Question 1

For the law of gravity equation, F = Gm₁m₂/r², to be dimensionally consistent, what must be the units of universal gravitational constant?

Accepted Answer

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Question 2

Using data from Table 3.3, calculate the ratio between the electrostatic force and the gravitational force between the proton and the electron in a hydrogen atom, if the distance between the electron and the proton is r = 5.3 × 10⁹ m. The universal gravitational constant G = 6.67 × 10^-11 m³ kg^-1 s^-2, and Coulomb's constant k = 9.0 × 10⁹ N m²/C².

Accepted Answer

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Question 3

Which one is stronger, the gravitational force with which you attract Earth or the gravitational force that Earth exerts on you?

Accepted Answer

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Question 4

The gravitational force with which Earth attracts you is larger than the force with which you attract Earth.

Accepted Answer

According to Newton's third law of motion, forces between two bodies are equal in magnitude and opposite in direction. Therefore, the gravitational force with which Earth attracts you is equal to the force with which you attract Earth.

Question 5

If you are trying to push a crate along a rough floor and it does not move, there is static friction acting between the floor and the crate. The magnitude of static friction is always larger than the applied force; therefore, the object does not move.

Accepted Answer

The magnitude of static friction adjusts up to its maximum value to match the applied force, preventing motion. It is not always larger than the applied force but can be equal to it, preventing the object from moving.

Question 6

We are trying to move a crate across a rough surface. We keep increasing the force until the crate starts moving. Since there is a range of magnitudes of a force, this means that kinetic friction has a range of values before the crate starts moving.

Accepted Answer

The range of magnitudes of force before the crate starts moving is related to static friction, not kinetic friction. Kinetic friction comes into play only after the object has started moving, and it typically has a single value for a given combination of surfaces and normal force.

Question 7

The origin of contact forces is electromagnetism.

Accepted Answer

Contact forces, such as friction, tension, and normal force, arise due to electromagnetic interactions between the atoms and molecules at the surfaces in contact. These forces are a result of the electromagnetic forces between charged particles (electrons and protons) within the atoms.

Question 8

We are trying to move a crate across a rough surface. We keep increasing the force until the crate starts moving. Since there is a range of magnitudes of force applied before the crate starts moving, does this mean that kinetic friction has a range of values as well?

Accepted Answer

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Question 9

Normal force is always a response of a surface to the entire weight of the body.

Accepted Answer

Normal force is a response to the component of the weight perpendicular to the surface, not necessarily the entire weight of the body, especially on inclined planes.

Question 10

Since viscous forces in fluid and air drag are the same in nature, both depend linearly on the object's velocity.

Accepted Answer

Viscous forces in fluid often follow a linear relationship with velocity at low speeds (laminar flow), but air drag (a form of viscous force encountered at higher speeds or in gases) can depend on the square of the velocity, especially at higher speeds where the flow becomes turbulent.

Question 11

From the data for Earth and the Moon in Table 3.2, how many times stronger is Earth's gravitational field than the Moon's?

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Question 12

What is the difference between mass and weight?

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Question 13

If a 50 kg skier is pulled by a rope up a 15° slope, what is the minimum tension in the rope? The coefficient of friction is 0.2.

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Question 14

Although gravity accelerates you when you fall freely, it is possible to achieve a constant velocity after you jump from an airplane.

Accepted Answer

When you jump from an airplane and fall freely, you initially accelerate due to gravity. However, as your velocity increases, the air resistance acting against you also increases. Eventually, the force of air resistance equals the force of gravity acting on you, resulting in no net force and thus no further acceleration. At this point, you reach terminal velocity and fall at a constant speed.

Question 15

If the weight of a chest is 560 N, and you try to push it along the floor with a force of 295 N, and the coefficient of static friction between the floor and the chest is µ = 0.5, will the chest move?

Accepted Answer

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Question 16

When a muscle contracts it can pull, and when it stretches it can push.

Accepted Answer

Muscles can only pull when they contract; they cannot push. When a muscle stretches, it is actually being elongated and does not exert a pushing force.

Question 17

You are standing on the floor, not moving. This means that there is no gravity acting on you.

Accepted Answer

Gravity is always acting on you, pulling you towards the Earth, which is why you are standing on the floor and not floating.

Question 18

Viscous force in a fluid is what is called drag in the air.

Accepted Answer

Viscous force in a fluid is indeed what causes drag in the air, as both refer to the resistance experienced by an object moving through a fluid (liquid or gas).

Question 19

The surface tension of water makes it possible for certain insects to walk on the surface of water. What is the origin of surface tension?

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Question 20

Can gravity act as a restoring force? Give an example.

Accepted Answer

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Quiz 3: Forces

For the law of gravity equation, F = Gm1m2/r2, to be dimensionally consistent, what must be the units of universal gravitational constant?