Forces and motion (Newton's laws)

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Forces and Their Effects

A force is a push or pull on an object; measured in newtons (N).
Forces can change the shape of an object (deformation), change motion (start, stop, speed up, slow down), and change direction.
Velocity is speed with direction; acceleration is the rate of change of velocity.
Forces act in pairs (action and reaction) on different objects, equal in strength and opposite in direction (Newton's Third Law).
When multiple forces act, they combine to produce a net force; if balanced, net force is 0 N; if unbalanced, there is a resultant force.

A person pushing and pulling a desk, illustrating how forces can change motion.

Types of Forces

Contact forces require objects to be touching (e.g., friction, normal force, tension, compression).
Non-contact (field) forces act over a distance (e.g., gravitational, magnetic, electrostatic).
Friction opposes motion and acts in the opposite direction to motion.
Normal force acts perpendicular to a surface, often balancing weight on a level surface.
Tension is a pulling force transmitted through a rope or string.
Compression is a force that squashes or deforms an object.

Newton's Laws of Motion

Newton's First Law: An object at rest stays at rest, and an object in motion stays in motion with constant velocity, unless acted on by an unbalanced force (inertia).
Newton's Second Law: The acceleration of an object depends on the net force and its mass; greater mass requires greater force for the same change in motion.
Newton's Third Law: For every action, there is an equal and opposite reaction; forces act on different objects.
Examples: kicking a ball (ball moves, you feel force), rocket launch (gas pushes down, rocket goes up).

Mass and Weight

Mass is the amount of matter in an object; measured in kilograms (kg); constant everywhere.
Weight is the gravitational force on an object; measured in newtons (N); varies with gravity.
Formula: W = m × g, where W is weight (N), m is mass (kg), g is gravitational acceleration (9.8 m/s² on Earth).
On the Moon, mass stays the same but weight is less because gravity is weaker.
Mass is a fundamental property; weight is a force.

Gravitational Force

Gravity is a non-contact force that attracts objects with mass.
The gravitational force between two objects depends on their masses and the distance between them.
Greater mass increases gravitational force; greater distance decreases it.
Weight is the gravitational force exerted on an object by a planet or moon.

Magnetic and Electrostatic Forces

Magnetic force is a non-contact force between magnets and magnetic materials (e.g., iron, cobalt, nickel).
Magnets have north and south poles; like poles repel, opposite poles attract.
Electrostatic force is between charged objects; like charges repel, opposite charges attract.
The electrostatic force decreases as distance between charges increases.
Lightning is an example of electrostatic discharge.

Representing Forces

Forces are represented by arrows in diagrams; length indicates size, direction shows direction.
Free-body diagrams show all forces acting on an object, drawn from the center.
Net force is calculated by adding forces in the same direction and subtracting opposite ones.
Example: 10 N left and 6 N right gives net force 4 N left.

Two people pushing a child on a tricycle, with arrows representing the forces F1 and F2.

Friction and Normal Force Relationship

Friction depends on the type of surface and the normal force.
The normal force is the support force perpendicular to a surface; on a level surface, it equals the object's weight.
To move an object, the applied force must exceed the static friction.
Friction can be beneficial (e.g., walking, car tires) and causes heating.

A wooden block loaded with masses, pulled sideways by a spring balance to measure the force needed to overcome friction.

Engineering Design and Newton's Third Law

Newton's Third Law explains collisions: when two objects collide, they exert equal and opposite forces on each other.
Damage in collisions depends on factors like mass, speed, and material strength.
Design solutions (e.g., car bumpers) aim to reduce damage by absorbing or distributing forces.
Engineering tasks involve identifying subsystems, interactions, and applying Newton's laws to solve problems.

A hand pushing on a brick wall: the push (action) and the wall's equal, opposite push back (reaction) are shown as labelled arrows.

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1.Which of the following is the correct definition of a force?
Easy
- AA push or a pull on an object
- BA measure of the amount of matter in an object
- CThe rate of change of an object's velocity
- DThe force that opposes motion between two surfaces
2.Which of the following is the unit of force?
Easy
- Anewton
- Bkilogram
- Cjoule
- Dwatt
3.A force can change the shape of an object.
Easy
True or false?
4.A force cannot make a motionless object move.
Easy
True or false?
5.Which of the following is NOT a field force?
Easy
- AFrictional force
- BGravitational force
- CElectrostatic force
- DMagnetic force
6.Which of the following substances is magnetic?
Easy
- ACobalt
- BAluminium
- CCopper
- DTin
7.According to Newton's First Law, an object in motion will continue moving at constant velocity unless acted on by an unbalanced force.
Easy
True or false?
8.If the forces acting on an object are balanced, the object must be at rest.
Easy
True or false?

