Page 41 of ISN

I can describe how gravity and air resistance affect the motion of a

falling object.

GRAVITY& AIR RESISTANCE

The physics of falling

The Force of Gravity

• Gravity is the force that pulls all objects down to the earth.

• Rain falls from the sky down to earth…

• If you drop a book, it falls to the ground…

• If you trip, you’ll fall down…

• Actually in science, gravity is a force of attraction that acts between ALL objects (the earth, you, the desk, a book)

• The force of gravity is much STRONGER for LARGER objects (more mass).

Universal Gravitation

Universal Gravitation• Because the Earth is by far, the largest

and closest object around, it has the greatest force of attraction...

• So, no matter where you are on earth, all things fall to the ground due to gravity…

What is “free fall”?

• When gravity is the ONLY force acting on an object, it is in free fall.

• In that case, gravity is an UNBALANCED FORCE which causes the object to accelerate.

Objects in Free Fall

• Do all objects fall at the same rate?

• If we dropped a bowling ball and a tennis ball from the same height, which would land first?

• Let’s try it!

Objects in Free Fall

• Do all objects fall at the same rate?

• ALL objects in free fall travel at the same rate, regardless of mass!

• In free fall, heavy objects and light objects fall at the same rate!

So, which will land first?• WHY does the penny

land first?

• Remember the force that opposes motion (slows things down)? FRICTION!

• Falling objects experience friction with the air called AIR RESISTANCE that slows them down.

Air Resistance• The larger the object (more surface

area), the more air resistance.

• That’s why parachutes work! The upward force of the air acting on the LARGE parachute slows you down as you fall.

Air Resistance• Draw a diagram

showing the forces…

• Downward force of gravity is same on both.

• Upward force of air resistance is greater on the feather.

• The net force (down) is greater on the penny.

Air Resistance

Gravity

Net Force on Feather

Net Force on Penny

Without air resistance, all objects would fall at the same rate…

http://www.planetseed.com/laboratory/galileo-drops-ball-virtual-experiment

Hammer and Feather Drop on the MoonSee video clip (1:22)

Gravity on the moon?• The force of gravity is much weaker on

the moon because…– It is much farther away from earth.– The moon is much smaller than earth.

That’s why astronauts weigh less on the moon!

Gravity Review

• Gravity is the force that pulls all objects down to the earth.

• When gravity is the ONLY force acting, ALL objects accelerate at a rate of 10 m/s2.

• Mass doesn’t matter – in free fall, heavy objects and light objects fall at the same rate!

Air Resistance Review

• Some objects take longer to fall – they are slowed down by FRICTION with the air called AIR RESISTANCE.

• The larger the surface area, the greater the force of air resistance pushing up.

• Without air resistance, all objects would fall at the same rate…

• Draw a free body diagram of the sky diver and label ALL the forces.Gravity = 1000 NAir Resistance = 800 N

• What is the net force?

Net Force = 200 N

Air Resistance

= 800 N

Gravity = 1000 N