Gravity and Air Resistance

Chapter 3Section 7-9

The Law of Gravitation

• You exert an attractive force on everything around you and everything is exerting an attractive force on you.

• This attractive force is called gravity.• Anything that has mass is attracted by

the force of gravity.• The Law of Gravitation states that any

two masses exert an attractive force on each other.

The Law of Gravitation

• Gravitation force depends on two things:– The mass of the two objects– The distance between the two objects

• Why do you suppose the Earth exerts a force on you that you can feel, but you can’t feel the force the desk is exerting on you?

Gravitational Acceleration

• When objects fall, the gravitational pull is 10 m/s2

• When a falling object is only affected by gravity it is said to be in free fall.

Gravitational Acceleration

• If you drop a a feather and a coin, which would hit the ground first?

• How about if there were no air?

Gravitational Acceleration

•Force of gravity is greater on the bowling ball because of its larger mass.

•The larger mass means it has a larger inertia so more force is needed to change its velocity.

•Gravitational force on the marble is smaller because it has a smaller mass

•The inertia on the marble is less and less force is needed to change the velocity.

•Therefore, all objects fall with the same acceleration!

Weight

• Even if you are standing still and you have zero acceleration, the force of gravity is still present.

• Weight is the gravitational force exerted on an object.

• Capital W stands for weight.• Gravitational force = mass x

(acceleration due to gravity)

Weight

• Because the gravitational force is the same as the weight and the acceleration due to gravity, the equation can also be written as:

• W = (m)(10 m/s2)

Weight & Mass

• Weight and mass are not the same thing.• Weight is a force• Mass is a measure of the amount of

matter an object contains.• The greater the weight, the greater the

attraction between the object and Earth.

Weightlessness

• To be nearly weightless, you would have to be far from the Earth.

• Astronauts experience this feeling when they are in space.

Air Resistance

• What two forces are acting on an object when it falls?

• Gravity• Air resistance

Air Resistance

• Imagine dropping two pieces of paper. One is crumpled and the other is flat.

• Which one will reach the ground faster and why?

• The crumpled one because it has less surface area.

Air Resistance

• When something falls, air resistance acts in the opposite direction as the force of gravity.

• Air resistance acts in the opposite direction of the object’s motion.

Air Resistance

• The amount of air resistance an object experiences depends on two things:

• Speed• Surface Area

Air Resistance

• Why do leaves, papers, and feathers fall at different speeds than acorns, pens, and glasses?

• Because of Air Resistance…• Air resistance not mass is

responsible for the difference in objects falling speed.

Terminal Velocity

• As an object falls, it accelerates and its speed increases.

• The force of air resistance increases with speed.

Terminal Velocity

• The force of air resistance increases until it becomes large enough to cancel the force of gravity.

• When the forces cancel each other out the object no longer accelerates.

• The object then falls at a constant speed called terminal velocity.

Terminal Velocity

• Terminal Velocity is the highest velocity that a falling object will reach.

• A low terminal velocity allows the skydiver to land safely.

Terminal Velocity

• Why would a skydiver want to lay out flat versus falling standing up?

• Think about our paper example… crumpled vs flat