3A3 Newton's Laws of Motion

Question 1

Define:

Newton’s laws of motion

Answer 1

Three fundamental laws explaining how forces affect motion.

They explain inertia, acceleration, and action-reaction forces in mechanics.

Question 2

Why are Newton’s laws of motion significant in physics?

Answer 2

They predict and explain how forces affect motion.

These laws are foundational in engineering, astronomy, and everyday problem-solving.

Question 3

True or false:

Newton’s laws apply to both microscopic and macroscopic systems.

Answer 3

True

They are used to describe everything from atomic interactions to planetary orbits.

Question 4

Do Newton’s laws hold true in all frames of reference?

Answer 4

No, they are valid only in inertial reference frames.

In non-inertial frames, fictitious forces must be accounted for.

Question 5

Define:

Inertia

Answer 5

It is an object’s resistance to changes in its state of motion.

The greater the mass, the greater the inertia, making it harder to change the object’s velocity.

Question 6

Which of Newton’s laws introduces the concept of inertia?

Answer 6

The first law of motion.

Inertia is an object’s tendency to resist changes in its state of motion.

Question 7

True or false:

The first law of motion states that an object will remain at rest or move at a constant velocity unless acted upon by an external force.

Answer 7

True

This principle is also known as the law of inertia.

Question 8

How does mass influence an object’s inertia?

Answer 8

The greater the mass, the greater the inertia, making the object more resistant to changes in motion.

Newton acknowledged that forces like friction can also change motion.

Question 9

Define:

External force

Answer 9

A force applied from outside a system’s boundaries.

They cause motion changes in a system.

Question 10

True or false:

A net force on a system requires an external force.

Answer 10

True

Internal forces within a system cannot change its overall motion; only external forces can produce a net force.

Question 11

Fill in the blank:

An object in motion stays in motion at a constant ______ unless acted upon by an external force.

Answer 11

velocity

This principle applies to objects at rest as well, as both conditions require an unbalanced force to change their state.

Question 12

Why do passengers lurch forward in a car when it suddenly stops?

Answer 12

Inertia keeps their bodies moving forward at the car’s original speed.

Seatbelts counter inertia, preventing forward motion.

Question 13

Which type of surface is best for demonstrating the first law of motion?

Answer 13

A nearly frictionless surface, such as an air hockey table.

Minimizing friction allows objects to better maintain their state of motion.

Question 14

What happens to an object if the forces acting on it are balanced?

Answer 14

The object remains at rest or moves with constant velocity.

Only unbalanced forces can change an object’s state of motion.

Question 15

What quantities are related in Newton’s Second Law?

Answer 15

It relates the net force (F) acting on an object to its mass (m) and acceleration (a), expressed as F=ma.

This law is key to understanding how forces cause changes in motion.

Question 16

Define:

Net Force

Answer 16

It is the vector sum of all forces acting on an object. It determines the object’s acceleration according to F_net=ma.

If the net force (Fnet) is zero, the object remains in equilibrium (no acceleration).

Question 17

Fill in the blank:

According to the second law, if the net force on an object increases, its acceleration _______.

Answer 17

increases

They cause an object to change velocity.

Question 18

Fill in the blank:

With constant force, increasing mass decreases _______ (Newton’s Second Law).

Answer 18

acceleration

This inverse relationship is a direct consequence of the second law.

Question 19

What is the purpose of a free-body diagram in the context of the second law?

Answer 19

To visually represent the magnitude and direction of all forces acting on an object, allowing the calculation of net force.

Each force should include both magnitude and direction for accuracy.

Question 20

What is the SI unit of force, and how is it defined?

Answer 20

The Newton (N), defined as the force required to accelerate a 1kg mass by 1m/s².

1N=1kg⋅m/s²

Question 21

A person pushes a shopping cart with a force of 50N. If the cart has a mass of 25kg, what is its acceleration?

Answer 21

Acceleration is 2m/s².

Using F=ma, solve a=F/m

Question 22

How does friction affect the net force in the second law of motion?

Answer 22

Friction opposes the motion, reducing the net force available to accelerate the object.

Always account for frictional forces when calculating net force.

Question 23

If two people push an object with forces of 30N and 40N in the same direction, and a friction force of 10N opposes them, what is the net force?

Answer 23

Net force is 60N.

Add forces in the same direction, subtract opposing forces: Fnet=30N+40N−10N

Question 24

Define:

Free fall

Answer 24

Motion of an object under the influence of gravity alone, with no other forces acting on it.

