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Forces and Motion
Describing Motion

An object is in motion when its distance
from a reference point is changing

Variables involved?
Speed and Velocity

Speed = distance ÷ time

Average speed = total distance
total time

Velocity is speed in a certain direction
Graphing Speed
Acceleration

The rate at which velocity changes
 Increasing
speed
 Decreasing speed
 Changing direction

Acceleration = Change in velocity
Change in time
Explaining Motion – Newton’s Laws
Galileo
→
Isaac Newton
Force





A push or pull; ex. Tug of War
A transfer of energy
Net Force = the sum of all forces on an object
Vector diagram – magnitude and direction
1 Newton = the force required to accelerate a
1 kg mass at 1 m/sec2
Newton’s First Law
(aka The Law of Inertia)

An object at rest will remain at rest and an
object in motion will remain in motion
unless acted upon by an unbalanced force
– resistance to change; an object’s
tendency to keep doing what it’s doing
What does mass have to do with inertia?
 Inertia
Types of Forces
Types of Forces
 Friction


Force that occurs when the surfaces of
matter move past each other
 Always opposes motion
 Produces heat
 Amount of friction depends on…….
Amount of force pushing objects together
 How rough/smooth the surfaces are

Friction Forces

Sliding

Rolling

Fluid

Static
Friction: Helpful or Harmful?

Ways to increase friction

Ways to decrease friction

Friction animation
Gravity or The Law of Universal
Gravitation

Force of attraction between objects
 Affected
by the mass of the objects and the
distance between them
 Weight = measure of the gravitational force
exerted on an object
 Falling objects ?
 Terminal velocity
 Freefall

Orbit

Projectile Motion
Newton’s 2nd Law

The acceleration of an object depends on
the mass of the object and the amount of
force applied

How is acceleration changed by force?

How is acceleration changed by mass?

Force = Mass X Acceleration
Newton = kg x m/sec2

What is the net force on a 1000 kg elevator
accelerating at 2m/sec2?

What is the net force required to accelerate a 55
kg shopping cart at 15 m/sec2?
Newton’s 3rd Law
aka The Action/Reaction Law
For every action, there is an equal and
opposite reaction
 When one object exerts a force on a
second object, the second object exerts a
force of equal strength in the opposite
direction

 All
forces act in pairs
 The forces are applied to two different objects
Momentum
A measure of an object’s “inertia of
motion”
 Momentum = Mass x velocity
kg·m/sec
Law of Conservation of Momentum: the total
momentum of objects that interact does
not change, unless acted on by an outside
force (eg. Friction)
Collision Animations

Object
Blackbird
Mass (kg)
Velocity (m/s)
0.04
19
Soccer player 100
10
Skier
60
20
Bullet
0.004
600
Frog
0.9
12
Meteorite
0.1
1,000
Baseball
0.14
30
Rocket
36,000
1,800
Wagon
2
3
Satellite
3,000
8,000
Momentum
(kg·m/s)