Download Newton`s Laws of Motion

Newton’s Laws of Motion
Use the Force Luke!
Force
Force
• A force is anything that can cause an
object with MASS
mass to accelerate
ACCELERATE
Mass
=
=
Acceleration =
How much stuff
Weight
Increasing or decreasing your
speed
If you are going 50 km/h and stay
that speed your acceleration = 0
Force and Mass
Mass is the measure of
how hard it is to change
an object’s velocity.
(Inertia)
Mass can also be
thought of as a measure
of the quantity of
matter in an object.
Force
• Measured in Newtons
• Weight: is a force
•
•
•
•
•
The effect of earths gravity on mass
Force = MASS * ACCELERATION (F=ma)
Mass = amount of stuff in the object
Acceleration = gravity
10N = 1kg (on earth)
On the Moon
• Mass stays the same
• Gravity is different
– Acceleration is different
• Gravity
• Moon = 1/6th that of earth
• So if you weigh 50lbs on earth, how
much will you weigh on the moon?
– Remember weight is a force, and
F=m*a
F = 50lbs * (1/6) = 8.35lbs
How many Newtons is this?
Newton’s Laws of Motion
• Law 1
• An object at rest will stay at rest
• An object in motion will stay in motion
• Unless acted upon by an unbalanced
force
Newton’s Laws of Motion
• Law 1
5-2 Newton’s First Law of
Motion
Question: If you stop pushing an object,
does it stop moving?
Answer: Only if there is friction! In the
absence of any net external force, an
object will keep moving at a constant speed
in a straight line, or remain at rest.
This is also known as the law of inertia.
INERTIA
• Inertia – an object’s resistance to a change in it’s
motion.
• Related to the mass of an object
• More mass = more inertia!
• Example: The heavier a shopping cart is, the
harder it is to get moving or stop moving.
Tablecloth
5-2 Newton’s First Law of Motion
In order to change the velocity of an
object – magnitude or direction – a net
force is required.
An inertial reference frame is one in
which the first law is true. Accelerating
reference frames are not inertial.
Not Inertial Frame of
Reference
• You are on a train that suddenly stops
while holding a tray with an ice cube on it.
From your point of view the ice cube
seems to have accelerated off the tray
even though no visible force has acted
upon it.
• This appears to violate the First Law of
Motion
Newton’s Laws of Motion
• Law 2
• The acceleration of an object is caused
by an unbalanced force
• Acceleration is directly proportional to
the force and in the same direction
•F=ma
5-3 Newton’s Second Law of Motion
Two equal weights exert twice the force of one;
this can be used for calibration of a spring:
5-3 Newton’s Second Law of Motion
5-3 Newton’s Second Law of Motion
Free-body diagrams:
A free-body diagram shows every force acting
on an object.
• Sketch the forces
• Isolate the object of interest
• Choose a convenient coordinate system
• Resolve the forces into components
• Apply Newton’s second law to each
coordinate direction
5-3 Newton’s Second Law of Motion
Example of a free-body diagram:
Newton’s Laws of Motion
• Law 3
• For every action there is an equal and
opposite reaction
• Example: Think Bumper Cars!
– When you crash into a bumper car, that car
pushes back on you, which is why you feel a
sudden stop and a “jerk”.
5-4 Newton’s Third Law of Motion
Some action-reaction pairs:
5-4 Newton’s Third Law of Motion
Although the forces are the same, the
accelerations will not be unless the objects
have the same mass.
Contact forces:
The force exerted by
one box on the other is
different depending on
which one you push.
Newton’s Laws of Motion
• While driving down the road, a firefly strikes the
windshield of a bus and makes a quite obvious mess in
front of the face of the driver. This is a clear case of
Newton's third law of motion. The firefly hit the bus
and the bus hits the firefly. Which of the two forces
is greater: the force on the firefly or the force on
the bus?
• Answer: trick question! They are equal
Momentum
• ALL moving objects have momentum
• Momentum depends on the mass of the
object and the velocity
• Momentum = mass x velocity
• Momentum is in the direction of the
velocity
Conservation of Momentum
• When two or more objects collide,
momentum is never lost!
• Momentum before collision =
momentum after collision
• Example: The momentum from a bat is
transferred to the ball once the ball has
been hit.
• Momentum of the bat = momentum of the
ball
Momentum
• Momentum = Mass * Velocity
P=M*V
Pi = 60 kg cm/s
Pf = 60 kg cm/s
Question?
• When fighting fires, a firefighter must use great
caution to hold a hose which emits large
amounts of water at high speeds. Why would
such a task be difficult?
• Answer: The hose is pushing lots of water (large
mass) forward at a high speed. This means the
water has a large forward momentum. In turn,
the hose must have an equally large backwards
momentum, making it difficult for the firefighters
to manage.
Problems
• Katie pushes a cart of physics text books
with a total mass of 40.0 kg. What is the
acceleration of the cart if the net force on
the cart is 60.0N?
• F = ma
• a=F/m
• a = 60.0N / 40.0g = 1.50 m/s2
Problem
• A 25 N force accelerates Garrett on a
bicycle at 0.5m/s2 What is the mass of
Garrett and the bicycle?
• F = ma
• m = F/a
• m = 25N/0.5m/s2 = 50kg
Collisions
• Elastic
– Bounces off each other
• Inelastic collisions
– Stick Together
Inelastic Collision
M1 V1 = M2 V2