Everyday objects often have several forces acting on them at the same time.
A book on a table experiences gravity and a support force.
A box being pushed across a floor may experience gravity, support, an applied push and friction.
In this lesson you will practise identifying those forces and representing them with simple force diagrams.
Goal: choose one object, identify the important forces acting on it, describe their directions and decide whether the forces balance or produce a nonzero net force.
Before you start
Review Forces and Motion in Everyday Life.
You should already understand that a force is a push or pull and that several forces can act on one object at the same time.
Materials
You will need:
- a notebook or worksheet
- a pencil
- a ruler if you want straight arrows
- several ordinary objects to observe
Suitable objects include a book, small box, ball, bag or toy car.
What is a force diagram?
A force diagram is a simple drawing that shows the important forces acting on one chosen object.
The object can be represented by a box, dot or simple shape.
Arrows represent forces.
An arrow tells us:
- which direction the force acts
- which force is being represented
- sometimes the relative strength of the force
Start with one object
This is the most important rule.
Before identifying forces, ask:
Which object am I analysing?
If you choose a book, include forces acting on the book.
Do not mix forces acting on the table, Earth or your hand into the same diagram unless they are forces that those objects exert on the book.
Example: A book resting on a table
Choose the book as the object.
Two important vertical forces act on it.
Gravity:
Earth pulls the book downward.
Support force:
The table pushes the book upward.
If the book remains at rest, these vertical forces balance.
How could you draw that?
Draw a simple box to represent the book.
Draw one arrow downward.
Label it:
gravitational force on the book
Draw one arrow upward.
Label it:
support force from the table on the book
If the forces are equal in strength, the arrows can be shown with equal lengths.
Why name the object in the label?
A label such as gravity is useful.
But a more complete label such as:
gravitational force on the book
helps remind you which object experiences the force.
This becomes especially useful when several objects interact.
Step 1: Choose the object
For every situation, begin by naming the object you want to study.
Examples:
- the book
- the ball
- the bicycle
- the box
- the person
- the toy car
Do not begin by listing every force in the whole scene.
Step 2: Ask what is touching the object
Objects in contact can exert forces.
Ask:
- Is a surface supporting it?
- Is a hand pushing it?
- Is a rope pulling it?
- Is another object pressing against it?
- Is friction important?
These questions help identify contact forces.
Step 3: Ask about forces that act without direct contact
Gravity can act without direct surface contact.
Magnetic forces can also act across a distance.
For many everyday force diagrams near Earth's surface, gravity is an important force to consider.
Step 4: Decide the direction of each force
Every force has a direction.
For example:
- gravity near Earth's surface acts downward
- a table can push upward on a book
- a hand may push a box to the right
- friction may oppose relative sliding
- a rope pulls along its length
The direction depends on the actual situation.
Step 5: Draw an arrow for each important force
Draw the arrow beginning at or near the object.
Point the arrow in the direction of the force.
Label it clearly.
You do not need a detailed picture of the object.
The purpose is to show forces, not create artwork.
Step 6: Compare opposite directions
Once the forces are drawn, ask:
Do forces in opposite directions balance?
If the combined effect is zero, the net force is zero.
If one direction has the greater combined force, there is a nonzero net force.
Practice 1: Book on a table
Object:
the book
Important forces:
- gravity downward
- support force upward
If the book remains at rest, the two vertical forces balance.
Net force:
zero
Practice 2: A hanging bag
Imagine a bag hanging motionless from a strong strap.
Choose the bag as the object.
Important forces include:
- gravity downward
- tension from the strap upward
If the bag remains still, the forces balance.
What is tension?
Tension is a pulling force transmitted through something such as a rope, cord or strap when it is pulled tight.
The strap pulls upward on the hanging bag.
The force acts along the direction of the strap.
Practice 3: A box being pushed
Imagine a box sliding to the right across a floor while a person pushes it.
Choose the box as the object.
Possible forces include:
- gravity downward
- support force upward
- applied push to the right
- friction to the left
If the applied push is stronger than the opposing friction, there is a net force to the right.
The box accelerates to the right.
Motion direction and force direction are different ideas
An object can move to the right while the net force points left.
In that situation, the object would slow down while still moving to the right for some time.
Do not automatically draw every force in the direction of motion.
Practice 4: A sliding book slowing down
A book slides to the right across a table after your hand is no longer touching it.
