Motion & Forces

Speed & Velocity
• Speed - the rate of change in
position over time (distance/time)
• Instantaneous speed
• Average speed
• How is velocity different from
speed? Velocity describes both
speed and direction

Change in Velocity
What makes an object
change velocity?

Force
• Push or Pull
• Forces can cause a change
in velocity
• Examples:
Gravity
Magnetism
Friction

Balanced Forces
Forces that
– act on the same object
– are opposite in direction
– are equal in size

Balanced Forces
Book on table
1. Weight of the book
is downward force
caused by gravity
2. Table pushes back
with an equal and
opposite force
Net force is zero

Unbalanced Forces
Forces that are NOT opposite
and equal
What do they cause?
CHANGE IN VELOCITY
ACCELERATION

Friction
Force that opposes
motion between 2
surfaces
Depends on the:
•types of surfaces
•force between the
surfaces

Types of Friction
• Static Friction: (Couch Potato)
–Force between objects at rest
• Sliding Friction: (Ice skating)
–Solid surfaces slide over each other
• Rolling Friction: (Rollerblading)
–An object rolls over a surface.
• Fluid friction: (Submarine)
–An object moves through a fluid

Friction
Friction is greater...
–between rough
surfaces
–when there’s greater
force between surfaces
(such as more weight)

Lubricant
Changes sliding friction
into fluid friction

Friction
Is friction always bad?

Gravity
The attractive force
between all objects
in the universe

Acceleration of
Gravity
Near the surface of the earth the
acceleration due to gravity is 9.8
m/s/s

Newton’s 1st
Law
An object at rest remains at
rest and an object in motion
will continue moving at a
constant velocity unless acted
on by unbalanced forces.

Newton’s Laws of
Motion

Newton’s 1st
Law
INERTIA
• tendency of an object to resist
any change in its motion
• increases as mass increases

Newton‘s 2nd
Law
If more force is applied, an
object will be accelerated _____more

Newton’s 2nd
Law
• More force means more
acceleration
• More mass means less
acceleration
Force = mass X acceleration
F = ma

Newton’s 3rd
Law
For every action, there is an equal
and opposite reaction.
Forces occur only in pairs
– Act on different objects
– Equal and opposite

Newton’s 3rd
Law
The hammer
exerts a force on
the nail to the
right.
The nail exerts an
equal but opposite
force on the
hammer to the
left.

Momentum

Terminal Velocity
The final speed of a
falling object.

Terminal Velocity
Air resistance is
offsetting the the
weight of the
object

Forces
End

Force
Aristotle's idea:
For an object to move at a
constant speed, a constant
force must be applied.

Force
Newton’s idea:
An object moving at a constant speed
will continue at that speed without
additional force being applied.

Balanced Forces