Q: A centrifuge reaches a maximum angular speed of $$1500\ \text{rad/s} in$$ $$5\ \text{s}$$, starting from rest. What is the angular acceleration required, assuming it is constant?

$$\alpha = 300\ \text{rad/s}^2$$

Q: An airplane is flying horizontally at $$50\ \text{m/s} at an $$altitude of $$500\ \text{m}. $$You jump from the plane and eventually land at $$10\ \text{m/s} ($$after opening your parachute). If your mass is $$80\ \text{kg}$$, what is the total work done by air drag during your fall?

$$W = 80 \times (\dfrac{1}{2} (10^2$ - 50^2$) + 9.8 \times 500)$$

Q: A centrifuge of radius $$8\ \text{cm}$$ reaches $$1500\ \text{rad/s} in$$ $$5\ \text{s}$$ from rest. How many revolutions does it make while spinning up?

$$\text{Revolutions} = \dfrac{1}{2} \alpha t^2$ / (2\pi)$$, with $$\alpha = 300\ \text{rad/s}^2$$

Q: A centrifuge reaches $$1500\ \text{rad/s} in$$ $$5\ \text{s}$$ from rest. What is the total angular displacement during this time?

$$\theta = \dfrac{1}{2} \alpha t^2$

Question 1

A centrifuge reaches a maximum angular speed of $$1500\ 	ext{rad/s} in$$ $$5\ 	ext{s}$$, starting from rest. What is the angular acceleration required, assuming it is constant?

Accepted Answer

$$\alpha = 300\ 	ext{rad/s}^2$$

Question 2

A bug of mass $$m$$ sits at a radius $$r on a $$rotating record player moving at speed $$v. $$What is the minimum coefficient of static friction $$\mu_s$$ required to prevent the bug from slipping?

Accepted Answer

$$\mu_s = \dfrac{v^2$}{r g}$$

Question 3

A uniform board of mass $$M$$ and length $$L is $$supported at its ends. A bear of mass $$m$$ sits at a distance $$\dfrac{L}{4}$$ from the left end. What is the normal force exerted on the board by the left support (support 1)?

Accepted Answer

$$N_1$$ = \dfrac{3 M g}{4} + \dfrac{3 m g}{4}$$

Question 4

An airplane is flying horizontally at $$50\ 	ext{m/s} at an $$altitude of $$500\ 	ext{m}. $$You jump from the plane and eventually land at $$10\ 	ext{m/s} ($$after opening your parachute). If your mass is $$80\ 	ext{kg}$$, what is the total work done by air drag during your fall?

Accepted Answer

$$W = 80 	imes (\dfrac{1}{2} (10^2$ - 50^2$) + 9.8 	imes 500)$$

Question 5

A roller coaster starts from rest at point A (top left of a track). At which of the marked locations (A, B, C, D, E) could the roller coaster be at some time later? (Select all that apply.)

Accepted Answer

B, C, D, E

Question 6

Three forces of equal magnitude are exerted on a door as shown: $$F_1$$ acts at the middle, $$F_2$$ and $$F_3 at $$the end, with $$F_3 at$$ $$45^\circ. If $$counterclockwise torques are positive, which correctly orders the torques?

Accepted Answer

torque 1 \u003c torque 3 \u003c torque 2

Question 7

A parachutist falls from rest off a cliff with an unopened parachute. Before opening the parachute, which statement is true?

Accepted Answer

At terminal velocity, the largest upward force on you is drag force.

Question 8

A kite is used to pull Harold up a slope. The wind blows horizontally at $$10\ 	ext{m/s}$$, air density is $$1.2\ 	ext{kg/m}^3$$, $$C_d = 0.4$$, Harold's mass is $$58\ 	ext{kg}$$, and $$\mu_k = 0.2. $$What is the magnitude of the drag force on the kite?

Accepted Answer

$$F_d = \dfrac{1}{2} \rho v^2$ C_d A$$

Question 9

A centrifuge of radius $$8\ 	ext{cm}$$ reaches $$1500\ 	ext{rad/s} in$$ $$5\ 	ext{s}$$ from rest. How many revolutions does it make while spinning up?

Accepted Answer

$$	ext{Revolutions} = \dfrac{1}{2} \alpha t^2$ / (2\pi)$$, with $$\alpha = 300\ 	ext{rad/s}^2$$

Question 10

A bug of mass $$m$$ sits at radius $$r on a $$rotating record player. What is the direction of the friction force acting on the bug?

Accepted Answer

Radially inward

Question 11

A uniform board of mass $$M$$ and length $$L is $$supported at its ends. A bear of mass $$m$$ sits at a distance $$\dfrac{L}{4}$$ from the left end. What is the normal force exerted on the board by the right support (support 2)?

Accepted Answer

Question 12

A parachutist falls from rest off a cliff. As you fall, but before reaching terminal velocity, what happens to the magnitude of your acceleration?

Accepted Answer

It decreases

Question 13

A roller coaster starts from rest at point A. Which of the following is true about its speed at point C compared to point B, assuming no friction?

Accepted Answer

Speed at C is less than at B

Question 14

A kite pulls Harold up a slope. If the coefficient of kinetic friction between Harold and the ground is $$\mu_k = 0.2$$, and Harold's mass is $$58\ 	ext{kg}$$, what is the frictional force acting on Harold?

Accepted Answer

$$F_f = \mu_k m g$$

Question 15

A centrifuge reaches $$1500\ 	ext{rad/s} in$$ $$5\ 	ext{s}$$ from rest. What is the total angular displacement during this time?

Accepted Answer

$$	heta = \dfrac{1}{2} \alpha t^2$

Question 16

A bug of mass $$m$$ sits at radius $$r on a $$rotating record player. What is the centripetal force acting on the bug?

Accepted Answer

$$F_c = m v^2$ / r$$

Question 17

A parachutist falls from rest off a cliff. After you reach terminal velocity, you open your parachute and your effective cross-sectional area increases. What happens to the magnitude of the drag force?

Accepted Answer

It increases

Midterm practice 2

Study Guide - Practice Questions

Study Guide - Flashcards