In free fall near Earth’s surface, the object accelerates at 9.8m/s², assuming no air resistance.

Question 25

Why does an object in **free fall** have the same acceleration regardless of its mass?

Answer 25

The **force of gravity is proportional to mass**, so the ratio Fnet/m (acceleration) remains constant. ## Footnote This is why objects fall at 9.8m/s² near Earth's surface, neglecting air resistance.

Question 26

If the **net force** acting on an object is **zero**, what can we infer about its motion?

Answer 26

The object is either at **rest** or moving with **constant velocity**. ## Footnote Zero net force implies no acceleration, as per Fnet =ma.

Question 27

A truck has a mass of **2000kg**. What **net force** is required to accelerate it at **2m/s²**?

Answer 27

Net force is 4000N. ## Footnote Use Fnet=ma =(2000kg)⋅(2m/s²)

Question 28

Which law explains why a **rocket propels forward** when gas is expelled backward?

Answer 28

The **third** law of motion. ## Footnote The force exerted by the gas on the rocket is matched by an equal and opposite force exerted by the rocket on the gas.

Question 29

# Define: The **third law** of motion

Answer 29

For every **action force**, there is an equal and opposite **reaction force**. ## Footnote This law explains interactions such as walking, where the foot pushes against the ground, and the ground pushes back.

Question 30

# Fill in the blanks: According to **Newton's Third Law**, if a hammer exerts a force on a nail, the nail exerts an \_\_\_\_\_\_ and \_\_\_\_\_\_ force on the hammer.

Answer 30

equal; opposite ## Footnote **Action-reaction** pairs always occur simultaneously and involve two separate objects.

Question 31

# True or false: Action and reaction forces **cancel** each other out because they are **equal in magnitude** and **opposite in direction**.

Answer 31

False ## Footnote The forces act on different objects, so they do not cancel out.

Question 32

# Fill in the blanks: Newton’s **third law** explains that forces always come in \_\_\_\_\_\_ and \_\_\_\_\_\_ pairs.

Answer 32

action; reaction ## Footnote These force pairs act on different objects and never cancel each other.

Question 33

What must be true for **forces to cancel each other**?

Answer 33

Forces must act **on the same object**, have the same magnitude, and act in opposite directions. ## Footnote If the forces act on different objects, they form an action-reaction pair and do not cancel each other.

Question 34

What happens when a bird **flaps** its wings **downward** in the air?

Answer 34

The air **pushes** the bird **upward** with an equal and opposite force. ## Footnote This reaction force enables the bird to fly.

Question 35

How does Newton’s **third law** apply to **walking**?

Answer 35

When you push backward on the ground with your feet, the **ground pushes you forward** with an equal and opposite force. ## Footnote This reaction force allows motion in the forward direction.

Question 36

How do Newton's **first** and **second** laws relate when analyzing a stationary **object that starts moving**?

Answer 36

The transition from rest to motion **connects inertia** (first law) with **force and acceleration** (second law). ## Footnote The first law explains why the object remains stationary until acted upon, and the second law determines how the object's acceleration depends on the applied net force and its mass.

Question 37

When you **push a box** across the floor, how do Newton's **second** and **third** laws interact?

Answer 37

The second law describes the box's **acceleration** due to the net force, while the third law states that the box exerts an equal and opposite **force on you**. ## Footnote These laws together describe both motion and interaction between objects.

Question 38

How do **Newton's three laws** explain the motion of a **rocket** launching into space?

Answer 38

* **First law**: The rocket remains stationary until a force (thrust) is applied. * **Second law**: The acceleration depends on the thrust and the rocket's mass. * **Third law**: The exhaust gases push downward, and the rocket moves upward with an equal and opposite force. ## Footnote The three laws work together to describe both the cause and behavior of motion.

Question 39

How do Newton's **first** and **third** laws help explain the **recoil** of a gun?

Answer 39

The first law explains the gun's tendency to stay at **rest** before firing. The third law accounts for the recoil as the bullet is propelled forward by an **equal and opposite force** on the gun. ## Footnote Inertia (first law) and interaction forces (third law) describe the event.

Question 40

In a **tug-of-war**, how do Newton's **second** and **third** laws describe the motion of the rope?

Answer 40

The third law explains that each team exerts **equal and opposite forces** on the rope, while the second law determines the rope's motion based on the **net force** applied by the stronger team. ## Footnote The combination of interaction forces (third law) and net forces (second law) determines motion.