Year 11 Physics: Newton’s Laws of Motion Explained

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🔍 Detailed Explanation of Newton’s Laws of Motion

Newton’s Laws of Motion are fundamental principles that describe how objects move and interact. These laws are essential in physics and are widely used to understand everyday movements and forces. In Year 11 Physics, it is important to know all three laws, their definitions, formulas, and examples to apply them correctly.

⚖️ Newton’s First Law of Motion (Law of Inertia)

Definition:
An object will remain at rest or continue moving at a constant velocity in a straight line unless acted upon by a resultant (unbalanced) force.

Explanation:
This means that if no external force acts on an object, it will not change its state of motion. The object resists changes in motion, which is called inertia.

Example:
If you slide a hockey puck on ice, it will keep moving in a straight line at a steady speed because there is very little friction. It only stops when a force (like friction or a player’s stick) acts on it.

🚀 Newton’s Second Law of Motion (Law of Acceleration)

Definition:
The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass. The formula is:

\[ F = m \times a \]

where

F = resultant force (in newtons, N),

m = mass (in kilograms, kg),

a = acceleration (in metres per second squared, \(m/s^2\)).

Explanation:
This law explains how the velocity of an object changes when a force is applied. A greater force means greater acceleration, but the heavier the object, the less acceleration it will produce from the same force.

Example:
If you push a shopping cart, it accelerates. If the cart is empty (less mass), it accelerates more easily than when it is full of groceries (more mass), even if you push with the same force.

↔️ Newton’s Third Law of Motion (Action and Reaction)

Definition:
For every action, there is an equal and opposite reaction.

Explanation:
This means that forces always come in pairs. When you push on something, it pushes back on you with the same force in the opposite direction.

Example:
When you jump off a boat onto a dock, you push the boat backwards (action), and the boat pushes you forwards onto the dock (reaction).

📊 Summary Table of Newton’s Laws

By understanding and applying Newton’s Laws of Motion, you can analyse many physical situations in everyday life and in physics problems at school. Remember to always identify the forces involved and use the correct formula to find acceleration, force, or mass when required.

📝 10 Examination-Style 1-Mark Questions with 1-Word Answers on Newton’s Laws of Motion

1. What is the force that opposes motion between two surfaces called?
   Answer: Friction
2. According to Newton’s First Law, an object at rest will stay at rest unless acted on by a ___?
   Answer: Force
3. What is the unit of force in the International System (SI)?
   Answer: Newton
4. Newton’s Second Law relates force, mass, and what other quantity?
   Answer: Acceleration
5. What type of force does Newton’s Third Law say acts in pairs?
   Answer: Action
6. What quantity remains constant if no external forces act on an object?
   Answer: Momentum
7. The tendency of an object to resist changes in its motion is called ___?
   Answer: Inertia
8. In Newton’s Third Law, if object A exerts a force on object B, object B exerts a force of equal ___ and opposite direction.
   Answer: Magnitude
9. Which law states that Force equals mass times acceleration?
   Answer: Second
10. An object moving at a constant velocity has zero ___ force according to Newton’s First Law?
    Answer: Net

🧠 10 Examination-Style 2-Mark Questions with 1-Sentence Answers on Newton’s Laws of Motion

1. Question: State Newton’s First Law of Motion.
   Answer: Newton’s First Law states that an object will remain at rest or in uniform motion unless acted on by an external resultant force.
2. Question: What is the relationship between force, mass, and acceleration according to Newton’s Second Law?
   Answer: Newton’s Second Law states that force equals mass times acceleration (F = ma).
3. Question: Describe Newton’s Third Law of Motion.
   Answer: Newton’s Third Law states that for every action there is an equal and opposite reaction.
4. Question: Why does a passenger lurch forward in a car when it suddenly stops?
   Answer: Because the passenger’s body tends to continue moving forward due to inertia as per Newton’s First Law.
5. Question: How does increasing the mass of an object affect its acceleration if the same force is applied?
   Answer: Increasing the mass decreases the acceleration because acceleration is inversely proportional to mass.
6. Question: What causes a rocket to move upwards according to Newton’s Third Law?
   Answer: The rocket moves upwards because the gases push downwards on the ground, and the ground pushes upwards on the rocket with equal force.
7. Question: An object in space is moving at a constant velocity; what force is acting on it?
   Answer: No net force is acting on it, as per Newton’s First Law.
8. Question: Why does a skater accelerate faster when they reduce their mass by throwing objects away?
   Answer: Because reducing mass increases acceleration for the same force applied, following Newton’s Second Law.
9. Question: What is the direction of the reaction force in Newton’s Third Law compared to the action force?
   Answer: The reaction force acts in the opposite direction to the action force.
10. Question: How does friction relate to Newton’s First Law?
    Answer: Friction is an external force that can stop a moving object, causing it not to keep moving indefinitely as per Newton’s First Law.

