Question 1

A man lifts a 20.0-kg bucket of concrete from the ground up to the top of a 20.0-m tall building. The bucket is initially at rest, but is traveling at 4.0 m/s when it reaches the top of the building. How much work was done by the man in lifting the bucket?

Accepted Answer

The work done by the man can be calculated using the work-energy principle, which states that the work done on an object is equal to its change in kinetic energy. The initial kinetic energy of the bucket is zero, so the work done by the man is equal to the change in potential energy of the bucket:

W = mgh

where W is the work done, m is the mass of the bucket (20.0 kg), g is the acceleration due to gravity (9.81 m/s^2), and h is the height moved (20.0 m). Plugging in the numbers:

W = (20.0 kg)(9.81 m/s^2)(20.0 m) = 3,920 J = 3.92 kJ

Therefore, the best answer choice is D.

Question 2

A ball is thrown upward with a speed and direction such that it reaches a maximum height of 16.0 m above the point it was released. At its maximum height it has a speed of 18.0 m/s. With what speed was the ball released?

Accepted Answer

At the maximum height, the ball's velocity is zero. So, using the equation of motion, v^2 = u^2 + 2as,we have,0 = u^2 - 2(9.8)(16)=> u^2 = 313.6=> u = 17.7 m/s.

Question 3

What is the horsepower needed to change the speed of a 1600-kg sport utility vehicle from 15.0 m/s to 40.0 m/s in 4.00 seconds?

Accepted Answer

The formula to calculate horsepower is:

Horsepower = (Force x Distance) / Time

We need to first calculate the force required to change the speed of the SUV:

Force = (mass x change in velocity) / time

∆v = 40 - 15 = 25 m/s

Force = (1600 kg x 25 m/s) / 4 s

Force = 10,000 N

Now we can calculate the horsepower:

Horsepower = (Force x Distance) / Time

We don't have the distance, but we can assume it's the average distance traveled during acceleration, which is:

Distance = (initial velocity + final velocity) / 2 x time

Distance = ((15 m/s + 40 m/s) / 2) x 4 s

Distance = 100 m

Horsepower = (10,000 N x 100 m) / 4 s

Horsepower = 250,000 J/s / 746 W/hp

Horsepower = 334.7 hp

Therefore, the answer is closest to choice A (369 hp).

Question 4

The ratio of power outputs of Jack and Jill is 2: 1 to complete a certain amount of work. If Jill takes 4 minutes to complete this work, how long will Jack take to finish the same work?

Accepted Answer

Since the power output ratio of Jack to Jill is 2:1, Jack is twice as powerful as Jill. Therefore, Jack will take half the time Jill takes to complete the work. Jill takes 4 minutes, so Jack will take 4/2 = 2 minutes.

Question 5

A car starts from rest and after 6.0 seconds it has a kinetic energy of 60,000 J. How much horsepower is needed for the car to acquire this kinetic energy?

Accepted Answer

Horsepower is a unit of power, and power is the rate of energy transfer. The car's kinetic energy is 60,000 J, and it takes 6 seconds to reach this energy. Power = Energy/Time = 60,000 J / 6 s = 10,000 W. 1 horsepower is approximately 746 W, so 10,000 W is about 13.4 hp, which rounds to 13 hp.

Question 6

A 20-N horizontal force pushes an object along a rough floor so that the object moves with a constant velocity of 6.0 m/s. How much power does this force deliver to the object?

Accepted Answer

The answer of A 20-N horizontal force pushes an object...

Question 7

A man drags a 130-kg object down an inclined plane at a speed of 1.40 m/s. The coefficient of friction between the object and the inclined plane is 0.800 and the angle of inclination with the horizontal is 23.0°. What is the amount of power this man expends in dragging this object? Use g = 10.0 m/s².

Accepted Answer

The answer of A man drags a 130-kg object down...

Question 8

An ordinary light bulb consumes 60 W of power. Through what height could a 1000-N object be lifted expending the same amount of power for 6 hours?

Accepted Answer

Power = F * v = m * g * h/t60 = 1000 * 9.8 * h/6 * 3600h = 1300 m

Question 9

What is the power needed to change the speed of a 1600-kg sport utility vehicle from 15.0 m/s to 40.0 m/s in 4.00 seconds?

Accepted Answer

The answer of What is the power needed to change...

Question 10

A car accelerates from rest to a speed of 19.0 m/s in 6.00 seconds. If the car weighs 16,000 N, what average power must the motor produce to cause this acceleration? Use g = 10 m/s².

Accepted Answer

The work done on the car to give it kinetic energy is:

W = (1/2)mv^2 = (1/2)(16000 kg)(19 m/s)^2 = 2883200 J

The power output of the motor is the work done divided by the time:

P = W/t = 2883200 J / 6 s = 480533.33 W

However, this is the instantaneous power, and we are asked for the average power over the entire acceleration.

The average power is the total work done divided by the time:

P_avg = W/t = 2883200 J / 6 s = 480533.33 W

Therefore, the motor must produce an average power of 49.1 kW. The correct answer is C.

Question 11

A 4.00-kg mass is attached to a spring hanging vertically and hangs at rest in the equilibrium position. The spring constant of the spring is 1.00 N/cm. The mass is pulled downward 2.00 cm and released. What is the speed of the mass when it is 1.00 cm above the point from which it was released?

Accepted Answer

The answer of A 4.00-kg mass is attached to a...

Question 12

A 220-kg object is lifted at a constant speed through a height of 42 m by a crane. If the crane takes a total time of 1.8 seconds to lift this object, what is the average power required? Use g = 10 m/s².

Accepted Answer

The answer of A 220-kg object is lifted at a...

Quiz 7: Work and Kinetic Energy