Newtons Laws of Motion

1
Newtons Laws of Motion
1st - Inertia. 2nd - F ma 3rd - Action/Reaction
2
Newtons First Law

• An object at rest tends to stay at rest and an
  object in motion tends to stay in motion unless
  acted upon by an unbalanced force.

3
What does this mean?

• Basically, an object will keep doing what it was
  doing unless acted on by an unbalanced force.
• If the object was sitting still, it will remain
  stationary. If it was moving at a constant
  velocity, it will keep moving.
• It takes force to change the motion of an object.

4
Some Examples from Real Life
A soccer ball is sitting at rest. It takes an
unbalanced force of a kick to change its motion.

• Two teams are playing tug of war. They are both
  exerting equal force on the rope in opposite
  directions. This balanced force results in no
  change of motion.

5
Newtons First Law is also called the Law of
Inertia

• Inertia the tendency of an object to resist
  changes in its state of motion
• The First Law states that all objects have
  inertia. The more mass an object has, the more
  inertia it has (and the harder it is to change
  its motion).
• So, which has more inertia? A bowling ball or a
  baseball?

6
Newtons 1st Law and You
Dont let this be you. Wear seat belts. Because
of inertia, objects (including you) resist
changes in their motion. When the car going 80
km/hour is stopped by the brick wall, your body
keeps moving at 80 m/hour.
7
If objects in motion tend to stay in motion, why
dont moving objects keep moving forever?
Things dont keep moving forever because theres
almost always an unbalanced force acting upon it.
A book sliding across a table slows down and
stops because of the force of friction.
If you throw a ball upwards it will eventually
slow down and fall because of the force of
gravity.
8
Newtons Second Law

• Force equals mass times acceleration.
• F ma

9
What does F ma say?
F ma basically means that the force of an
object comes from its mass and its acceleration.
Something very massive (high mass) thats
changing speed very slowly (low acceleration),
like a glacier, can still have great force.
Something very small (low mass) thats changing
speed very quickly (high acceleration), like a
bullet, can still have a great force. Something
very small changing speed very slowly will have a
very weak force.
10
2nd Law (F m x a)

• How much force is needed to accelerate a 1400 kg
  car 2 m/s2?
• Write the formula
• F m x a
• Fill in given numbers and units
• F 1400 kg x 2 m/s2
• Solve for the unknown
• 2800 kg-m/s2 or 2800 N

11
If mass remains constant, doubling the
acceleration, doubles the force. If force remains
constant, doubling the mass, halves the
acceleration.
12
Newtons Third Law

• For every action there is an equal and opposite
  reaction.

13
What does this mean?
For every force acting on an object, there is an
equal force acting in the opposite direction.
Right now, gravity is pulling you down in your
seat, but Newtons Third Law says your seat is
pushing up against you with equal force. This
is why you are not moving. There is a balanced
force acting on you gravity pulling down, your
seat pushing up.
14
Action and Reaction forces act on DIFFERENT
OBJECTS!

• Action Your foot pushes on skateboard
• Reaction The skateboard pushes on your foot.
• Remember
• There are always 2 forces
• They are always equal
• They act on different objects

15
Review
Newtons First Law
Objects in motion tend to stay in motion and
objects at rest tend to stay at rest unless acted
upon by an unbalanced force.
Newtons Second Law
Force equals mass times acceleration (F ma).
Newtons Third Law
For every action there is an equal and opposite
reaction.
16
Other Motion Concepts

• Gravity
• Freefall
• Weight
• Momentum

17
What is Gravity?

• GRAVITY - attraction force between all masses
• Newtons universal law of gravitation Every
  object in the universe exerts a gravitational
  attraction to all other objects in the universe
• Gravity depends on the mass of the objects and
  the distance between the objects

18
What is Gravity?

• The greater the mass, the greater the force
• The greater the distance, the less the force
• Acceleration due to gravity 9.8 m/s/s or 9.8
  m/s2

19
Free Fall
The rate of falling increases by 9.8 m/s every
second.
A ball thrown horizontally will fall at the same
rate as a ball dropped directly.
20
Air Resistance Effects Free Fall

• In air
• A stone falls faster than a feather
• Air resistance affects stone less
• In a vacuum
• A stone and a feather will fall at the same speed.

21
Terminal Velocity

• Free Fall
• A person in free fall reaches a terminal velocity
  of around 54 m/s
• With a parachute, terminal velocity is only 6.3
  m/s
• Allows a safe landing

22
Weight vs. mass

• Weight and mass are not the same.
• Mass is a fundamental property of matter measured
  in kilograms (kg).
• Weight is a force measured in newtons (N).
• Weight depends on mass and gravity.

23
Weight depends on mass and gravity
A 10-kilogram rock has the same mass no matter
where it is in the universe. On Earth, the10 kg.
rock weighs 98 N.. On the moon, the same rock
only weighs 16 N.
24
Which of Newtons Law does this look like?
25

• Weight is a measure of the gravitational force
  between two objects
• The greater the mass the greater the force
  (weight)
• Measured in units called Newtons (N)

26

• Weightlessness free from the effects of gravity

27

• Momentum The quantity of motion
• A property of moving objects
• Calculated by P mv
• (p momentum, m mass, v velocity)
• Law of conservation of momentum the total
  amount of momentum of a group of objects does not
  change unless outside forces act on the objects