🧩 10 Examination-Style 4-Mark Questions on Newton’s Laws of Motion with Detailed 6-Sentence Answers

Question 1:

Explain Newton’s First Law of Motion and give an example of it in everyday life.

Answer:
Newton’s First Law of Motion states that an object will remain at rest or continue to move at a constant velocity unless acted upon by a resultant external force. This is also called the law of inertia. For example, a book lying on a table will stay there until someone pushes it or picks it up. Similarly, a car moving at a steady speed will keep moving unless the driver applies brakes or the car hits an obstacle. This law shows that objects resist changes in their state of motion. It helps us understand why seat belts are important in cars, to stop passengers from moving forward during sudden stops.

Question 2:

What does Newton’s Second Law of Motion tell us about the relationship between force, mass, and acceleration?

Answer:
Newton’s Second Law of Motion states that the acceleration of an object depends directly on the net force acting upon it and inversely on its mass. Mathematically, it is expressed as \(F = ma\). This means if the force increases, the acceleration increases proportionally, but if the mass increases, acceleration decreases. For example, pushing a heavy shopping cart requires more force to accelerate it compared to an empty one. This law explains why heavier objects are harder to move or stop. It provides a way to calculate the force needed for a certain acceleration.

Question 3:

Describe Newton’s Third Law of Motion with an everyday example.

Answer:
Newton’s Third Law states that for every action, there is an equal and opposite reaction. This means that forces always come in pairs, acting on two different objects. For example, when you push against a wall, the wall pushes back against you with an equal force. Another example is when a rocket launches, the gases push downwards (action) and the rocket is pushed upwards (reaction). This law explains how birds fly by pushing air downwards with their wings and receiving an upward force. Understanding this helps us see why forces never act alone.

Question 4:

A soccer ball is kicked with a force of 20 N and accelerates at 5 m/s². What is the mass of the ball? Use Newton’s Second Law.

Answer:
According to Newton’s Second Law, \( F = ma \), where \( F \) is force, \( m \) is mass, and \( a \) is acceleration. Rearranging the equation to find mass gives \( m = \frac{F}{a} \). Substituting the given values, mass \( m = \frac{20 \, N}{5 \, m/s^2} = 4 \, kg \). Therefore, the mass of the soccer ball is 4 kilograms. This calculation shows how force and acceleration relate to mass. It also helps in understanding how different forces affect motion.

Question 5:

Why do passengers feel pushed forward in a car when it suddenly stops? Explain using Newton’s Laws.

Answer:
When a car suddenly stops, the deceleration is a result of a large force applied on the car. According to Newton’s First Law, the passengers tend to continue moving forward at the same speed even though the car stops. The car seat applies a force to slow down the passengers, but if they aren’t wearing seat belts, no external force acts directly on them. Newton’s Second Law explains that the change in velocity requires a force, which the seatbelt provides. Newton’s Third Law shows the seatbelt applies a backward force on the passenger, and the passenger pushes forward on the belt with equal force. This is why wearing seat belts is important for safety.

Question 6:

Explain why a heavy object and a light object fall at the same rate in a vacuum.

Answer:
In a vacuum, there is no air resistance to slow down falling objects. Newton’s Second Law tells us that acceleration depends on the net force and mass, but the force of gravity on an object is proportional to its mass. When the force of gravity (weight) is divided by mass to find acceleration, the mass cancels out. This means both heavy and light objects experience the same acceleration due to gravity. Therefore, without air resistance, they fall at the same rate. This explains why a feather and a hammer dropped on the Moon fall together.

Question 7:

How does friction affect the motion of an object according to Newton’s Laws?

Answer:
Friction is a force that opposes the motion of an object, acting opposite to the direction of movement. According to Newton’s First Law, an object in motion will slow down and eventually stop unless a force overcomes friction. Newton’s Second Law says that the net force on an object determines its acceleration, so friction reduces the net force and slows acceleration. For example, pushing a box on a rough surface requires more force because friction opposes the motion. Friction can also generate heat, and it is essential for things like walking or car tyres gripping the road. Understanding friction helps in designing better systems to reduce or increase it as needed.

Question 8:

A 10 kg object accelerates at 3 m/s². Calculate the force acting on the object using Newton’s Second Law.

Answer:
Newton’s Second Law is \( F = ma \), where \( F \) is force, \( m \) is mass, and \( a \) is acceleration. Substituting the values, force \( F = 10 \, kg \times 3 \, m/s^2 = 30 \, N \). So, the force acting on the object is 30 Newtons. This calculation shows how force causes acceleration for a given mass. It helps understand how much effort is needed to move or stop objects. Knowing this relationship is important in physics problems involving motion.

Question 9:

Explain what would happen if there was no friction when you tried to walk. Use Newton’s Laws in your answer.

