3.1 Force

In kinematics, we studied how objects move. Now, we ask a deeper question: why do objects move the way they do? What causes an object at rest to start moving? What makes a moving object speed up, slow down, or change direction? The answer to all these questions is force. The study of the relationship between force and motion is called dynamics.

Definition

• Start or stop its motion.
• Speed it up or slow it down.
• Change the direction of its motion.

Main Idea

The main idea is that force is the cause of changes in motion. An object will not change its state of motion on its own; a force must act upon it. Whether it's a push, a pull, or an interaction at a distance, a force is responsible for every change in an object's velocity. Understanding forces is essential for explaining the physical world, from why a ball falls to the ground to how a rocket launches into space.

Core Concept

• Magnitude: The strength of the force (measured in newtons, N).
• Direction: The direction in which the force acts.

A force is often represented by an arrow pointing in the direction of the force, with the length of the arrow representing the magnitude.

• Pushing a trolley: You apply a force to start it moving.
• Pulling a rope: You apply a force to pull an object towards you.
• Gravity: The Earth applies a force on all objects, pulling them downwards.

Detailed Explanation

The text introduces force by asking key questions: "How an object at rest begin to move? What causes an object to accelerate or decelerate? What makes an object to moves in a curved path? The simple answer to all these questions is force."

Force is what causes an object to change its velocity. This means force is the cause of acceleration. If an object is at rest, a force can start it moving. If it is moving, a force can speed it up, slow it down, or change its direction.

The text states: "Every motion you observe or experience is related to a force." Whether you are pushing a trolley, throwing a ball, or simply walking, forces are at play.

The SI unit of force is the newton (N). One newton is the force required to give a mass of one kilogram an acceleration of one metre per second squared (1 N = 1 kg·m/s²).

Intermediate Level

• Change its speed: Speeding up (acceleration) or slowing down (deceleration).
• Change its direction: Causing it to move in a curved path.
• Change its shape: Deforming the object (e.g., stretching a spring).

The total force acting on an object is called the net force or resultant force. If the net force is zero, the object is in equilibrium and its velocity does not change (it stays at rest or moves at constant velocity). If the net force is non-zero, the object accelerates in the direction of the net force. This relationship is encapsulated in Newton's second law of motion.

Important Points

• - Force is a vector quantity (magnitude and direction).
• - Force is the cause of changes in motion.
• - It can start or stop motion, speed up or slow down, or change direction.
• - The SI unit of force is the newton (N).
• - The net force (or resultant force) is the vector sum of all forces acting on an object.

Remember

• - Force: A push or pull on an object.
• - Vector: Has magnitude and direction.
• - Unit: Newton (N).
• - Effect: Causes acceleration, deceleration, or changes in direction.

Common Mistakes

• - Thinking force is always needed to keep an object moving: An object in motion will continue to move at constant velocity if no net force acts on it (Newton's first law).
• - Confusing force with velocity or acceleration: Force is the cause; acceleration is the effect. Velocity is the result of motion.
• - Forgetting that force is a vector: Direction matters. A force of 10 N to the right is different from a force of 10 N to the left.

Quick Revision

Force is a vector quantity that causes changes in the state of motion of an object. It can start or stop motion, speed it up or slow it down, or change its direction. The SI unit of force is the newton (N). Force is the fundamental cause of all changes in motion.

60-Second Revision

• Effect: Changes or tends to change motion.
• Unit: Newton (N).
• Cause of acceleration: Net force causes acceleration.
• Formula: F = ma.

Frequently Asked Questions (FAQs)

Q1. What is force? Answer: Force is a vector quantity that changes or tends to change the state of motion of a body. It can start or stop motion, speed it up or slow it down, or change its direction.

• Q2. Is force a scalar or a vector?
• Answer: Force is a vector quantity. It has both magnitude and direction.

• Q3. What is the SI unit of force?
• Answer: The SI unit of force is the newton (N). One newton is the force that gives a mass of 1 kg an acceleration of 1 m/s².

• Start motion from rest.
• Stop a moving object.
• Increase or decrease the speed of an object.
• Change the direction of motion.
• Change the shape or size of an object.

• Q5. What is net force?
• Answer: Net force (or resultant force) is the vector sum of all the individual forces acting on an object. It determines the actual acceleration of the object.

