Q: A record player spins a disk-shaped record (mass $$0.150\ \mathrm{kg}$$, radius $$0.15\ \mathrm{m}) at$$ $$42\ \mathrm{rpm}. $$What torque must the player provide to keep it spinning at this rate?

$$7.4 \times 10$$^{-1}$$\ \mathrm{N\cdot m}$$

Q: After the record player is unplugged and the record stops, how much mechanical energy is lost if it took $$10$$ seconds to come to rest?

$$1.6 \times 10$$^{-1}$$\ \mathrm{J}$$

Question 1

A tennis ball of mass $$m$$ rebounds from a racquet with the same speed $$v as it $$had initially, but in the opposite direction, making an angle $$	heta$$ with the normal as shown. What is the magnitude of the momentum change of the ball?

Accepted Answer

$$2mv \cos 	heta$$

Question 2

A 2.0-kg ball traveling at 3.0 m/s hits a vertical wall and rebounds with the same speed. If the contact time with the wall is $$1.0 	imes 10$$^{-2}$$ s, what is the magnitude of the force exerted on the ball?

Accepted Answer

$$1,200\ \mathrm{N}$$

Question 3

A block of mass $$m = 3.0\ \mathrm{kg}$$, moving on a frictionless surface with a speed $$v_i = 9.0\ \mathrm{m/s}$$, makes a perfectly elastic collision with a stationary block of mass $$M. $$Just after the collision, the $$3.0\ \mathrm{kg}$$ block recoils with a speed of $$|v_f| = 3.0\ \mathrm{m/s}. $$What is the speed $$V of $$the other block?

Accepted Answer

$$6.0\ \mathrm{m/s}$$

Question 4

A firecracker explodes inside a watermelon, splitting it into two chunks, $$A$$ and $$B$$, with $$m_A = 3.0\ \mathrm{kg}$$ and $$m_B = 12\ \mathrm{kg}. If$$ $$120\ \mathrm{J} of $$energy is converted into kinetic energy, what are the speeds of the two chunks?

Accepted Answer

$$v_A = 8.0\ \mathrm{m/s},\ v_B = 2.0\ \mathrm{m/s}$$

Question 5

A CD player reads data at a constant linear speed of $$1.25\ \mathrm{m/s} at a $$radius $$R = 5.0\ \mathrm{cm}. $$What is the rotational frequency of the CD in rpm?

Accepted Answer

$$240\ \mathrm{rpm}$$

Question 6

A wheel of radius $$0.5\ \mathrm{m}$$ rolls without slipping on a horizontal surface, starting from rest with a constant angular acceleration of $$6\ \mathrm{$$rad/s^2$$}. $$What is the distance traveled by the center of the wheel in the first $$3$$ seconds?

Accepted Answer

$$14\ \mathrm{m}$$

Question 7

A forearm accelerates a $$3.6\ \mathrm{kg}$$ ball at $$7.0\ \mathrm{m/s^2$} by $$means of the triceps muscle, as shown. What is the force that must be exerted by the triceps muscle if the distance from the elbow to the ball is $$31\ \mathrm{cm}$$ and the triceps attaches $$2.5\ \mathrm{cm}$$ from the elbow?

Accepted Answer

$$310\ \mathrm{N}$$

Question 8

An L-shaped object consists of three small masses connected by light rods. How much work must be done to accelerate the object from rest to an angular speed of $$3.25\ \mathrm{rad/s}$$ about the $$x-$$axis if its moment of inertia is $$6.48\ \mathrm{kg\cdot m^2$}$$?

Accepted Answer

$$34.2\ \mathrm{J}$$

Question 9

A block of mass $$m = 4.0\ \mathrm{kg} is $$hung from a string wrapped around a frictionless pulley (solid cylinder) of radius $$0.12\ \mathrm{m}. If $$the block is falling at $$3.0\ \mathrm{m/s}$$, what is the angular speed of the pulley?

Accepted Answer

$$25\ \mathrm{rad/s}$$

Question 10

If the block in the previous problem accelerates downward at $$g/2$$ when released, what is the mass $$M of $$the pulley?

Accepted Answer

$$8.0\ \mathrm{kg}$$

Question 11

A record player spins a disk-shaped record (mass $$0.150\ \mathrm{kg}$$, radius $$0.15\ \mathrm{m}) at$$ $$42\ \mathrm{rpm}. $$What torque must the player provide to keep it spinning at this rate?

Accepted Answer

$$7.4 	imes 10$$^{-1}$$\ \mathrm{N\cdot m}$$

Question 12

After the record player is unplugged and the record stops, how much mechanical energy is lost if it took $$10$$ seconds to come to rest?

Accepted Answer

$$1.6 	imes 10$$^{-1}$$\ \mathrm{J}$$

Question 13

A dancer spins initially at $$1.5\ \mathrm{rad/s}$$ with her arms and one leg extended. When she draws her arms and legs in, her moment of inertia becomes $$0.90\ \mathrm{kg\cdot m^2$}$$ and her angular speed increases to $$4.5\ \mathrm{rad/s}. $$What was her initial moment of inertia?

Accepted Answer

$$2.7\ \mathrm{kg\cdot m^2$}$$

Question 14

How much work did the dancer do to alter her motion as described in the previous problem?

Accepted Answer

$$6.1\ \mathrm{J}$$

Question 15

A store sign of mass $$M = 20\ \mathrm{kg}$$ and length $$L = 3.0\ \mathrm{m} is $$supported by a bolt at one end and a wire at the other, making an angle $$	heta = 25^\circ$$ with the horizontal. What is the tension in the wire?

Accepted Answer

$$230\ \mathrm{N}$$

Question 16

How much vertical force does the wall provide to the store sign described above?

Accepted Answer

$$98\ \mathrm{N}$$

Question 17

A $$0.50\ \mathrm{kg}$$ block slides along a frictionless surface at $$2.0\ \mathrm{m/s}$$ and is brought to rest by compressing a spring with $$k = 800\ \mathrm{N/m}. $$What is the time between impact and maximum compression?

Accepted Answer

$$0.039\ \mathrm{s}$$

Question 18

A simple pendulum on planet X has a period of $$1.66\ \mathrm{s}$$ and a length of $$1.00\ \mathrm{m}. $$What is the acceleration due to gravity on planet X?

Accepted Answer

$$14.3\ \mathrm{m/s^2$}$$

PHY121 Exam 3 Practice – Step-by-Step Physics Guidance

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