Q: A crane lifts a $$425\ \mathrm{kg}$$ steel beam vertically a distance of $$12.0\ \mathrm{m}. $$How much work does the crane do on the beam if the beam accelerates upward at $$1.8\ \mathrm{m/s^2$}$$? Neglect frictional forces.

$$4.9 \times 10^5$\ \mathrm{J}$$

Question 1

Two men, Joel and Jerry, push against a wall. Jerry stops after 10 min, while Joel is able to push for 5.0 min longer. Compare the work they do.

Accepted Answer

Neither of them does any work.

Question 2

Consider a plot of the displacement $$x as a $$function of the applied force $$F$$ for an ideal elastic spring. What does the slope of the curve represent?

Accepted Answer

the reciprocal of the spring constant

Question 3

A 4.0-kg object is moving with speed 2.0 m/s. A 1.0-kg object is moving with speed 4.0 m/s. Both objects encounter the same constant braking force and are brought to rest. Which object travels the greater distance before stopping?

Accepted Answer

the 1.0-kg object

Question 4

Two objects, one of mass $$m$$ and the other of mass $$2m$$, are dropped from the top of a building. When they hit the ground, which statement is correct?

Accepted Answer

the heavier one will have twice the kinetic energy of the lighter one.

Question 5

Is it possible for a system to have negative potential energy?

Accepted Answer

Yes, since the choice of the zero of potential energy is arbitrary.

Question 6

You carry a 7.0 kg bag of groceries above the level floor at a constant velocity of 0.75 m/s across a room that is 9.0 m wide. How much work do you do on the bag in the process?

Accepted Answer

0.0 J

Question 7

Refer to the force vs. position graph provided. What is the work done as the object moves from $$x = 4\ \mathrm{m} to$$ $$x = 6\ \mathrm{m}$$?

Accepted Answer

$$20\ \mathrm{J}$$

Question 8

A force $$\vec{F} = 12\ \mathrm{N}\,\hat{i} - 10\ \mathrm{N}\,\hat{j}$$ acts on an object. How much work does this force do as the object moves from the origin to the point $$(2.0\ \mathrm{m}, 2.0\ \mathrm{m})$$?

Accepted Answer

$$46\ \mathrm{J}$$

Question 9

Refer to the box and force diagram (second image). A constant external force $$P = 160\ \mathrm{N} is $$applied to a $$20.0\ \mathrm{kg}$$ box, which is on a rough horizontal surface. While the force pushes the box a distance of $$8.00\ \mathrm{m}$$, the speed changes from $$0.500\ \mathrm{m/s} to$$ $$2.60\ \mathrm{m/s}. $$The work done by friction during this process is closest to:

Accepted Answer

$$-1170\ \mathrm{J}$$

Question 10

A traveler pulls on a suitcase strap at an angle $$36^\circ$$ above the horizontal. If $$900\ \mathrm{J} of $$work are done by the strap while moving the suitcase a horizontal distance of $$15\ \mathrm{m}$$, what is the tension in the strap?

Accepted Answer

$$61\ \mathrm{N}$$

Question 11

A crane lifts a $$425\ \mathrm{kg}$$ steel beam vertically a distance of $$12.0\ \mathrm{m}. $$How much work does the crane do on the beam if the beam accelerates upward at $$1.8\ \mathrm{m/s^2$}$$? Neglect frictional forces.

Accepted Answer

$$4.9 	imes 10^5$\ \mathrm{J}$$

Question 12

A student slides her $$80.0\ \mathrm{kg}$$ desk across the level floor of her dormitory room a distance $$4.00\ \mathrm{m} at $$constant speed. If the coefficient of kinetic friction between the desk and the floor is $$0.400$$, how much work did she do?

Accepted Answer

$$1.26\ \mathrm{kJ}$$

Question 13

A spring stretches by $$0.20\ \mathrm{m}$$ when a $$50.0\ \mathrm{N}$$ object is attached. What is the weight of a fish that would stretch the spring by $$0.40\ \mathrm{m}$$?

Accepted Answer

$$91.0\ \mathrm{N}$$

Question 14

An object attached to an ideal massless spring is pulled across a frictionless surface. If the spring constant is $$45\ \mathrm{N/m}$$ and the spring is stretched by $$0.88\ \mathrm{m}$$ when the object is accelerating at $$2.3\ \mathrm{m/s^2$}$$, what is the mass of the object?

Accepted Answer

$$17\ \mathrm{kg}$$

Question 15

It requires $$49\ \mathrm{J} of $$work to stretch an ideal very light spring from a length of $$1.4\ \mathrm{m} to a $$length of $$2.9\ \mathrm{m}. $$What is the value of the spring constant of this spring?

Accepted Answer

$$44\ \mathrm{N/m}$$

Question 16

A force $$F = bx^3$ acts in the x$ direction, where the value of b$ is 3.7$$\ \mathrm{$$N/m^3$$}$$. How much work is done by this force in moving an object from x = 1.0$$\ \mathrm{m}$$ to x = 2.0$$\ \mathrm{m}$$?

Accepted Answer

$$42\ \mathrm{J}$$

Question 17

A $$1000.0\ \mathrm{kg}$$ car is moving at $$30.0\ \mathrm{km/h}. If a $$truck has $$18$$ times the kinetic energy of the car, how fast is the truck moving?

Accepted Answer

$$63\ \mathrm{km/h}$$

Question 18

How much energy is needed to change the speed of a $$1600\ \mathrm{kg}$$ sport utility vehicle from $$15.0\ \mathrm{m/s} to$$ $$40.0\ \mathrm{m/s}$$?

Accepted Answer

$$1.10\ \mathrm{MJ}$$

Question 19

A worker lifts a $$20.0\ \mathrm{kg}$$ bucket of concrete from the ground up to the top of a $$20.0\ \mathrm{m}$$ tall building. The bucket is initially at rest, but is traveling at $$4.0\ \mathrm{m/s}$$ when it reaches the top of the building. What is the minimum amount of work that the worker did in lifting the bucket?

Accepted Answer

$$3.92\ \mathrm{kJ}$$

Question 20

Refer to the inclined plane diagram (third image). An $$8.0\ \mathrm{kg}$$ block is released from rest ($$v_1 = 0.00$$\ \mathrm{m/s}$$) on a rough incline. The block moves a distance of 1.6$$\ \mathrm{m}$$ down the incline, in a time interval of 0.80$$\ \mathrm{s}$$, and acquires a velocity of v_2$ = 4.0\ \mathrm{m/s}. $$How much work does gravity do on the block during this process?

Accepted Answer

$$+120\ \mathrm{J}$$

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