Answer:
If there was no friction between your feet and the ground, your feet would slip when you try to push backwards. According to Newton’s Third Law, when you push back on the ground, the ground must push forward on you with an equal force. Without friction, the ground cannot apply this forward force, so you wouldn’t move forward. Newton’s First Law means you would continue in your initial position or slide instead of walking. Friction between shoes and ground is necessary to generate the reaction force that allows walking. This shows the importance of friction in everyday movement.

Question 10:

Describe why astronauts feel weightless in space using Newton’s Laws of Motion.

Answer:
Astronauts feel weightless because they are in free fall around the Earth, experiencing continuous acceleration downward due to gravity. Newton’s First Law means they and their spacecraft move together at the same rate without any external force stopping the fall. Although gravity is still acting on them, there is no normal contact force pushing up, which usually gives us the sensation of weight. Newton’s Second Law shows they accelerate due to gravity, but no friction or air resistance acts to change this motion. This creates the sensation of weightlessness. Understanding this helps explain conditions experienced in orbiting spacecraft.

✍️ 10 Examination-Style 6-Mark Questions on Newton’s Laws of Motion with 10-Sentence Answers

Question 1:

Explain Newton’s First Law of Motion and give an example of where it applies in everyday life.

Answer:
Newton’s First Law of Motion states that an object will remain at rest or continue to move at a constant velocity unless acted upon by a resultant external force. This law is also known as the law of inertia. It means objects resist changes to their state of motion. For example, a book resting on a table will stay still unless someone moves it. Similarly, a hockey puck sliding on ice moves in a straight line at constant speed until friction or another force slows it down. The tendency of an object to keep doing what it is already doing is called inertia. This law explains why passengers feel pushed forward when a car suddenly stops. It also applies when you jump on a trampoline; your body continues moving upwards due to inertia after your feet leave the mat. Without external forces like friction or air resistance, motion would stay unchanged. This law helps us understand the behaviour of stationary and moving objects in everyday life.

Question 2:

Describe how Newton’s Second Law of Motion relates force, mass, and acceleration. Include a practical example.

Answer:
Newton’s Second Law of Motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The mathematical expression is \(F = ma\), where \(F\) is force, \(m\) is mass, and \(a\) is acceleration. This means if you apply the same force to lighter and heavier objects, the lighter one will accelerate more. For example, pushing an empty shopping cart requires less force to accelerate than a full cart. In this case, the mass of the cart changes, so the acceleration changes for the same force. If you double the force on an object, its acceleration doubles, assuming the mass stays the same. The law helps explain why heavier vehicles need more engine power to speed up. It also shows why it’s harder to stop a heavy object, like a truck, because it has more mass and momentum. This law allows us to calculate how an object will move when forces act on it. Understanding this helps in designing vehicles, sports equipment, and safety features like airbags.

Question 3:

Using Newton’s Third Law of Motion, explain why rockets can move in space despite the absence of air.

Answer:
Newton’s Third Law of Motion says that for every action, there is an equal and opposite reaction. This means when one object exerts a force on a second object, the second exerts an equal force back in the opposite direction. In the case of rockets in space, they push gas out of their engines at high speed. The action is the force of the rocket pushing gases backwards. The reaction is the gases pushing the rocket forwards with equal force. This propels the rocket forward even in the vacuum of space where there is no air for it to push against. This is why rockets don’t need air or wings to move; they rely on the interaction of forces between the expelled gas and the rocket itself. The rocket’s acceleration is caused by the reaction force from the gases. This principle is crucial for space exploration and satellite launches. Newton’s Third Law explains how movement can occur in environments without air resistance.

Question 4:

Explain why passengers in a car feel pushed back into their seats when the car accelerates suddenly.

Answer:
When a car accelerates suddenly, the car moves forward, but the passengers’ bodies initially tend to stay at rest due to inertia. Newton’s First Law tells us that objects resist changes in motion. The seat applies a force on the passenger’s back to accelerate them forward with the car. Because the passenger’s body resists this acceleration, they feel a force pushing them back against the seat. This sensation is the result of the seat exerting a force that changes the passenger’s velocity. The feeling of being pushed is actually the seat pushing forward, but the passenger’s body inertia makes it seem as though they are pushed back. This interaction is an example of forces described by Newton’s Laws. Without seat belts, passengers could be thrown forward due to this sudden acceleration force. Seat belts help by applying a force to keep passengers safely in place. Understanding this helps in improving vehicle safety designs.

Question 5:

A force of 10 N is applied to a 2 kg object. What acceleration does it experience? Show your calculation and explain.