複習筆記

## Forces and Motion (Newton's Laws) — Quick Revision ### Newton's Three Laws 1. **First Law (Inertia):** An object at rest stays at rest, and an object in motion stays in motion at constant speed in a straight line, unless acted on by a net force. - *Example:* A book on a table stays still until you push it. 2. **Second Law (F = ma):** The net force on an object equals its mass times its acceleration. - Formula: **F_net = m × a** (where F_net is net force in newtons, m is mass in kg, a is acceleration in m/s²). - *Example:* Pushing a shopping cart: more force → more acceleration; heavier cart → less acceleration for same force. 3. **Third Law (Action-Reaction):** For every action force, there is an equal and opposite reaction force. - *Example:* When you jump, you push down on the ground (action), and the ground pushes you up (reaction). ### Balanced vs. Unbalanced Forces - **Balanced forces** (net force = 0) → no change in motion (object at rest or moving at constant speed). - **Unbalanced forces** (net force ≠ 0) → change in motion (acceleration). ### Key Concepts - **Force:** A push or pull measured in newtons (N). - **Mass:** Amount of matter in an object (kg). - **Acceleration:** Change in velocity over time (m/s²). - **Inertia:** Tendency of an object to resist changes in motion; depends on mass. - **Momentum:** Mass × velocity (kg·m/s). Newton's second law can also be stated as: net force = rate of change of momentum. ### Collisions (Third Law) - In a collision, forces on each object are equal in size and opposite in direction. - The objects may have different accelerations depending on their masses (F = ma). ### Quick Tips - Always identify the **net force** (sum of all forces). - Remember: forces come in pairs (action-reaction) but act on **different objects**. - Use F = ma to solve for any one variable if you know the other two.

Newton's first law: A body remains at rest, or in motion at a constant speed in a straight line, unless it is acted upon by a force.
Newton's second law: The net force on a body is equal to the body's acceleration multiplied by its mass (F = ma).
Newton's third law: If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.
Net force: The vector sum of all forces acting on an object.
Balanced forces: Forces that cancel each other out, resulting in no change in motion.
Unbalanced forces: Forces that do not cancel, causing a change in motion (acceleration).
Inertia: The tendency of an object to resist changes in its state of motion.
Mass: A measure of the amount of matter in an object, measured in kilograms.
Acceleration: The rate of change of velocity with time, measured in m/s².
Momentum: The product of an object's mass and its velocity.

字卡

What does Newton's first law state?

An object at rest stays at rest, and an object in motion stays in motion at constant speed in a straight line, unless acted on by an unbalanced force.

What is another name for Newton's first law?

The law of inertia.

What is inertia?

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

What does Newton's second law state?

The net force on an object equals its mass times its acceleration (F = ma).

How is acceleration related to net force and mass?

Acceleration is directly proportional to net force and inversely proportional to mass.

What does Newton's third law state?

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

If you push a wall with 10 N of force, what force does the wall exert on you?

The wall exerts a 10 N force back on you in the opposite direction.

What is a force?

A force is a push or pull that can cause an object to change its velocity.

What is net force?

The net force is the vector sum of all forces acting on an object.

What are balanced forces?

Balanced forces are forces that cancel each other out, resulting in no change in motion.

What are unbalanced forces?

Unbalanced forces are forces that do not cancel, causing a change in motion (acceleration).

What is the relationship between mass and inertia?

Mass is a measure of inertia; more mass means more inertia and more resistance to change in motion.

If a 2 kg object accelerates at 3 m/s², what is the net force?

F = ma = 2 kg × 3 m/s² = 6 N.

What is the unit of force in the SI system?

The newton (N). 1 N = 1 kg·m/s².

When a rocket launches, what is the action and reaction?

Action: rocket pushes exhaust gases downward. Reaction: exhaust gases push rocket upward.

Why do passengers lurch forward when a car suddenly stops?

Due to inertia, passengers continue moving forward at the original speed until a force (seatbelt) acts on them.

What is the difference between mass and weight?

Mass is the amount of matter in an object; weight is the force of gravity on that mass (weight = mass × g).

If an object is moving at constant velocity, what can you say about the net force?

The net force is zero; forces are balanced.

What is the formula for Newton's second law?

F_net = m × a, where F_net is net force, m is mass, and a is acceleration.

What happens to the acceleration if the mass is doubled while net force stays the same?

Acceleration is halved (a = F/m, so doubling m halves a).

What is the reaction force when you walk?

Your foot pushes backward on the ground; the ground pushes forward on your foot.

Can an object be in motion if no forces act on it?

Yes, according to Newton's first law, an object in motion stays in motion at constant velocity if no net force acts.

What does 'net force' mean in the context of Newton's second law?

Net force is the total force resulting from all individual forces acting on an object.

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