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Chapter 11
Section 1: Laws of Motion
Section 2: Gravity
Section 3: Newton’s Third Law
Key Term
Inertia
Newton’s First Law
Every motion you observe or experience is related to force.
Newton’s First Law of motion states that an object at rest
remains at rest and an object in motion maintains its
velocity unless it experiences an unbalanced force.
Objects tend to maintain their state of motion. This
tendency is known as inertia. All objects resist changes in
motion, so all objects have inertia.
How does the mass of an object relate to inertia?
Small mass = less inertia
Large mass = more inertia
Newton’s first law is sometimes called the law of inertia.
This law describes what happens when the net force acting
on an object is zero
Newton’s Second Law
Newton’s second law describes the effect of an
unbalanced force on the motion of an object
Force equal mass times acceleration
This law states that the unbalanced force acting on an
object equals the object’s mass times its acceleration.
Force = mass x acceleration
F = ma
Force is measured in newtons. This is the
SI unit for force (N)
1N = 1kg x 1m/s²
Key Terms
Free Fall
Terminal Velocity
Projectile Motion
Law of Universal Gravitation
Sir Isaac Newton came up with a law that helps explain
why objects fall towards the Earth.
This law states that all objects in the universe attract each
other through gravitational force.
This equation says that the gravitational force increases as
one or both masses increase and the force decreases as the
distance between the masses increase.
All matter is affected by gravity no matter how big or how
small
Gravitational force increases as mass increases
Gravitational force decrease as distance increases
Gravitational force is weaker than other types of forces,
even though it holds the planets, stars, and other galaxies
together
Free Fall and Weight
When gravity is the only force acting on an object, the
object is said to be in free fall.
Free fall is the motion of a body when only the force of
gravity is acting on the body.
Where is the acceleration of free fall directed?
Towards the center of the Earth.
Free fall acceleration near Earth’s surface is constant
Without air resistance, all objects on Earth accelerate at
9.8m/s²
Weight is equal to mass times free fall acceleration
The force on an object due to gravity is called weight
Why do astronauts appear to be weightless in space?
Because the astronauts and their surroundings all
accelerate at the same rate.
Weight is different from mass
Mass is a measure of the amount of matter in an object
Weight is the gravitational force an object experiences
because of mass
Mass remains the same everywhere, but weight changes as
the gravitational force acting on an object changes.
Free Fall and Motion
Why is it incorrect to say that skydivers are free falling
before the open their parachute?
Because air resistance is a force, free fall can occur
only where there is no air
Projectile Motion and Gravity
The orbit of a space shuttle around Earth is an
example of projectile motion
Projectile motion is the curved path that
an object follows with thrown, launched, or
otherwise projected near the surface of Earth
Projectile motion has some horizontal motion
and vertical motion.
Key Terms
Momentum
Action and Reaction Forces
Newton’s third law says that for every action force, there is
an equal and opposite reaction force.
If you kick a soccer ball the action force is your foot hitting
the ball and the reaction force is the ball hitting your foot.
Forces always occur in pairs
Do action and reaction force occur when there is no
motion?
Yes. When you sit on a chair, your weight pushes
down on the chair (action force), and the chair pushing
back up (reaction force)
Action and reaction forces occur at the same time
Equal forces don’t always have equal effects
Momentum
Momentum is a property of all moving objects.
Momentum is a quantity defined as the product of the
mass and velocity of an object
Momentum = mass times velocity
(p = mv)
The more mass an object has, the greater its momentum.
Also the more velocity an object has, the greater its
momentum
If an object is not moving, its momentum is zero
Like velocity momentum has direction. The direction is
the same as its velocity.
Force is related to change in momentum
If the time period of the momentum’s change becomes
longer, the force need to change the momentum is less.