Answer:
According to Newton’s Second Law, \( F = ma \), where \( F = 10 \, \text{N} \) and \( m = 2 \, \text{kg} \). To find the acceleration \( a \), rearrange the formula to \( a = \frac{F}{m} \). Substitute the values to get \( a = \frac{10}{2} = 5 \, \text{m/s}^2 \). This means the object accelerates at 5 meters per second squared when a force of 10 newtons is applied. The acceleration is directly proportional to the force applied and inversely proportional to the mass. A larger force would cause a greater acceleration, while a larger mass would reduce the acceleration. This calculation shows how much the velocity of the object changes every second. It helps predict the motion of objects when forces are known. This type of problem is common in physics to apply the relationship between force, mass, and acceleration.

Question 6:

Describe how friction affects the motion of an object sliding on a surface using Newton’s Laws of Motion.

Answer:
Friction is a force that opposes the motion of objects sliding over each other. According to Newton’s First Law, an object would continue moving at a constant speed if there were no forces acting on it. However, friction acts as a resistive force, opposing motion, causing the object to slow down and eventually stop. Newton’s Second Law explains that friction reduces the net force in the direction of motion, resulting in a lower acceleration or negative acceleration (deceleration). For example, when sliding a box on the floor, friction between the floor and the box slows it down. Newton’s Third Law states that the box also applies an equal force to the floor, but the friction force opposes the motion. Friction depends on the surfaces in contact and the force pushing them together. It transforms kinetic energy into heat energy, causing objects to lose speed. Friction is necessary for walking or driving because it provides grip, but it also causes objects to stop moving. Understanding friction helps us design machines and materials to control motion effectively.

Question 7:

Explain why heavier objects do not always fall faster than lighter objects despite having more weight.

Answer:
Heavier objects have a greater weight because weight is the force due to gravity, and it depends on mass. However, according to Newton’s Second Law, acceleration depends on both force and mass. The force on a falling object is its weight, while the resistance from air is friction acting upwards. For objects falling under gravity with no air resistance, all experience the same acceleration (about 9.8 m/s²), regardless of mass. This happens because although heavier objects have more weight (force), they also have more mass, meaning the acceleration produced by the force is the same. For example, a bowling ball and a tennis ball dropped from the same height fall at the same rate if air resistance is ignored. Air resistance affects lighter objects more, causing them to fall slower. Newton’s Laws explain this balance between force and mass. Hence, heavier objects do not necessarily fall faster than lighter ones in free fall.

Question 8:

How does seatbelt use in cars demonstrate Newton’s First and Third Laws of Motion?

Answer:
Seatbelts protect passengers by applying Newton’s First Law of Motion. When a car suddenly stops, passengers’ bodies tend to keep moving forward due to inertia. The seatbelt provides the external force needed to change the passengers’ motion and stop them safely. Without seatbelts, passengers could continue moving and be injured. Newton’s Third Law is demonstrated because when the seatbelt pulls back on the passenger, the passenger exerts an equal and opposite force on the belt. This interaction stops the forward motion and prevents passengers from hitting the dashboard or windshield. The seatbelt stretches slightly to reduce the force and lessen injury. It distributes the stopping force across the stronger parts of the body, like the pelvis and chest. This shows how forces come in pairs and help control motion safely. Understanding these laws helps in designing effective safety features in vehicles.

Question 9:

A car of mass 800 kg accelerates at 2 m/s². What is the resultant force acting on the car? Show your working clearly.

Answer:
Using Newton’s Second Law \(F = ma\), where \(m = 800 \, \text{kg}\) and \(a = 2 \, \text{m/s}^2\), the resultant force \(F\) can be calculated as follows: \(F = 800 \times 2 = 1600 \, \text{N}\). This means the car experiences a net force of 1600 newtons in the direction of acceleration. The force could be from the engine pushing the car forward through the wheels. This force overcomes resistive forces like friction and air resistance to accelerate the car at 2 m/s². By knowing the mass and acceleration, we can find the exact force causing the car to speed up. This calculation helps engineers design engines and braking systems. Newton’s Second Law is essential to understand the relationship between forces and motion in vehicles. It lets us predict how cars respond to different driving conditions.

Question 10:

Explain why astronauts feel weightless in orbit despite gravity still acting on them.

Answer:
Astronauts feel weightless in orbit because they are in a state of free fall towards Earth, moving forward at the same rate as the gravitational pull. Newton’s First Law says objects continue in motion unless acted upon by a force, and gravity acts as the centripetal force pulling the spacecraft towards Earth. The spacecraft and the astronauts inside fall together, creating a sensation of weightlessness since they do not push against anything. Although gravity is still acting, the astronauts have no contact force pushing up on them, which is what we usually experience as weight. This condition is often called microgravity. Newton’s Laws explain that without a normal force counteracting gravity, the feeling of weight disappears. The astronauts float inside the spacecraft because both they and the craft accelerate downwards at the same rate. This principle is important for understanding motion in space and designing space missions. Weightlessness does not mean gravity is absent but that no force acts to give a sensation of weight.

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