Newton's Laws of Motion

1. Forces and Motion
Chapter 3.1
Force, Mass, and Acceleration

3. *Newton’s Laws of Motion
• *First Law: An object at rest will stay at
rest unless acted on by an unbalanced
force. An object in motion will continue
with constant speed and direction, unless
acted on by an unbalanced force.
• Second Law: Force causes an object to
accelerate while the object’s mass resists
acceleration.
• Third Law: For every action, there is an
equal and opposite reaction.

4. How would you move a heavy box?

5. Force
• Since every object in a state of rest will
stay at rest- you need to apply force to
cause motion.
• *What is force?
– *Force is any action that has the ability to
change motion
• Example: Push or Pull

6. How do we measure force?
• *There are two units of measurements of
force:
• *Pounds
• *Newtons
– 1 newton = 0.228 pounds
– 1 pound = 4.448 newtons
– A person who weighs 100 pounds would
weigh 444.8 newtons.

7. Difference Between
Force and Mass
• A Force is a push or pulling action that
causes change in motion
• *Mass- is the amount of “stuff” in an
object.
– *Mass resists the action of forces making
objects harder to move.

8. What is the difference between
mass and weight?
• *Weight is the amount of force pulling on
an object due to gravity.
• Mass is the amount of matter in an object.
• Example:
A child weighs 30 pounds on Earth with a
mass of 14 kilograms.
In space the child weighs only 4 pounds,
but still has a mass of 14 kilograms.
Why??

9. Mass and Inertia
• *Newton’s First Law of motion is also
called the Law of Inertia.
• *Inertia is the property of an object to
resist changing its state of motion.
An object with a lot of inertia takes a lot of force
to start or stop.

10. Answer these review questions
in your own words:
1. *What is an example of force?
2. *What is a newton?
3. *What is the difference between weight
and mass?
4. *What is inertia?
5. *Which is easier to get moving, a
basketball or a bowling ball? Which is
easier to stop? Why?

11. Part 2:
Remember the Law of Inertia:

12. Law of Inertia
• *First Law of Motion: An object at rest
will stay at rest unless acted on by an
unbalanced force. An object in motion will
continue with constant speed and
direction, unless acted on by an
unbalanced force.

13. *What is this unbalanced force that acts
on an object in motion?
• *There are four main types of friction:
– *Sliding friction: two surfaces rub against each
other
– *Rolling friction: object rolling over another
– *Viscous friction (air or liquid): air or water
resistance
– *Air friction: air moving around an object
creates opposing force

14. Friction
• Objects on earth, unlike the frictionless
space the moon travels through, are under
the influence of friction.

16. Newton’s Laws of Motion
• First Law: An object at rest will stay at rest
unless acted on by an unbalanced force. An
object in motion will continue with constant
speed and direction, unless acted on by an
unbalanced force.
• *Second Law of Motion: Force causes
an object to accelerate while the object’s
mass resists acceleration.
• Third Law: For every action, there is an
equal and opposite reaction.

17. Newton’s Second Law
• A force causes an object to accelerate,
while an object’s mass resists
acceleration.
– Force causes acceleration
– Mass resists acceleration
– The acceleration you get is equal to the ratio
of force over mass
a=F/m a=acceleration F=force m=mass

18. What if??
http://www.physicsclassroom.com/mmedia/newtlaws/efar.cfm
http://www.physicsclassroom.com/mmedia/newtlaws/efff.cfm
• You drop an elephant and a feather at
the same time off a building. Which
will land first?
No animals were harmed in the making of this PowerPoint!

19. Elephant is first?
http://www.physicsclassroom.com/mmedia/newtlaws/efar.cfm

20. They land at the same time?
http://www.physicsclassroom.com/mmedia/newtlaws/efff.cfm

21. What happened??
• The feather reached its terminal velocity
first- in other words: the feather was
slowed down by the upward push of air
resistance. Also called: air friction.
• Since the elephant has so much more
mass, it needed much more air to slow it
down and reach it’s terminal velocity.
There wasn’t enough time, so the elephant
continued to gain speed until he landed.

22. What about gravity?
• Gravity worked on both the elephant and
the feather.
• The pull of gravity was GREATER on the
elephant, but the mass is also greater.
• The increased mass resists acceleration
at the same rate as gravity pulls it.
• They cancel each other out- and the
elephant falls at the same speed as the
feather! (In a world with no air)

23. So…
• Force causes an object to accelerate, while an
object’s mass resists acceleration.
• In a world with NO AIR, therefore NO friction-
everything falls at the same rate!
• But we will notice in our world, friction plays a
big part in the rate of objects falling!

24. Newton’s Third Law:
• *Newton’s Third Law of Motion:
– For every action, there is an equal and
opposite reaction.

25. What does it mean?
• As you sit on your chair- you exert
pressure on your chair, but your chair
pushes back equally with pressure on you.
• Action-Reaction Pair of Forces

26. Short Video