Question 1

How many Joules of energy are in one kilowatt-hour?

Accepted Answer

One kilowatt-hour is equivalent to 3.6 million Joules. This is calculated by multiplying the number of watts in a kilowatt (1,000 watts) by the number of seconds in an hour (3,600 seconds), giving 3,600,000 Joules or 3.6 million Joules.

Question 2

A large metal ball is shot from a cannon with a short barrel. If the same ball were to be shot from cannon with a longer barrel, its muzzle velocity would be

Accepted Answer

The longer barrel allows the expanding gases from the propellant to act on the ball for a longer period of time, increasing its acceleration and resulting in a higher muzzle velocity.

Question 3

On a sunny day about 500 watts of solar power are received by each of the four solar cells on the roof of a solar-powered automobile.. If the solar cells were 100 % efficient, they would deliver

Accepted Answer

1 horsepower (hp) is equivalent to approximately 746 watts. If each of the four solar cells receives 500 watts, the total power received is 2000 watts. Dividing 2000 watts by 746 watts per hp gives approximately 2.7 hp.

Question 4

The power required to exert 1 N force, over a distance of 1 m in 1 second is

Accepted Answer

The power required is calculated using the formula Power = Work / Time. Work is calculated as Force x Distance. Here, Work = 1 N x 1 m = 1 Joule, and Time = 1 second. Therefore, Power = 1 Joule / 1 second = 1 Watt.

Question 5

If an object is raised twice as high, its potential energy will be

Accepted Answer

Potential energy is directly proportional to height, so if the height is doubled, the potential energy will also be doubled.

Question 6

A 1-kg chunk of putty moving at 1 m/s collides with and sticks to a 5-kg bowling ball initially at rest. The bowling ball and putty then move with a momentum of

Accepted Answer

The answer of A 1-kg chunk of putty moving at...

Question 7

Compared to the recoiling rifle, the bullet fired has

Accepted Answer

The bullet has a greater kinetic energy because, although the momentum of the bullet and rifle are equal and opposite (conservation of momentum), the bullet's smaller mass and much higher speed result in a greater kinetic energy.

Question 8

When you jump from an elevated position you usually bend your knees upon reaching the ground. By doing this, you make the time of the impact about 10 times as great as for a stiff-legged landing. In this way the average force your body experiences is

Accepted Answer

The force experienced by the body during impact is inversely proportional to the time over which the impact occurs. By increasing the time of impact (by bending the knees), the average force experienced is reduced. According to the impulse-momentum theorem, $F \Delta t = \Delta p$, where $F$ is the average force, $\Delta t$ is the time over which the force acts, and $\Delta p$ is the change in momentum. Increasing $\Delta t$ decreases $F$, making it about 1/10 as great for a 10 times increase in impact time.

Question 9

According to the impulse-momentum equation Ft = change in (mv), a person will suffer less injury falling on a wooden floor which "gives" than on a more rigid cement floor. The "F" in the above equation stands for the force exerted on the

Accepted Answer

The impulse-momentum equation Ft = Δ(mv) involves the force exerted during the interaction, which can be considered as the force on the person by the floor or the force on the floor by the person due to Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. Thus, the force can be viewed from either perspective.

Question 10

A ball rolling down an incline has its maximum kinetic energy at

Accepted Answer

The kinetic energy of a ball rolling down an incline is maximum at the bottom because it has converted the maximum amount of its potential energy into kinetic energy by that point.

Question 11

You're driving down the highway and a bug spatters into your windshield. Which undergoes the greater change in momentum during the time of contact?

Accepted Answer

The law of conservation of momentum states that in the absence of external forces, the total momentum of a system remains constant. When the bug and the car collide, they exert equal and opposite forces on each other for the same amount of time, leading to changes in momentum of equal magnitude but in opposite directions.

Question 12

A ball is projected into the air with 100 J of kinetic energy which is transformed to gravitational potential energy at the top of its trajectory. When it returns to its original level after encountering air resistance, its kinetic energy is

Accepted Answer

Due to air resistance, some of the ball's energy is converted into other forms, such as heat, resulting in less kinetic energy when it returns to its original level compared to when it was projected.

