Notes: Chapter 11.2Gravity and Projectile

Motion

Gravity/ Law of Universal Gravitation

• All objects in universe exert gravity on each other

• Formula F= G m1m2

d2

• The force between two objects depends on their masses and the distance between them

• As mass increases, so does the gravity between them

• As distance increases, the gravity between them decreases

Gravity/ Law of Universal Gravitation

• As mass increases, so does the gravity between them (direct proportion)

• As distance increases, gravity decreases

(inverse proportion)

Free Fall and Weight

• Free fall: objects move with only gravity (not evenfriction)– can only happen in vacuum (space)

• Objects fall with the same acceleration due to gravity – (Earth = 9.8 m/s2, we usually round to 10.0 m/s2)

• Effect of gravity on a mass is called its weight • Weight = mass x g (accel. gravity)• Weight and mass are not the same. Mass is

the amount of matter, weight is how gravity affects that matter. On the Moon you have the same mass as on Earth, but weigh a different amount (gravity is different)

Terminal Velocity, Orbit

• An object reaches a point as it falls where the effects of air resistance balance the gravity. This is called terminal velocity.

• It’s not that objects will not fall anymore, it’s that they won’t fall any faster (accelerate)

• Orbiting objects are in free fall (only gravity).

• They also have a forward momentum, so they don’t fall back into earth right away.

• Gravity and Orbit

What hits first?

• A bowling ball or a golf ball?– Heavier objects get pulled more by

gravity, but have more inertia. The inertia and pull of gravity balance each other out.

– They hit at the same time.

• Why don’t we see this with a piece of paper?

– Because different objects have different amounts of air resistance.

Projectile Motion

• Note: these are velocity vectors, not force vectors.

• Objects have forward motion as well as downward motion.

• These two forces do not affect each other. They are independent.

• In fact, the speed with which it falls will increase (accelerate), while the forward velocity is constant (object in motion…)

Projectile Motion• Notice how on

just falls down, where the other goes forward as it’s falling?

• The addition of the forward motion does not

affect when they hit.

• They both hit at the same time.

Projectile Motion• Although both forces do not affect each other, they

do both affect the motion of the object. • It will follow a curved path (parabola). • In other words how far you throw something has no effect on how long it takes to fall. It will affect where it will land, though.• Projectile Motion