Download Newtons Laws 2014 ppt

START A NEW WARM UP PAGE FOR
UNIT 4
• Put today’s date on the 1st line
(10/8) and then write this prompt
and your answer:
• In the next three minutes, write
down anything you remember or
think you remember about forces
• You can write in bullets or complete
sentences
FORCES AND
NEWTON’S LAWS OF
MOTION
FORCES
• In order to make an object at
rest move, you need to apply
a push or a pull, otherwise
known as a force.
• A force can make an object:
 Speed up
 Slow down
 Change direction
…AKA change velocity or
accelerate
Force: any
influence that
tends to
accelerate an
object.
Forces produce
accelerations
Ex: a push or a pull
SI unit: Newtons (N)
NEWTON’S FIRST LAW
Newton’s 1st Law of Motion:
An object in motion will continue in
motion at the same speed and in the
same direction or an object at rest will
remain at rest until another force changes
its motion
• aka the Law of Inertia
• An object at rest stays at rest; an object in
motion stays in motion
• Ex: An object moving in space will keep
moving at a constant speed
INERTIA
Inertia: the property of a body to resist
change in motion.
 Things tend to keep doing what they’re
already doing.
 SI Unit: kilograms (kg)
 Ex: you keep moving when a Marta train stops
b/c you have inertia
• Objects in a state of rest, stays at rest.
 Ex: pulling the tablecloth out from under a
table full of plates and cups
 Table cloth trick video
• Objects in motion, stay in motion.
 Only if moving at a constant velocity in a
straight line.
 Ex: A car you are sitting in stops, but you keep
moving forward (this is why we wear seatbelts)
MASS AND INERTIA
Mass:
the amount of matter present
in an object
 The more mass, the greater the
inertia…and the greater the force it
takes to change the state of motion.
 Mass remains the same wherever you
are
 SI units: kilograms (kg)
 Ex: your mass is 10 kg on Earth and on
the moon
CHECK YOUR UNDERSTANDING
Why is it more difficult to stop a rolling car than a
rolling toy car?
The car has more mass and therefore more inertia.
The inertia an object has, the more is needed to
change its state of motion (liking making something
stop).
WEIGHT
Weight:
the force of gravity on an
object’s mass
 Ex: your weight on Earth is 100 N, but on
the moon it is 16 N
 SI unit: Newtons (N)
 Equation:
W = mg
• W = weight (N)
• m = mass (kg)
• g = acceleration due to gravity (10m/s2)
MASS AND WEIGHT
• Mass and weight are used
interchangeably in everyday
language, but they are NOT the
same thing in physics!
• Weight varies with location,
based on gravity.
• Mass is the same everywhere;
weight is not.
CHECK YOUR UNDERSTANDING
A ball has a mass of 10 kg on Earth. Will
its mass be more or less on the moon?
Neither, the mass will be the same in
both locations because the mass of an
object does not change.
What about the weight?
The ball will weigh more on the Earth
than the moon because there is more
gravity on Earth.
CHECK YOUR UNDERSTANDING
A girl has a mass of 25 kg. What is her weight on
Earth?
W = ?
m = 25 kg
g = 10 m/s2
W = mg
W = (25)(10)
W = 250 N
WARM UP 10/10 – REMEMBER TO WRITE
THE QUESTION BEFORE YOU ANSWER
• 1. A space shuttle with a mass of 250 kg
on Earth lands on the moon, what is it's
mass on the moon?
• 2. What is the same shuttle's weight on
Earth? What is it's weight on the moon
(the moon's gravity is 1.6 m/s/s)?
NET FORCE
• A force is a vector
 It takes into account direction
Net force:
the combination of all forces
acting on an object.
 It is the net force that changes an object’s
state of motion.
 SI Unit: Newtons (N)
 Ex: If you push on a cart with 10 N of force
and someone else pushes in the opposite
direction with 4 N, the net force is 6 N.
• If an object is resting on the table, the table
is pushing on it with the same force that the
book is pushing on the table, the object is
in equilibrium.
