Transcript Newtons Laws ppt

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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
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Force: any influence that tends to accelerate an
object.
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Forces produce accelerations
Ex: a push or a pull
SI unit: Newtons (N)
Newton’s 1st Law of Motion:
An object at rest will stay at rest
and an object in motion will
stay in motion …. Unless acting
on by an outside force !!!
AKA:the Law of Inertia
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Ex: An object moving in space will keep moving at a
constant speed
Inertia: the property of an object to resist a
change in motion.
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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
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Objects in a state of rest, stays at rest.
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Ex: pulling the tablecloth out from under a table full
of plates and cups
Objects in motion, stay in motion.
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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:
the amount of matter present in an
object
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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
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:
the force of gravity on an object’s
mass
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Ex: your weight on Earth is 100 N, but on the moon
it is 16 N
SI unit: Newtons (N)
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Equation:
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W = mg
 W = weight (N)
 m = mass (kg)
 g = acceleration due to gravity (10m/s2)
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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.
A ball has a mass of 10 kg on Earth. Will its
mass be more or less on the moon?
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Neither, the mass will be the same in both
locations because the mass of an object does not
change.
What about the weight?
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The ball will weigh more on the Earth than the
moon because there is more gravity on Earth.
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
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A force is a vector
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It takes into account direction
Net force:
the combination of all forces acting
on an object.
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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.
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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.
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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.
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?
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Since they are going in opposite directions, you
subtract the forces to find the net force.
10 N – 5 N = 5N
A hockey player hits a hockey puck across the
ice. 10 seconds after he hits it and 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.
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If a force is applied to an object at rest, it starts
to move
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The object accelerates because it changed motion
(from rest to movement)
Once it is moving at a CONSTANT VELOCITY
(on a frictionless surface) no force is needed.
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Because of INERTIA!
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To increase the acceleration of the object, more
force needs to be applied
Net force is directly proportional to
acceleration
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2F = 2a
3F = 3a
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As the mass increases, the acceleration
decreases if the same amount of force is
applied to the object
Mass resists acceleration
Mass and acceleration are inversely
proportional
2m, ½a
 ½m, 2a
 10m, 1/10a
 1/10m, 10a
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Newton’s 2nd Law:
The acceleration
produced by a net force on an object is
directly proportional to the magnitude of the
net force, is in the same direction as the net
force, and is inversely proportional to the
mass of the object.
Car Test Dummies Video Clip
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Forces produce accelerations
Ex: The harder you push something,
the faster it goes
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Equation: F = ma
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 F = force (N)
 m = mass (kg)
 a = acceleration (m/s2)
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
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A car is set on cruise control and moving at a
constant 50 m/s headed east. If the car’s mass is
1000 kg, what is its acceleration?
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Because the car is not CHANGING its velocity, the
acceleration is zero
What is the force?
F=?
m = 1000 kg
a = 0 m/s2
F = ma
F = 1000 (0)
F=0N
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Everything in nature comes in pairs.
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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.
In an interaction between objects, there are a
pair of forces (one acting on each object).
Does a stick of dynamite contain force?
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No. Force is not something an object has, like
mass. Force is an interaction between two
objects. An object may have the capability to
exert a force on another object, but it cannot
possess force as a thing in itself.
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.
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aka The Law of Action and Reaction
For every action, there is an equal and opposite
reaction.
Ex: You hit the wall, but the wall is hitting you back
(hence why it hurts)
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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.
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?
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Both! They are part of the same interaction.
They pull on each other in an action-reaction
pair.
Identify the action and reaction pair of forces for
the case of a bat interacting with a ball.
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The bat pushes on the ball and the ball pushes
on the bat.
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Action: Object A exerts a force on object B
Reaction: Object B exerts a force on object A
The forces are equal and in opposite directions,
even if the masses are very different.
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According to Newton’s 2nd law ( F = ma), if
mass is not proportional to the pair of forces,
neither is the acceleration…but the masses are
still inversely proportional to the accelerations.
F=F
ma = ma
m a
a=m
Can you identify the action and reaction forces
of an object falling in a vacuum of outer
space?
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The interaction is the gravitational attraction
between the falling object and another object in
space, possibly a distant planet. So the planet
pulls down the object (action) while the object
pulls up on the planet (reaction).
According to Newton’s 3rd law of motion, the
action and reaction forces are equal. Does
that mean that the planet is actually moving
towards the object?
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Yes. The same thing happens when you jump
on Earth; the Earth moves towards you. But in
both cases the mass of the planet is so large
compared to the mass of the object/person, the
acceleration would be infinitesimally small,
almost zero. That’s why we don’t feel Earth
move to one side every time a person jumps.