Chapter 2: Forces and Motion
Section 1: Gravity and Motion
In Ancient Times, around 400 BC, a philosopher named Aristotle, thought that the rate objects fell depended on their
mass.
Example: he thought a baseball would fall faster than a marble
In the late 1500’s a young Italian scientist named Galileo Galilei questioned this idea.
He thought that the mass of an object did not matter.
To prove this, he dropped two different cannonballs off of the Leaning Tower of Pisa in Italy 
Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects!!!.
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Acceleration depends on both force and mass
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A heavier object experiences a greater gravitational force than a lighter object does
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But a heavier object is also harder to accelerate because it has more mass
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The extra mass of the heavier object exactly balances the additional gravitational force so they fall at the
same rate
Acceleration is the rate at which velocity changes over time the rate at which velocity changes over time
All objects accelerate toward Earth at a rate of 9.8 m/s2.
 So for every second an object falls, the object’s downward velocity increases by 9.8m/s
You can calculate the change in velocity (∆v) of a falling object by using the following equation:
∆𝑣 = 𝑔 × 𝑡
 In this equation, g is the acceleration due to gravity on Earth (9.8 m/s2)
 T is the time the object takes to fall (in seconds)
 The change in velocity is the difference between the final velocity and the starting velocity
Air resistance: the force that opposes the motion of objects through air
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Amount depends on size, shape, and speed of object
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As speed increases, air resistance increases until it is equal to the downward force of gravity
Terminal Velocity: the constant velocity of a falling object when the force of air resistance is equal in magnitude and
opposite in direction to the force of gravity
Free Fall: the motion of a body of when only the force of gravity is acting on the body
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Can only occur if there is NO AIR!!!!!
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This is only in space and in a vacuum.
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Astronauts are not weightless, but they are in Free Fall due to the lack of air
Orbiting: when an object is traveling around another object in space. (See figure 7 on page 40)
Centripetal Force: The unbalanced force that causes objects to move in a circular path
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Gravity provides the centripetal force that keeps objects in orbit
Projectile Motion: the curved path that an object follows when thrown, launched, or otherwise projected near the
surface of Earth
Has Two components:
1. Horizontal motion – parallel to the ground
2. Vertical motion – perpendicular to the ground
These components are independent of one another, so they have no effect on each other
When the two motions are combined, they form a curved path
Examples of projectile motion:
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Frog leaping
Water sprayed by a sprinkler
Arrow shot by an archer
Section 2: Newton’s Laws of Motion
Newton’s First Law of Motion  An object at rest remains at rest, and an object in motion remains in motion as a
constant speed and in a straight line unless acted upon by an outside force
An object at rest is not moving. These objects will not move until a push or pull is exerted on them.
A moving object stops eventually because of the opposing force of friction.
Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or
direction until an outside force acts on the object
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Newton’s First Law is sometimes called the Law of Inertia.
Mass is a measure of inertia
Newton’s Second Law of Motion  The acceleration of an object depends on the mass of the object and the amount of
force applied
Acceleration depends on Mass: the acceleration of an object decreases as its mass increases and vice versa
Acceleration Depends on Force: acceleration increases as the force of the object increases and vice versa; acceleration
of an object is always in the same direction as the force applied
Newton’s second Law Mathematically:
𝐹
𝑎=𝑚
or
𝐹 =𝑚 ×𝑎
Newton’s Third 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.
Simply stated, all forces act in pairs; they are action and reaction forces.
Example: You sit on a chair. Your weight pushing down on the chair is an action force. The reaction force is the
force exerted by the chair that pushes up on your body. The force is equal to your weight.
These forces do not always equal though or nothing would ever move!!
When the action and reaction forces are unbalanced, there is movement.
Section 3: Momentum
Momentum: a quantity defined as the product of the mass and velocity of an object
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The more momentum an object has the harder it is to stop the object or change its direction
Calculating momentum (p):
𝑝 = 𝑚 ×𝑣
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m = the mass of the object in kilograms
v = the object’s velocity in meters per second
units of momentum will be kg∙m/s
Like velocity, momentum has a direction and its direction is always the same as the direction of the object’s velocity
The Law of Conservation of Momentum: states that any time objects collide, the total amount of momentum stays the
same
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is true for any collision if no other forces act on the colliding objects
law applies whether the objects stick together or bounce off each other after they collide
Objects sticking together:
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after objects stick together they move together as one object (like a dog catching a ball)
the mass of the combined objects is equal to the masses of the two objects added together
When mass changes, velocity must change also
Objects Bouncing off Each other:
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Momentum is usually transferred from one object to another
The transfer causes the objects to move in different directions at different speeds; however the total
momentum of all the objects will remain the same before and after the collision
When action and reaction forces are equal and opposite, momentum is neither gained nor lost.