleration
They are directly proportional
Newton’s Second Law states that
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a = F/m
F = ma
F is measured in newtons (N)
1 N = 1kg.m/s2
Net Force (Fnet)
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Vector sum of all forces
See Figure 4-5 page 92
Newton’s First Law
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An object that is at rest will remain at
rest and an object that is moving will
continue to move in a straight line with
constant speed, if and only if the net
force acting on the object is zero
Also known as inertia (the tendency of
an object to resist any change in
motion)
Equilibrium
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If no net forces are acting on an object
is said to be at equilibrium
4.2 Using Newton’s Second Law
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Fg = mg
Fg (Force of gravity) is weight
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Fg is always (-)
Weight is always (-)
Weight changes with location because
gravity differs…that’s why we weigh less
on the moon
Mass does not change with location
because it doesn’t depend on gravity
Apparent Weight
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Equal to the weight of the object plus
the net force acting on it
See Problem 2 on page 99
Weightlessness is a factor of apparent
weight. This does not mean that the
object has no weight, rather there are
no upward contact forces acting on it.
Fscale = mg + ma (going up)
Fscale = mg – ma (going down)
Drag Force
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Drag force is experienced by any
object moving through a fluid
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Gas (like air)
Liquid (like water)
The magnitude of the drag force
increases with the speed of the object
moving through the fluid
Terminal Velocity
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Object is dropped and accelerates as
gravity acts upon it
As the velocity increases, so does the
drag force of the air acting against
gravity
When gravity pulling down equals drag
force pushing up the object stops
accelerating
This constant and final velocity is
called “terminal velocity”
4-3 Interaction Forces
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Newton’s Third Law
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All forces come in pairs
FAonB = -FBonA
Interaction pair
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Also called action-reaction pair
Two forces that are opposite in direction
but equal in magnitude
Example: Action-Reaction Pair
Forces of Ropes & Strings
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Tension – a force exerted by a string or
rope
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Is equal to the weight of the object
hanging on the rope/string
Fnet = Sum of the positive force of a rope
pulling up and the negative force of
gravity pulling down
Fnet = FT + (-Fg)
Normal Force
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Perpendicular contact force exerted by
one surface on another
May be equal to the weight of an
object or…
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Less than the weight if an additional
upward force is exerted
More than the weight if an additional
downward force is exerted
See Figure 4-16 on page 107