Question 1

An apple weighing 3 newtons hangs, motionless, from a tree branch. What are the forces by the environment that act on this apple?

Accepted Answer

The apple is motionless, so the net force on it must be zero. The only two forces acting on the apple are its weight (downward) and the upward lift of the branch. These two forces must be equal in magnitude and opposite in direction in order for the apple to be motionless.

Question 2

Marcia places a lead block and an iron block on the table, and hits both of them with the same force. The lead block is twice as massive as the iron block. The acceleration of the lead block is

Accepted Answer

According to Newton's second law, acceleration is inversely proportional to mass when the same force is applied. Since the lead block is twice as massive, its acceleration will be half that of the iron block.

Question 3

A "force" could be best described as

Accepted Answer

A force is an interaction that, when unopposed, will change the motion of an object, typically causing it to accelerate.

Question 4

When you pull an object, such as a table, across the floor, the forward force by you on the table is equal but opposite to the backward force by the table on you. It seems as though these two forces should balance out, so that the table can't be moved. But the table does move, because

Accepted Answer

The force you apply acts on the table, while the equal and opposite force acts on you, not the table. Therefore, these forces do not cancel each other out on the table.

Question 5

An apple weighing 3 newtons hangs, motionless, from a tree branch. The net force on the apple is

Accepted Answer

The apple is motionless, meaning it is in equilibrium. The net force on the apple is zero because the gravitational force downward is balanced by the tension force upward from the branch.

Question 6

The basic physical principal principle that explains the operation of rocket engines is

Accepted Answer

Rocket engines operate based on Newton's third law of motion, which is the law of action and reaction (force pairs). When a rocket expels gas out of its engine, it experiences an equal and opposite force that propels it forward.

Question 7

The "drive force" that pushes a car forward is

Accepted Answer

The driving force that propels a car forward is exerted by the road pushing forward against the tires due to friction.

Question 8

You would be richest if your hunk of gold had a mass of one kilogram

Accepted Answer

The value of gold is determined by its mass and market price, not by its location.

Question 9

An auto moves at a steady 50 mph down a straight level road. The net force on the auto is

Accepted Answer

The net force is zero because the car is moving at a constant speed in a straight line, indicating that all forces are balanced.

Question 10

Can two pool balls, both of them moving, have a total momentum of zero?

Accepted Answer

If two pool balls are moving in opposite directions with the same speed and have equal mass, their momenta are equal in magnitude but opposite in direction, resulting in a total momentum of zero.

Question 11

A car weighing 5000 N is traveling along a straight level road powered by a forward drive force of 500 N. The force of air resistance is 200 N, and rolling resistance is 100 N. The net force on the car is

Accepted Answer

The net force is calculated by subtracting the resistive forces (air resistance and rolling resistance) from the forward drive force: 500 N - 200 N - 100 N = 200 N forward.

Question 12

Which of the following principles explains why guns recoil?

Accepted Answer

The recoil of a gun is explained by the conservation of momentum, which states that the total momentum of a closed system remains constant. When a bullet is fired, the gun and bullet exert equal and opposite forces on each other, resulting in the gun's recoil.

Question 13

A one kilogram grapefruit weighs ten newtons. Suppose we take this grapefruit to the moon. The force of gravity on the grapefruit will then be

Accepted Answer

The force of gravity on the moon is about 1/6th of that on Earth, so the grapefruit will weigh less than ten newtons on the moon.

Question 14

It's harder to stop or turn a moving supertanker than a small speedboat because

Accepted Answer

The supertanker has a larger momentum due to its much greater mass, making it harder to stop or turn compared to a small speedboat.

Question 15

A certain grapefruit on Earth has a mass of one kilogram and a weight of one newton. When the grapefruit is on the moon its mass is

Accepted Answer

Mass is a measure of the amount of matter in an object and does not change with location. Therefore, the grapefruit's mass remains one kilogram on the moon.

Question 16

One newton is about equal to

Accepted Answer

One newton is the force required to accelerate a one-kilogram mass at a rate of one meter per second squared. So, one newton is about equal to one-fourth pound.

Question 17

Suppose that you are isolated and "floating" in outer space and that you hold a wooden block and a lead block, one in each hand, and that the two blocks look and feel identical. How could you tell which one is the lead block?

Accepted Answer

The lead block has more mass, so when you tap it, it will accelerate less compared to the wooden block due to its greater inertia.

Question 18

An auto weighing 3000 newtons has a forward drive force of 1000 newtons. Air resistance on the car is 100 newtons, and road resistance is 100 newtons. The auto moves along a straight level road. The net force on this auto is

Accepted Answer

The net force is calculated by subtracting the total resistance forces (air resistance + road resistance) from the forward drive force: 1000 N - (100 N + 100 N) = 800 N forward.

Question 19

Mary pushes horizontally on a large, heavy table which is standing alone in the middle of a room, and notes that the table does not move despite the fact that she is pushing on it (see the figure). Why doesn't the table move?

Accepted Answer

The answer of  Mary pushes horizontally on a large,...

Question 20

A car weighing 5000 N is traveling along a straight level road powered by a forward drive force of 500 N. The force of air resistance is 200 N, and rolling resistance is 100 N. The car's acceleration is

Accepted Answer

The net force acting on the car is the forward drive force minus the sum of air resistance and rolling resistance (500 N - 200 N - 100 N = 200 N). Since the net force is positive and in the forward direction, the car's acceleration is also in the forward direction.

Quiz 4: Why Things Move As They Do