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.