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# Ch.

3 & 4
Motion & Forces

## III. Defining Force

Force
Newtons First Law
Friction
A. Force
Force
a push or pull that one body exerts on another
What forces are being
exerted on the football?

Fkick

Fgrav
A. Force
Balanced Forces
forces acting on
an object that
are opposite in
direction and
equal in size
no change in
velocity
A. Force
Net Force
unbalanced forces that are not
opposite and equal
velocity changes (object accelerates)
Fnet

Ffriction Fpull

N N
W
B. Newtons First Law

## Newtons First Law of Motion

An object at rest will remain at
rest and an object in motion
will continue moving at a
constant velocity unless acted
upon by a net force.
B. Newtons First Law
Newtons First Law of Motion
Law of Inertia

Inertia
tendency of an object to resist any change
in its motion
increases as mass increases
ConcepTest 1

TRUE or FALSE?
The object shown in the diagram must
be at rest since there is no net force
acting on it.
FALSE! A net force does not cause
motion. A net force causes a
change in motion, or acceleration.

## Taken from The Physics Classroom Tom Henderson, 1996-2001.

ConcepTest 2
You are a passenger in a car and not wearing your
seat belt.
Without increasing or decreasing its speed, the car
makes a sharp left turn, and you find yourself
colliding with the right-hand door.
Which is the correct analysis of the situation? ...
ConcepTest 2
1. Before and after the collision, there is a rightward
force pushing you into the door.
2. Starting at the time of collision, the door exerts a
leftward force on you.
2.both
3. Starting at the
of the above time of collision, the
door ofexerts
4. neither a leftward force on you.
the above
C. Friction
Friction

## force that opposes motion between

2 surfaces
depends on the:
types of surfaces
force between the surfaces
C. Friction
Friction is greater...
between rough surfaces
when theres a greater
force between the
surfaces
(e.g. more weight)

## Pros and Cons?

FRICTION

https://youtu.be/U5rSklsvNH8
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