Gravity and Air Resistance

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 NO! 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)

2 Ways to Increase Air Resistance 1. Increase SURFACE AREA Larger surface area  collide with more air molecules  needs to push more air molecules out of the way

2 Ways to Increase Air Resistance 1. Increase SPEED Faster speed  collide with air molecules more frequently  hit air molecules with more force

Which has more air resistance? A B A has more air resistance It has a larger surface area

Which has more air resistance? 20 m/s 50 m/s A B B has more air resistance It has a faster speed

Air Resistance and Net Force As an object falls it  ACCELERATES  The object’s speed  INCREASES  The air resistance of the object  INCREASES  Air resistance (friction)

Air Resistance and Net Force The force of friction in the  UPWARDS direction of motion  INCREASES  The downwards force of gravity  DOES NOT CHANGE 

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 

Air Resistance and Net Force The net force acting on the object  DECREASES  The acceleration of the object  DECREASES  The rate at which the speed changes  DECREASES  science8lewis.weebly.com

Air Resistance and Net Force The object’s speed still  INCREASES  but…  SLOWER  Time (sec) Speed (m/s) The increase in speed each second is getter smaller, therefore the acceleration is getting smaller

In Summary The speed of a falling object INCREASES  the air resistance acting on the object INCREASES  the net force acting on the object DECREASES  the object’s acceleration DECREASES

In Summary until the net force acting on the object = 0 and therefore the acceleration of the object = 0 designintheclassroom.com

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

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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

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

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To Change Terminal Velocity **Click on picture to go to interactive activity** To decrease:  Less mass (less weight)  more surface area To Increase:  More mass (more weight)  Less surface area