Transcript 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:
Every
object continues in a state of rest, or of motion in
a straight line at a constant speed, unless it is
compelled to change that state by forces exerted
upon it.
 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.
 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
 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: a mass of 25 kg on Earth also has a mass of 25 kg 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 more inertia an object has, the more force is
needed to change its state of motion (like making
something stop).
Weight
Weight:
the force of gravity on an object
 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 and is 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 less on the moon than the Earth
because there is less gravity on the moon.
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
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.
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.
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
frictionless ice. 10 seconds after he hits it and it is
still moving down the ice, is the puck in
equilibrium? Ignore air resistance.
 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.
Force and Acceleration
 If a force is applied to an object at rest, it starts to move
 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.
 Because of INERTIA!
 To increase the acceleration of the object, more force
needs to be applied
 Net force is directly proportional to acceleration
 2F = 2a
 3F = 3a
Mass and Acceleration
 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
Newton’s Second Law of Motion
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
 Forces produce accelerations
 Ex: The harder you push something, the
faster it goes
 Equation: F



= ma
F = force (N)
m = mass (kg)
a = acceleration (m/s2)
Check Your Understanding
A girl pushes on a 9 kg cart and causes it to
accelerate at 3 m/s2
. What force did she exert on the cart?
F=
N
m = 9 kg
a = 3 m/s2
F = ma
= (9)3
F = 27 N
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
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?
 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
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.
 In an interaction between objects, there are a pair of
forces (one acting on each object).
Check Your Understanding
Does a stick of dynamite contain force?
 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 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
 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)
 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.
Interacting Forces
 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.
 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
Check Your Understanding
Can you identify the action and reaction forces of
an object falling in a vacuum of outer space?
 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?
 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.