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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.