4.12 Drag, Lift and Terminal Speed

Question 1

What is friction?

Answer 1

Friction is a force that opposes motion

Question 2

What are the two main types of friction?

Answer 2

1. Contact friction
2. Fluid friction

Question 3

When does contact friction happen?

Answer 3

Contact friction happens between solid surfaces

Question 4

When is fluid friction known as?

Answer 4

Fluid friction is known as drag or air resistance

Question 5

What three things affect fluid friction?

Answer 5

1. The force depends on the thickness/viscosity of the fluid
2. The force increases as the speed increases
3. The force depends on the shape of the object moving through it - the larger the area pushing against the fluid, the greater the resistance force

Question 6

What are the three things you need to remember about frictional forces?

Answer 6

1. They always act in the opposite direction to the motion of the object
2. They can never speed things up or make something move
3. They convert kinetic energy into heat

Question 7

What is lift?

Answer 7

An upwards force on an object moving through a fluid

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

At what direction does lift act?

Answer 9

Lift acts perpendicular to the direction in which the fluid is flowing

Question 10

What is terminal speed?

Answer 10

Terminal speed happens when frictional forces equal the driving force

Question 11

What does an object need to reach terminal speed?

Answer 11

It there is a driving force that stays the same all the time, and a frictional force that increases with speed

Question 12

How do you increase a vehicles maximum speed?

Answer 12

• increase the driving force
• reduce the frictional force (e.g. by making the body more streamlined)

Question 13

What do the velocity-time and acceleration time graphs look like for terminal velocity?

Answer 13

Velocity-time: sharp increase then bottoms out as object reaches terminal velocity
Acceleration-time: decrease until it reaches zero (at terminal velocity)

Question 14

How would you set up an experiment to calculate the terminal velocity of a ball bearing in a viscous liquid?

Answer 14

• put elastic bands around the tube of viscous liquid at fixed distances using a ruler, then drop a ball bearing into the tube and use a stopwatch to record the time at which it reaches each band
• repeat and average to reduce the effect of random errors (use a strong magnet to remove the ball bearing from the tube)
• calculate the time taken for the ball bearing to travel between consecutive elastic bands and calculate an average for each reading
• use the average times and the distance between bands to calculate the average velocity between each pair of elastic bands

Question 15

How do parachutists survive falling through air?

Answer 15

• a skydiver leaves a plane and will accelerate until the air resistance equals his weight
• they are travelling at terminal speed, but this terminal speed is too high
• they open their parachute, which increase the air resistance so it is now bigger than their weight
• this slows them down until their speed has dropped enough for the air resistance to equal their weight again. This terminal speed is small enough to survive landing

Question 16

What does the velocity-time graph for a parachutist look like?

Answer 16

• velocity increases at first until it flattens out (terminal velocity reached)
• parachute is opened and velocity decreases sharply
• line is lower and flat as terminal velocity is reached