1.6 Air resistance Grade 7 A B Learning objectives • Explain what affects air resistance. • Describe what is meant by terminal velocity. • Explain how and why the speed of skydiver changes. What is Air resistance ? • Most objects falling through air experience a type of fluid friction called air resistance. • Air resistance: the force on an object that is moving through the air , causing it to slow down ( also known as DRAG ) Gravity and Air Resistance • Most objects falling through air experience a type of fluid friction called air resistance. • Air resistance is not the same for all objects. • The greater the surface area of the object the greater the air resistance. (paper drop) • However, since the elephant has more mass, it has more downward force of gravity and falls faster. Video Reducing air resistance • Air resistance can be problem . It is a form of friction that slows things down. • Air resistance is less if the area in contact with air reduced. • Streamlining: designing a shape designed to reduce resistance to motion from air or liquid Using air resistance • Air resistance can be very useful for slowing things down. • A parachute increases the area that is in contact with the air and therefore increases the air resistance. Terminal velocity • What are the forces on a skydiver? How do these forces change (think about when they first jump out; during free fall; and when the parachute has opened)? Terminal velocity Skydiving Stages • Stage 1 – after just jumping from the plane the skydiver is not moving very fast – their weight is a bigger force than their air resistance, so they accelerate downwards Terminal velocity Skydiving Stages • Stage 2 – eventually the force of the air resistance has increased so much that it is the same size as the skydiver’s weight – the forces are balanced and the speed remains constant (this is terminal velocity) Terminal velocity Skydiving Stages • Stage 3 – when the chute opens air resistance increases dramatically: the air resistance force is much greater than the weight force, so the skydiver slows down Terminal velocity Skydiving Stages • Stage 4 – as the skydiver slows, the air resistance force from the chute is reduced, until it is the same size as the weight force – the forces are balanced and the speed remains constant (this is a new terminal velocity) This graph shows how the speed of a parachutist changes as he falls. Parachute opens – diver slows down Velocity Speed increases… Terminal velocity reached… New, lower terminal velocity reached Time Diver hits the ground Do heavier things fall faster ? • When you see things fall on Earth , like a feather or a hummer , the heavier object (hummer) falls faster ,why? Air resistance affects the motion of the feather much more than the motion of the hummer. Do heavier things fall faster ? • If you remove the air resistance or do the experiment somewhere where there is no air like the moon , the hummer and feather hit the ground at same time . • If there is no air resistance all objects fall at same rate.