Question 13

A bow is drawn so that it has 40 J of potential energy. When fired, the arrow will ideally have a kinetic energy that is

Accepted Answer

In an ideal situation, without any energy losses due to factors like air resistance or friction, the potential energy stored in the drawn bow (40 J) would be completely converted into kinetic energy of the arrow upon release.

Question 14

A 2500-N pile-driver ram falls 10 m and drives a post 0.1 m into the ground. The average impact force on the ram is

Accepted Answer

The average impact force can be calculated using the work-energy principle. The work done by the impact force is equal to the change in kinetic energy. Initially, the ram has potential energy (PE = mgh) which is converted into kinetic energy (KE) just before impact. At impact, this energy is used to do work in driving the post into the ground. The work done (W) is equal to the force (F) times the distance (d), so W = Fd. Given that the ram falls 10 m, the potential energy converted into work is PE = mgh = 2500 N * 10 m = 25,000 J. The work done to drive the post 0.1 m into the ground is thus 25,000 J, and since W = Fd, we have 25,000 = F * 0.1 m, solving for F gives F = 250,000 N.

Question 15

A ball rolling down an incline has its minimum speed

Accepted Answer

The ball has its minimum speed near the top of the incline because it starts from rest and gains speed as it rolls down due to gravitational acceleration.

Question 16

Two billiard balls having the same mass and speed roll toward each other. What is their combined momentum after they meet ?

Accepted Answer

Since momentum is a vector quantity and the balls have equal mass and speed but are moving in opposite directions, their momentums cancel each other out, resulting in a combined momentum of 0 after they meet.

Question 17

A block of ice sliding down an incline has its maximum speed at

Accepted Answer

The block of ice will have its maximum speed at the bottom of the incline because as it slides down, it accelerates due to gravity, converting potential energy into kinetic energy, reaching maximum speed at the lowest point.

Question 18

Two pool balls, each moving at 2 m/s, roll toward each other and collide. Suppose after bouncing apart, each moves at 4 m/s. This collision violates conservation of

Accepted Answer

The collision violates conservation of energy because the total kinetic energy of the system increases, which is not possible in an elastic collision without an external force acting on the system. Conservation of momentum would still hold as the momentum before and after the collision can be equal and opposite, but the increase in speed (and thus kinetic energy) cannot happen without an external energy input.

Question 19

A car's engine is 20% efficient. When cruising, the car encounters an average retarding force of 1000 N. If the energy content of gasoline is 40 megajoules per liter, how many kilometers per liter does
The car get?

Accepted Answer

The car's engine efficiency is 20%, meaning only 20% of the energy from gasoline is used for work. The work done against the retarding force is force times distance. With 40 MJ/L of energy in gasoline, only 8 MJ/L is used for work (20% of 40 MJ). The distance per liter is 8,000,000 J divided by 1,000 N, which equals 8,000 meters or 8 kilometers.

Question 20

A motorcycle with a mass of 100 kilograms, slowly rolls over the edge of a cliff and falls for three seconds before it reaches the bottom of a gully. Its momentum ( in units of kilogram meters per second) just before it hit bottom and became burned rubber and scrap metal was

Accepted Answer

The motorcycle's momentum just before hitting the bottom can be calculated using the formula for momentum, $p = mv$, where $m$ is the mass and $v$ is the velocity. To find the velocity, we use the formula for free fall, $v = gt$, where $g$ is the acceleration due to gravity (approximately $9.8 \, \text{m/s}^2$) and $t$ is the time. Substituting $g = 9.8 \, \text{m/s}^2$ and $t = 3 \, \text{s}$, we get $v = 9.8 \times 3 = 29.4 \, \text{m/s}$. The mass of the motorcycle is $100 \, \text{kg}$, so the momentum is $100 \times 29.4 = 2940 \, \text{kg m/s}$, which rounds to $3000 \, \text{kg m/s}$, making option C correct.

Quiz 5: Momentum and Energy