Q6. Does an object at rest have force acting on it? Answer: Yes. Even an object at rest can have forces acting on it (e.g., gravity and the normal force from the surface), but the net force is zero, so it remains at rest.

Q7. What is the relationship between force and acceleration? Answer: According to Newton's second law of motion, force is directly proportional to acceleration (F = ma). A greater net force causes a greater acceleration.

Multiple Choice Questions (MCQs)

• Q1. Force is a:
• A. Scalar quantity
• B. Vector quantity
• C. Base quantity
• D. Derived quantity

• Correct Answer: B
• Explanation: Force is a vector quantity because it has both magnitude and direction.

• Q2. The SI unit of force is the:
• A. Joule
• B. Newton
• C. Watt
• D. Pascal

• Correct Answer: B
• Explanation: The SI unit of force is the newton (N).

• Q3. Which of the following is NOT an effect of force?
• A. Starting motion
• B. Changing direction
• C. Changing mass
• D. Slowing down motion

• Correct Answer: C
• Explanation: Force can change motion, but it does not change the mass of an object. Mass is an intrinsic property.

• Q4. A force can:
• A. Only start motion
• B. Only stop motion
• C. Start or stop motion, speed up or slow down, or change direction
• D. Only change direction

• Correct Answer: C
• Explanation: The text states that force can start or stop motion, speed it up or slow it down, and change the direction of motion.

• Q5. One newton is the force that gives a mass of 1 kg an acceleration of:
• A. 1 m/s
• B. 1 m/s²
• C. 9.8 m/s²
• D. 1 m²/s²

• Correct Answer: B
• Explanation: 1 N = 1 kg·m/s², meaning it gives 1 kg mass an acceleration of 1 m/s².

• Q6. Which of the following is a force?
• A. Mass
• B. Velocity
• C. Weight
• D. Speed

• Correct Answer: C
• Explanation: Weight is a force (the gravitational force on an object). Mass, velocity, and speed are not forces.

• Q7. The net force on an object is zero. The object:
• A. Must be at rest
• B. Must be moving
• C. Is either at rest or moving with constant velocity
• D. Is accelerating

• Correct Answer: C
• Explanation: If the net force is zero, the object is in equilibrium. It will either remain at rest or continue moving with constant velocity.

• Q8. Force is the cause of:
• A. Mass
• B. Acceleration
• C. Volume
• D. Density

• Correct Answer: B
• Explanation: Force causes acceleration. According to Newton's second law, F = ma.

• Q9. If two forces act on an object in opposite directions, the net force is:
• A. The sum of the magnitudes
• B. The difference of the magnitudes
• C. Always zero
• D. Always maximum

Correct Answer: B Explanation: The net force is the vector sum. If forces are in opposite directions, they subtract. The net force is the difference between their magnitudes, in the direction of the larger force.

• Q10. The text states that every motion you observe or experience is related to:
• A. Mass
• B. Volume
• C. Force
• D. Density

• Correct Answer: C
• Explanation: The text states, "Every motion you observe or experience is related to a force."

Frequently Asked Questions (FAQs)

Q1. What is the difference between force and mass? Answer: Mass is the amount of matter in an object and is a scalar quantity (measured in kg). Force is a push or pull that can change an object's motion and is a vector quantity (measured in N).

• Q2. What is the difference between force and weight?
• Answer: Weight is a specific type of force—the gravitational force exerted on an object by the Earth. Weight = mass × acceleration due to gravity (W = mg).

Q3. Can force change the mass of an object? Answer: No. Force can change motion and shape, but mass is an intrinsic property of the object and does not change with force (unless the object is moving at relativistic speeds).

Q4. What is the difference between applied force and net force? Answer: Applied force is a specific force applied to an object (e.g., a push). Net force is the vector sum of all forces acting on the object, which determines the resulting acceleration.

• First law: No net force means no change in motion.
• Second law: Net force equals mass times acceleration (F = ma).
• Third law: For every action, there is an equal and opposite reaction.

Q6. What is a free-body diagram? Answer: A free-body diagram is a sketch showing all the forces acting on an object. It is a useful tool for analyzing the net force and solving dynamics problems.

Q7. Why is force considered the cause of motion? Answer: Force is the cause of changes in motion (acceleration). Without a net force, an object will not change its state of motion (it will remain at rest or move at constant velocity).

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