4: Mechanics - Drag, lift and terminal speed

Question 1

What is friction?

Answer 1

a force that opposes motion

Question 2

What are the two main types of friction?

Answer 2

• Contact friction
• Fluid friction

Question 3

When does contact friction happen?

Answer 3

Between solid surfaces

Question 4

What is fluid friction?

Answer 4

known as drag, fluid resistance or air resistance

Question 5

What three things affect fluid friction?

Answer 5

• the force depends on the thickness (or viscosity) of the fluid
• The force increases as speed increases
• the force depends on the shape of the object moving through it (larger the area, greater the force)

Question 6

What are three things to remember about friction?

Answer 6

• they always act in the opposite direction to the motion of the object
• they can never speed things up or start something moving
• they convert kinetic energy into heat

Question 7

What is lift?

Answer 7

An upwards force on an object moving through a fluid.
the force acts perpendicular to the direction in which the fluid is flowing

Question 8

when does lift happen?

Answer 8

when the shape of an object causes the fluid flowing over it to change direction

Question 9

When does terminal speed/velocity happen?

Answer 9

When frictional forces equal the driving force

Question 10

When will an object reach terminal velocity?

Answer 10

if there is a constant driving force, and a frictional or drag force that increases with speed.

Question 11

What are the three main stages of reaching terminal velocity?
(car example)

Answer 11

1. Car accelerates from rest using a constant driving force
2. As the speed increases, the resistance forces increase. This reduces the resultant force on the car and hence reduces its acceleration
3. Eventually, the car reaches a speed at which the resistance forces are equal to the driving force. There is now no resultant force and no acceleration, so the car carries on at a constant speed

Question 12

What are the two ways of increasing a vehicle’s maximum speed?

Answer 12

• increasing the driving force (eg by increasing engine size)
• reducing frictional force (eg. by making body more streamlined)

Question 13

What does the velocity-time graph look like for the terminal velocity situation?

Answer 13

increases exponentially then levels off

Question 14

What does the acceleration-time graph look like for the terminal velocity situation?

Answer 14

Decreases rapidly, then slow down rate of decrease then hits 0

Question 15

How would a parachutist reach terminal velocity?

Answer 15

• they accelerate until the air resistance equals his weight
• they are then travelling at terminal speed
• they then open their parachute, which immediately increases air resistance so it is now bigger than their weight
• this slows them down so their speed is low enough for the air resistance to be equal to his weight, but this time the terminal speed is small enough to survive landing