Equilibrium: when the net force is equal
to zero.
 An object in equilibrium has only inertia, no
force acting on it and is moving at a constant
velocity or not moving at all
 Ex: You push on the wall with 10 N of force, the
wall pushes on you with 10 N of force to equal 0
N.
CHECK YOUR UNDERSTANDING
A girl pushes a cart with a force of 10 N, however her
mischievous friend pushes the same cart in the
opposite direction with 5 N of force. What is the net
force?
Since they are going in opposite directions, you
subtract the forces to find the net force.
10 N – 5 N = 5N
CHECK YOUR UNDERSTANDING
A hockey player hits a hockey puck across the ice.
10 seconds after he hits it, it is still moving down the
ice. Is the puck in equilibrium?
Yes! Even though it is still moving, there is no net
force being exerted on it, so it is moving at a
constant velocity and only inertia is allowing it to
keep moving.
• Net force is directly proportional
to acceleration
• This means that as one increases,
the other increases by the same
amount
2F = 2a
3F = 3a
MASS AND ACCELERATION
• Mass and acceleration are inversely
proportional
• As one increases, the other decreases by the
same amount
 2m, ½a
 ½m, 2a
 10m, 1/10a
 1/10m, 10a
GET A DRY ERASE MARKER FROM MY
DESK THEN COMPLETE THIS WARM UP:
•What happens to an object’s
acceleration when you apply
more force to the object?
•What happens to an object’s
acceleration when you
increase the mass of the
object?
NEWTON’S SECOND LAW OF
MOTION
Newton’s
nd
2
Law:
The
acceleration on an object is
directly proportional to the
magnitude of the net force,
and is inversely proportional to
the mass of the object.
Forces produce accelerations
Ex: The harder you push
something, the faster it goes
Equation: a = f/m
• F = force (N)
• m = mass (kg)
• a = acceleration (m/s2)
CHECK YOUR UNDERSTANDING
A boy pushes on a 10 kg cart with a force of
50 N. What is the acceleration of the cart?
a = ?
F = 50 N
m = 10 kg
F = ma
50 = (10)a
a = 50/10
a = 5 m/s2
CHECK YOUR UNDERSTANDING
How much force is needed to accelerate a bike
10 m/s/s if the bike has a mass of 50 kg?
WARM UP 10/12
**Have homework on desk to be
checked
• How much force is needed to
make a toy car increase its
speed from 2 m/s to 4 m/s in 4
seconds if the car has a mass of
5 kg?
BALANCE IN NATURE
• Everything in nature comes in pairs.
 Forces also always come in pairs.
• A force is not just a push or pull, but a
part of an interaction between one
thing and another.
• One object is acting on the other object
NEWTON’S THIRD LAW
Newton’s 3rd Law of
Motion: Whenever one object
exerts a force on a second object, the
second object exerts an equal and
opposite force on the first object.
 aka The Law of Action and Reaction
 Ex: You hit the wall, but the wall is hitting
you back (hence why it hurts)
• One force is the action force
and the other force is the
reaction force.
• Neither coexists without the
other.
• You can’t touch something
without being touched.
CHECK YOUR UNDERSTANDING
Earth and the moon are “connected” to each other
by a gravitational force. Is Earth pulling on the
moon, or is the moon pulling on Earth?
Both! They are part of the same interaction. They
pull on each other in an action-reaction pair.
CHECK YOUR UNDERSTANDING
Identify the action and reaction pair of forces for the
case of a bat interacting with a ball.
The bat pushes on the ball and the ball pushes on
the bat.
Action/reaction of a ball falling towards the Earth
Question: when a ball falls towards the Earth is it
pulling up on the Earth?
Answer: Yes! The action force is gravity pulling down
on the ball, the reaction is the ball pulling up on the
Earth
Question: Why can’t we see the Earth accelerating
(moving) up to meet the ball then?
Answer: Think about Newton’s second law:
acceleration is inversely related to mass – the Earth
is so massive that its acceleration towards the ball is
tiny and we can’t see it