Transcript The Laws of Motion Chapter 4 - Belle Vernon Area School District

The Laws of Motion
Chapter 4
2014-2015
The First Two Laws of Motion
Section 4-1
The British Scientist Isaac Newton published a
set of three rules in 1687 that explained how
forces and motion are related
The First Law of Motion
• If the forces acting on an object are balanced
(net force is zero) then the velocity of the object
does not change
• If the object is at rest it will stay at rest.
• If the object is moving in a straight line, it will
continue moving in a straight line with constant
speed
First Law Simply Stated
• If the object is at rest it will stay at rest.
• If the object is moving in a straight line, it will
continue moving in a straight line with
constant speed
“An object At rest will
remain at rest and an
object in motion will
remain in motion unless
acted upon by an outside
unbAlAnced force”
Golden Rule #1
ONLY UNBALANCED
FORCES CHANGE
VELOCITY/MOTION
Example
• Suppose a toy car is at rest, it does not move unless
you give it a push. The velocity (speed and
direction) increases as long as you are pushing it.
(forces are balanced)
• When you stop pushing frictional force will slow it
down and stop it (forces become unbalanced)
Inertia and Mass
• All objects have a property called
inertia, it is the tendency of that object
to resist a change in its motion
• The inertia of an object depends on the
objects mass
Example
Bike wanting to move in a straight line when you want
to turn (force needed to make it turn against inertia)
Example
Which has more inertia and why?
vs.
Inertia and the First Law of Motion
What Happens in a Crash?
• When the car that you are in suddenly stops,
your body want to keep moving at the speed
that you were traveling (inertia)
• The car stops due to unbalanced forces, but
you keep moving unless you are wearing a
seat belt!
The Second Law of Motion
• Newton’s second law describes how
the net force of an object, its mass,
and its acceleration are related.
Force and Acceleration
• The greater the force applied to an object,
the greater the acceleration
• Recall acceleration is the change in velocity
divided by the time
• 𝒂 = 𝜟𝒗/𝜟𝒕
Example
Which ball will have greater acceleration and
why?
vs.
Mass and Acceleration
• Acceleration depends on the objects mass
and the net force exerted on it
• Net force, acceleration, and mass are related
Which will have a greater acceleration
and why?
vs.
The Second Law in Equation
• The acceleration of an object is in the same
direction as the net force on the object
• Net force, acceleration, and mass are related
Algebraic Manipulation
m = F/a
Example Problem #1
• You push a friend on a sled. Your friend and
the sled together have a mass of 70kg. If the
net force on the sled is 35N (newton's), what
is the sleds acceleration?
• Identify what the known values are and
choose the proper equation…
Example Problem #2
• If the mass of a helicopter is 4,500kg,
and the net force on it is 18,000N, what
is the helicopters acceleration?
Example Problem #3
• What is the net force on a dragster with
a mass of 900kg if it acceleration is 32.0
m/s2 ?
Example Problem #4
• A car is being pulled by a tow truck.
What is the cars mass if the net force on
the car is 3,000N and it has an
acceleration of 2.0 m/s2 ?
Gravity
Section 4-2
What is Gravity?
• Gravity is an attractive force between
any two objects that depends on the
masses of the objects and the distances
between them
The Four Basic Forces
•
•
•
•
Gravitational Force
Electromagnetic Force (electricity)
Strong Nuclear Force (nuclei)
Weak Nuclear Force (nuclei)
The Law of Universal Gravitation
d
The Limits of Gravity
• The gravitational forces between two objects
decreases rapidly as the distance between
the two objects increases
• The larger the mass the more pull it exerts,
the smaller the less pull it exerts
Gravitational Acceleration
• If air resistance can be eliminated, all
objects will fall at the same rate of
speed toward the Earth
9.8 m/s2
Weight and Mass
• Mass is the amount of matter in an
object
• Weight is a measure of the amount of
gravitational force exerted on an object
Moon vs. Earth
• The gravitational force of the moon is 1/6 as
strong as that of the Earth, so you would
weight 1/6 as much on the moon
• Ex. 200 lbs. on Earth is 33lbs. on the moon
Why doesn’t the Earth pull you apart?
• The force of gravity pulling you down is offset
by the floor, ground etc…pushing you back
up.
• Balanced forces keep us together…
Projectile Motion
• Anything thrown or shot through the air
is called a projectile.
• The Earth’s gravity causes projectiles to
follow a curved path
Pitching a Ball
• The pitcher releases the ball
horizontally, but the force of gravity
gradually pulls the ball downward giving
it a curved path
Centripetal Acceleration
• Acceleration toward the center of a
curved or circular path.
• Remember that acceleration is not the
same as speed. Acceleration is a vector
that is made up of speed and direction
Centripetal Force
• The net force exerted toward the center
of a curved path.
• Anything that moves in a circle is doing
so because this force is accelerating it
toward the center
Newton’s Third Law of Motion
Section 4-3
Action and Reaction
• When a force is applied in nature, a reaction
force occurs at the same time.
• “For every action, there is an equal and
opposite reaction”
Action and reaction forces are not
always equal!!!
• Sometimes forces cancel each other and no
movement occurs, but other times the forces
can combine to create motion…
Rocket Propulsion
• Rockets use Newton’s third law to propel
themselves forward. The gases expelled by
the rocket act to push the rocket up.
Momentum
• Momentum is related to how much force is
needed to change an objects motion
• 𝒑 = 𝒎𝒗
• 𝒎=
𝒑
𝒗
• 𝒗 = 𝒑/𝒎
• p stands for momentum
Example Problem #1
• At the end of a race, a sprinter with a
mass of 80kg has a speed of 10m/s.
What is the sprinters momentum?
Example Problem #2
• What is the momentum of a car with a
mass of 1,300kg traveling a speed of
28m/s?
Example Problem #3
• What is the mass of a person walking a
speed of 0.8m/s if there momentum is
52.0kg-m/s?
Example Problem #4
• A baseball thrown by a pitcher has a
momentum of 6.0kg-m/s. If the baseballs
mass is 15.0kg, what is the baseballs speed?
Conservation of Momentum
• If a group of objects exerts forces only on
each other, their total momentum does not
change.
• Momentum gained equals momentum lost!