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C2.1 · Use force, mass, acceleration, friction, gravity, and normal-force terminology

Learn to use force, mass, acceleration, friction, gravity, and normal-force terminology through clear examples and targeted practice.

Ontario Grade 11 Physics

Forces

A Grade 11 guide to describing forces and motion clearly

A clear physics description begins by naming the object being discussed. This object is the system. For example, if you describe a book on a table, the book is the system. Choose a positive direction when you need to use signs, such as right or upward. State the direction in words as well. This lesson focuses on describing force, mass, acceleration, friction, gravity, and normal force. The goal is to use these terms correctly, not to calculate a motion from them.

What you will learn

Use force, mass, and acceleration terminology accurately.
Describe gravity as a force and distinguish it from mass.
Identify friction and normal force by their directions and contact with surfaces.
Label forces acting on a selected object in a simple force diagram.

1. Mass, force, and acceleration

Mass describes how much matter an object has. Its SI unit is the kilogram, written as kg. For example, a bag can have a mass of 4.0 kg. Mass is not the same as the force of gravity on the bag.

A force is a push or pull on an object. A force may change an object’s motion or shape. Force is measured in newtons, written as N. When you describe a force, name the object receiving it and give its direction: “The hand pushes the cart to the right.” This is clearer than saying only “there is a push.”

Acceleration describes a change in motion. More precisely, it is a change in velocity over time. Velocity means how quickly an object moves and the direction in which it moves. An object accelerates when it speeds up, slows down, or changes direction. Acceleration is measured in metres per second squared, written as m/s².

Keep mass, force, and acceleration distinct. Mass is described in kilograms. A force is a push or pull, and acceleration describes a change in motion. Do not use “force” as another name for motion, or “mass” as another name for weight.

Mass describes how much matter an object has; its unit is kg.
Force means a push or pull; its unit is N.
Acceleration describes a change in velocity; its unit is m/s².
When describing a force, name the object it acts on and its direction.

2. Gravity and normal force

Gravity is an attractive force between objects that have mass. Near Earth’s surface, Earth’s gravity pulls objects toward Earth. The force of gravity acting on an object is often called its weight. Weight is a force, so it is measured in newtons. Mass and weight are not interchangeable terms: mass is measured in kilograms, while weight is a gravitational force measured in newtons.

A normal force is a contact force exerted by a surface on an object touching it. The normal force acts at a right angle to the surface. “Perpendicular” is another way to say “at a right angle.” A level table exerts an upward normal force on a book resting on it. A sloped surface exerts a normal force at a right angle to that slope, not automatically straight upward.

A normal force is not automatically equal in size to the force of gravity. In a simple situation where a book rests on a level table and does not accelerate vertically, the upward normal force balances the downward force of gravity. That description applies to this situation; it is not a general rule for every object touching a surface.

A free-body diagram is a simple drawing that shows the forces acting on one chosen object. Draw the object as a box or dot. Draw an arrow for each force acting on it, and label the arrow. The arrow points in the direction of the force. Do not draw a force on the object merely because the object exerts a force on something else.

Near Earth, gravity on an object points toward Earth.
Weight is the force of gravity on an object, not another name for mass.
A normal force is exerted by a touching surface and is perpendicular to that surface.
Show only forces acting on the selected system in its free-body diagram.

3. Friction and clear force descriptions

Friction is a contact force between surfaces that touch. It acts along the surfaces, rather than at a right angle to them. Friction opposes sliding, or the tendency for the surfaces to slide, relative to one another. For example, if a box slides right across a floor, friction on the box points left.

Do not decide the direction of friction just by asking which way the object moves. Consider how the touching surfaces slide, or would begin to slide, relative to one another. Then state the force’s direction on the chosen object. Friction is a force, so its unit is the newton.

A useful force description names the system, the force, and the direction. For example: “The floor exerts friction on the box toward the left.” For the same box, “Earth exerts a gravitational force downward” and “the floor exerts a normal force upward” identify the other forces in a simple level-floor situation.

A labelled force diagram can help organize this description. The diagram below represents a box on a level floor, with a push to the right. It shows possible forces on the box; it does not claim that the forces have equal sizes.

Box: push →; friction ←; normal force ↑; gravity ↓.

Friction is a contact force between touching surfaces.
Friction acts along the surfaces and opposes sliding or the tendency to slide.
The normal force is at a right angle to a surface; friction acts along it.
Label each force as acting on the chosen object and include its direction.

Worked example

Describing a sliding box

A box slides to the right across a level floor. Identify the system and describe the directions of friction, gravity, and the normal force.

