P1.1: Introduction to forces

Lesson focus: are interactions represented by directed, scaled arrows.

Learning objective

Identify forces, draw and calculate simple resultants.

You will be successful when you can…

Name contact and non-contact forces.
Draw arrows on one chosen object.
Use arrow direction and relative length.
Calculate a one-dimensional resultant.

KEY VOCABULARY

Words that carry the science

force: A push or pull caused by an interaction.
newton: SI unit of force, symbol N.
force diagram: Diagram showing forces acting on one chosen object.
resultant force: Single force with the same overall effect as all forces combined.
contact force: Force requiring interacting surfaces or materials to touch.

BUILD YOUR UNDERSTANDING

Represent the interaction before predicting motion

01Name the interaction

Contact examples include friction, drag, tension and normal force. Non-contact examples include gravitational, magnetic and electrostatic .

02Draw on one object

Each arrow begins on the chosen object, points in force direction and is labelled. Longer arrows represent larger forces on the same scale.

03Zero does not mean none

Equal opposite forces sum to zero. The object can remain still or continue at constant velocity while forces still act.

Figure · Draw every arrow from the object being analysed. Arrow direction shows force direction and arrow length represents magnitude; forces are measured in newtons.

Interactive model

Force arrows and the resultant force

Forces are vectors: they have a size and a direction. Add them tip to tail to find the resultant force. A zero resultant means the object keeps doing what it was doing; otherwise it speeds up in the direction of the resultant.

Controls

Forces

In a lineAt an angle

F₁ (to the right)12 N

F₂8 N

F₂ points

Opposite waySame way

Resultant force4.0 N at 0°

Speed of the trolley0.0 m/s

Trolley mass 4 kg, no friction (illustrative). Reset to stop the trolley: balanced forces will not stop it.

WORKED EXAMPLE

Find a resultant

A trolley has 60 N drive right and 30 N drag left.

Choose right as positive.
Write +60 N and −30 N.
Add: 60 − 30 = 30 N.
Resultant is 30 N to the right.

How to make the reasoning visible: A resultant needs both magnitude and direction.

LEARN BY DOING

Force-diagram audit

All arrows refer to the named object.

Object	Forces	Resultant	Motion evidence
Book on table	20 N up; 20 N down	0 N	No acceleration
Trolley	60 N right; 30 N left	30 N right	Accelerates right
Hanging lamp	15 N tension up; 15 N weight down	0 N	Stationary
Ball falling	8 N down; 3 N drag up	5 N down	Accelerates downward

Can a moving object have zero resultant?
Your answer
Why must arrows use one scale?
Your answer
Use the table. Work out the resultant force on the trolley and its direction.
Your answer

RETRIEVE · APPLY · CHECK

Check your understanding

Answer from memory, then check against the suggested answer.

What is force?

What is newton?

What is force diagram?

What is a contact force? Give an example.

What does a longer arrow on a force diagram show?

Apply the idea

A cyclist travels straight at constant speed. What can you infer?

Stretch and challenge

Why can a wall exert force on a stationary person pushing it?

A book rests on a table. Name the two forces acting on it and explain why it does not move.

Exam Question Practice

P1.1Calculate4 marks

Exam Question Practice

Mark points shown after marking

A box is pushed with a force of 45 N to the right. A force of 18 N acts on it to the left.

(a)

Calculate the size of the resultant force.

(2)

AnswerN

(b)

State the direction of the resultant force and predict the change in the motion of the box.

(2)

Total for this question: 4 marks

Command word: State magnitude, unit and direction before describing motion.