Chapter 2, Section 1 Notes

Gravity and Motion

History

Aristotle was the first to discuss gravity.

He believed that the amount of gravity was dependent on an

object’s mass.

Galileo Galilei challenged Aristotle

He argued that mass of an object does NOT affect the time it takes that object to fall to the ground.

He tested this by dropping two cannon balls of different masses to the ground. They hit at the

same time!

Gravity and Acceleration

Acceleration is the rate at which velocity changes over time.

Objects fall to the ground at the same rate because the

acceleration due to gravity is the same for all objects.

Acceleration of all object toward Earth= 9.8 m/s2

Acceleration depends on force and mass.

An object with a lot of mass does have a larger force acting upon it, but it is harder to accelerate. This is why objects of different mass

can fall with the same acceleration

Velocity of Falling objects

• To calculate the change of velocity of a falling object, use the following equation

• V= g x t• • = change• V= velocity• g= acceleration due to gravity, 9.8m/s2

• t= time( seconds)

Air Resistance and Falling Objects

Air resistance is the force that opposes the motion of objects

through air.

Air resistance is dependent on size, shape and speed of an object. For

example, air resistance would affect a flat piece of paper more than a

crumpled pieces of paper.

Increased speed = more air resistance

Air resistance will increase until it is balanced with the downward

force of gravity.

Once the net force is 0 Newtons, the object will fall at a constant velocity known as its terminal

velocity

If there is no air resistance, and object is said to be in free fall.

Free Fall occurs if the only force acting on

an object is gravity.

Free fall can only occur where there is no air, such as in a

vacuum.

Orbiting Objects are in Free Fall

An object orbits when it travels around another object in space.

A spacecraft that orbits Earth moves forward, but is also in free fall towards Earth, this causes the

spacecraft to orbit.

The 2 forces ( forward + down) allows the shuttle to follow the

curve of the Earth’s surface, and allows it to orbit.

The astronauts inside a spacecraft are also in free fall, which is why

they float.

Other objects also orbit in space.

( the moon, planets, star, etc)

Objects that orbit are in a constant circular motion, and are always

changing direction. This is caused by a constant unbalanced force, known

as centripetal force. This force is caused by gravity.

Projectile Motion and Gravity

Projectile motion is the curved path an object follows when it is

thrown or propelled near the surface of Earth.

There are two components to projectile motion

• horizontal motion: motion that is parallel to the ground

• vertical motion: motion that is perpendicular to the ground(gravity)

Examples of projectile motion:

• frog leaping• diving into a pool• shooting an arrow