Name the system
The system is the object whose forces are being described. Here, that object is the box.
Describe friction
The box slides right relative to the floor. Friction on the box acts along the floor and opposes that sliding, so it points left.
Describe the vertical forces
Earth’s gravity pulls the box downward. The floor exerts a normal force upward, at a right angle to the level floor.

Answer: The system is the box. Friction on it points left, gravity points downward, and the floor’s normal force points upward.

Check: The friction arrow is along the floor, while the normal-force arrow is perpendicular to it. The directions match the stated situation.

Worked example

Distinguishing mass from weight

A student says, “The toolbox has a weight of 7.0 kg.” Correct the description using force and mass terminology.

Identify the quantity and unit
Kilograms measure mass, not force. The student has used the word “weight” with a mass unit.
Rewrite the statement
Use “mass” with kilograms. Use “weight” for the force of gravity, which is measured in newtons. The problem gives no value for that force, so do not invent one.

Answer: The toolbox has a mass of 7.0 kg. Its weight is the force of gravity acting on it and would be described in newtons.

Check: The corrected sentence matches each quantity to its proper unit and does not claim an unsupported numerical weight.

Worked example

Describing a book on a sloped surface

A book rests on a sloped surface. Describe the direction of the normal force and gravity on the book. Do not assume that the normal force points upward.

Name the system
The book is the system. Describe forces acting on the book, rather than forces the book exerts on the surface.
Describe gravity
Near Earth, gravity on the book points toward Earth, which is downward in the usual description.
Describe the normal force
The sloped surface exerts a normal force at a right angle to the surface. Since the surface is sloped, this force is not straight upward.

Answer: Gravity on the book points downward toward Earth. The surface’s normal force on the book points perpendicular to the slope and away from the surface.

Check: The directions distinguish gravity from the normal force. The normal force is perpendicular to the sloped surface, as required by its definition.

Common mistakes and how to avoid them

Using mass and weight as if they mean the same thing.

Correction: Mass describes how much matter an object has and is measured in kilograms. Weight is the force of gravity and is measured in newtons.

Assuming the normal force always points upward.

Correction: The normal force is perpendicular to the touching surface. Its direction depends on the surface’s orientation.

Assuming friction always points opposite the object’s overall motion.

Correction: Friction opposes sliding or the tendency to slide between the touching surfaces. Describe the surfaces’ relative sliding.

Drawing a force that the system exerts on another object as though it acts on the system.

Correction: A free-body diagram includes only forces acting on the selected system. Name the object receiving each force.

Leaving the direction out of a force description.

Correction: State the direction in words or show it with a labelled arrow.

Lesson summary

Mass describes how much matter an object has and is measured in kg.
Force is a push or pull and is measured in N.
Acceleration describes a change in velocity and is measured in m/s².
Gravity pulls objects toward Earth; weight is the force of gravity on an object.
A normal force is exerted by a touching surface at a right angle to that surface.
Friction acts along touching surfaces and opposes sliding or the tendency to slide.

Check your understanding

Question 1

Which unit is used for mass?

Kilogram (kg)
Newton (N)
Metre per second squared (m/s²)
Metre (m)

Show answer and explanation

Kilogram (kg)

Mass is measured in kilograms. Newtons measure force, and metres per second squared measure acceleration.

Question 2

A box slides left across a floor. In the usual description of friction opposing the sliding between the box and floor, which direction does friction on the box point?

Left, along the floor
Right, along the floor
Upward, perpendicular to the floor
Downward, perpendicular to the floor

Show answer and explanation

Right, along the floor

Friction acts along the touching surfaces and opposes their relative sliding. For the box sliding left, friction on it points right.

Question 3

A book rests on a sloped surface. Which statement describes the normal force correctly?

It points straight upward in every situation.
It points toward Earth, just like gravity.
It is perpendicular to the sloped surface.
It acts along the slope and is the same as friction.

Show answer and explanation

It is perpendicular to the sloped surface.

The normal force is exerted by the surface and acts at a right angle to that surface. Friction acts along the surface.

Key terms

Acceleration: A change in velocity over time; it describes a change in an object’s motion.
Force: A push or pull on an object, measured in newtons.
Friction: A contact force along touching surfaces that opposes sliding or the tendency to slide.
Gravity: An attractive force between objects that have mass; near Earth, it pulls objects toward Earth.
Mass: A measure of how much matter an object has, measured in kilograms.
Normal force: A contact force exerted by a surface on an object, perpendicular to the surface.
System: The object or group of objects being described.
Weight: The force of gravity acting on an object, measured in newtons.

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About this lesson and its review

Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Physics (SPH3U), expectation C2.1. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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