Gravity and Air Resistance

Gravity

The force that pulls objects towards each other – a force of attraction between objects!

Law of Universal Gravitation

The force of gravity acts between all objects in the universe.

This means that all objects attract each other!

Factors Influencing Gravity

Both Mass and Distance affect gravity.

Increasing Gravity

Increase the mass of 1 or both of the objects

Move the objects closer together

Weight

Weight is a force! The force of gravity between an

object and planet it’s onWeight depends on mass and the

force of gravity at a specific location

Weight and Mass

NO! Weight is not the same thing as mass.

Mass is the amount of matter in an object and weight is the force of attraction between a mass and a planet.

Weight changes depending on where you are in the solar system. Mass does not change based on location.

Unit for Weight

Weight is FORCEThe unit is a NEWTONYou WEIGH NEWTONS (not pounds)

Free Fall

When falling the only force acting on an object is gravity

Type of force when in free fall:unbalanced force

Objects in free fall accelerate!

Acceleration in Free Fall

Falling objects on Earth accelerate at 9.8 m/s/s

Mass does NOT affect the acceleration of objects in free fall. All objects accelerate at the same rate due to the Earth’s gravity.

Air Resistance

In a vacuum (on Earth) all objects fall at the same rate. But on Earth there is air so there is FLUID friction, which is called AIR RESISTANCE. This force is exerted in the UPWARDS direction on a falling object.

Forces on the object:

Weight (gravity)

Air resistance (friction)

Surface Area

Objects with a larger surface area have higher air resistance

An object with a larger surface area needs to push more air molecules out of the way than a smaller object

When an object falls…

When an object falls it ACCELERATES, so the air resistance INCREASES.

Which has more air resistance?

A B

A has more air resistance since it has a larger surface area

Which has more air resistance?

20 m/s

50 m/s A B

B has more air resistance since it is moving at a faster speed

Ways to Increase Air Resistance

Increase surface areaIncrease speed

Air Resistance and Falling Objects

Air resistance increases as an object falls

As object falls object accelerates object gains speed air resistance increases

Air resistance (friction)

Air Resistance and Net Force

Air resistance reduces the net force on a falling object

Air resistance acts in opposite direction as weight, to find net force you need to subtract!

Weight = 55 N

Air resistance = 10 N

Net Force = 45 N

Increase in Air Resistance and Net Force

More air resistance = more force opposite motion = smaller net force

Weight = 55 N

Air resistance = 10 N

Net Force = 45 N

Weight = 55 N

Air resistance = 30 N

Net Force = 25 N

Increased Air Resistance and Acceleration

More air resistance = smaller net force on object = lower acceleration

Time (sec)

Speed (m/s)

0 0

1 9.8

2 17.15

3 22.66

4 26.79

5 29.89

6 32.22

7 33.96

The increase in speed each second is getter smaller, therefore the acceleration is getting smaller

Increased Air Resistance and Speed

More air resistance = lower accelerationThe object will still gain speed as it is

falling, but at a smaller rate (the increase in speed will be getting smaller)

And remember more speed = more air resistance = smaller net force = smaller acceleration… this will continue until the object reaches terminal velocity and stops accelerating

Terminal Velocity

Terminal Velocity = The maximum speed an object can reach while falling through the air.

Type of force: Balanced Forces (net force = 0 N)

Objects at terminal velocity have a constant velocity (they are no longer accelerating)

This occurs when: Force of gravity = force of air resistance

To Change Terminal Velocity

To decrease:Less mass (less weight)more surface area

To Increase:More mass (more weight)Less surface area

Falling Boxes

How much air resistance is needed to reach terminal velocity?

A B

Air Resistance Needed: 20 N 2 N_

20 N

2 N

Falling Boxes

Which will have a higher terminal velocity?

A B

A. Since it has more weight there needs to be more air resistance to have balanced forces. It will need to reach a higher speed to reach that higher air resistance. Box A will therefore accelerate for a longer time than box B.

20 N

2 N

Falling Boxes

How much air resistance is needed to reach terminal velocity?

A B

Air Resistance Needed: 10 N 10 N_

10 N 10 N

Falling Boxes

Which will have a higher terminal velocity?

A

B

B. Since it has less surface area it has less “inherent” air resistance and will need to reach a higher speed to reach 10 N of air resistance. Therefore Box B will accelerate for a longer time than box A.

10 N 10 N

Projectile Motion

Projectile: An object that is thrown – has a horizontal force as well as the downwards force of gravity

Will fall in an arc

Forces on the Cannon Ball

Force of cannon

Air resistance

(friction)Gravity

Direction of motion

Cannon Balls

The cannon ball will be pulled down by gravity.

Gravity causes the cannon ball to slow down in the upwards direction and eventually will cause the upwards motion to stop and become downwards motion.