Multiple-Choice Questions

Animals

Falling Cats

Falling Cats Question 1 Which is the correct free body diagram for a cat immediately after falling off of a cliff?

b) a) c)

Falling Cats

b) a) c)

Force of Gravity is constant.

D= ¼ A v2, so D increases as v increases

Which is the correct free body diagram for a cat immediately after falling off of a cliff?

Question 1 Solution

Falling Cats

Which is the correct free body diagram for a cat that has reached terminal velocity ?

Question 2

b) a) c)

Falling Cats

b) a) c)

For v = vterm, D = Fg

Question 2 Solution Which is the correct free body diagram for a cat that has reached terminal velocity ?

Falling Cats Question 3 An object with smaller cross sectional area A has a larger terminal velocity than an object with larger A.

a) always true

b) sometimes true

c) never true

Falling Cats

An object with smaller cross sectional area A has a larger terminal velocity than an object with larger A.

a) always true

b) sometimes true

c) never true

Need to compare the ratio (m/A), not just A alone. (Note: It will also depend on how aerodynamic the objects are in case there is a big difference in their drag coefficients.)

€

vterm =4mgρA

Question 3 Solution

Falling Cats Question 4 Which of the following describes what happens to a meteor crashing through the atmosphere travelling faster than its terminal velocity?

a) The meteor decelerates

b) The meteor stays at constant v

c) The meteor accelerates

Falling Cats

Which of the following describes what happens to a meteor crashing through the atmosphere travelling faster than its terminal velocity?

a) The meteor decelerates

b) The meteor stays at constant v

c) The meteor accelerates When v > vterm , the drag force is larger than the gravitational force meaning the net force is opposite to the motion causing deceleration until v = vterm. (Note: The work done by the drag force is converted into heat, which usually causes meteorites to burn up in the atmosphere.)

Question 4 Solution

Falling Cats

vair vbike

Question 5

a) D = ¼ ρ A (vbike)2

c) D = ¼ ρ A (vbike+ vair)2

b) D = ¼ ρ A (vbike-vair)2

The magnitude of the drag on a cyclist encountering a head wind is given by

Falling Cats

a) D = ¼ ρ A (vbike)2

c) D = ¼ ρ A (vbike+ vair)2

b) D = ¼ ρ A (vbike-vair)2

D = ¼ ρ Av2

where v is the speed of the object relative to the air

The magnitude of the drag on a cyclist encountering a head wind is given by

vair vbike

Question 5 Solution

Falling Cats

c) D = ¼ ρ A (vbike+ vair)2

b) D = ¼ ρ A (vbike-vair)2

The magnitude of the drag on a cyclist encountering a tail wind is given by

vbike vair

a) D = ¼ ρ A (vbike)2

Question 6

Falling Cats

a) D = ¼ ρ A (vbike)2

c) D = ¼ ρ A (vbike+ vair)2

b) D = ¼ ρ A (vbike-vair)2

D = ¼ ρ Av2

where v is the speed of the object relative to the air. (Note: If vair > vobj, the drag force changes direction and the cyclist will be pushed by the wind.)

The magnitude of the drag on a cyclist encountering a tail wind is given by

vbike vair

Question 6 Solution