Choose the book as the object.
Important forces include:
- gravity downward
- support force upward
- friction to the left
There is no longer an applied force from your hand.
The horizontal net force points left.
The book slows while it is still moving to the right.
A common mistake: Drawing a force because an object is moving
Motion itself is not a force.
If a ball is moving to the right, you do not automatically draw a force called:
motion force
Instead, identify actual interactions acting on the ball.
Practice 5: A falling ball
Imagine a ball falling through the air.
Choose the ball as the object.
Important forces may include:
- gravity downward
- air resistance upward
If gravity is stronger than air resistance, the net force is downward.
What is air resistance?
Air resistance is a force caused by interaction between an object and the air through which it moves.
It often acts opposite the object's motion relative to the air.
Its size depends on factors such as speed, shape and surface area.
Can air resistance become important?
Yes.
For a slowly moving dense object, it may be relatively small.
For a parachute or rapidly moving object, it can become very important.
Practice 6: A person standing still
Choose the person as the object.
Important vertical forces include:
- gravity downward
- support force from the floor upward
If the person remains still, the forces balance.
Do not confuse the force on the person with the force on the floor
The floor pushes upward on the person.
The person also pushes downward on the floor.
These forces belong to an interaction pair, but they act on different objects.
If your diagram is for the person, show the force from the floor on the person.
Practice 7: A person jumping
While a person pushes against the ground during a jump, the ground exerts an upward force on the person.
Choose the person as the object.
Important forces may include:
- gravity downward
- support force from the ground upward while contact continues
If the upward force is greater than the downward gravitational force, the net force is upward.
What happens after the feet leave the ground?
Once the person is no longer touching the ground, the support force disappears.
Gravity still acts downward.
Air resistance may also act.
The force diagram therefore changes when the interaction changes.
Force diagrams describe a particular moment
A situation can change over time.
A ball while being kicked has a different set of contact forces from the same ball after it leaves the foot.
A person standing on the ground has a different force diagram from the same person after jumping into the air.
Always identify the moment you are analysing.
Practice 8: A ball while it is being kicked
During contact with the foot, the ball experiences a force from the foot.
Gravity also acts downward.
The ground may exert forces if the ball is still touching it.
After the ball leaves the foot, the force from the foot no longer acts.
A common mistake: Keeping a contact force after contact ends
A hand cannot continue pushing an object after the hand is no longer touching it unless another interaction provides the force.
The object may continue moving because of inertia.
Continued motion does not prove that the original push is still acting.
Practice 9: A toy car on a level floor
Imagine a toy car moving to the right after being released.
Choose the car as the object.
Important forces might include:
- gravity downward
- support force upward
- rolling resistance and friction opposing motion
- air resistance if significant
If resistance produces a net force opposite the motion, the car slows.
Practice 10: A bicycle moving at constant speed
Imagine a bicycle travelling along a level road at constant speed in a straight line.
The rider continues pedalling.
Why can there still be zero net force?
The forward driving effects can balance the backward resistive forces.
Constant velocity means the net force is zero.
Balanced forces do not mean there are no forces
This idea is worth repeating.
Balanced forces mean the combined force is zero.
Several real forces can still be acting.
A stationary book and a bicycle moving at constant velocity can both have zero net force.
Vertical and horizontal forces can be considered separately
For many simple situations, it helps to examine forces by direction.
For a sliding box:
Vertical:
- gravity downward
- support force upward
These may balance.
Horizontal:
- push to the right
- friction to the left
These may balance or may produce a horizontal net force.
Arrow length can represent relative force strength
If your diagram uses arrow length to represent force strength, keep the scale consistent.
Equal forces should have arrows of equal length.
A stronger force should have a longer arrow.
If you are not comparing force strengths, clear direction and labels are more important than exact arrow length.
Do force arrows show the path of an object?
No.
A force arrow shows the direction of a force.
It does not show the route travelled by the object.
A motion path and a force diagram answer different questions.
Gravity and weight
Near Earth's surface, gravity acts downward on ordinary objects.
The gravitational force acting on an object is its weight.
In a force diagram you may label this:
gravitational force
or:
weight
Use one clear term consistently.
The support force is not always equal to weight
For a book resting quietly on a level table, the upward support force can equal the downward weight.
But this is not automatically true in every situation.
If an object is accelerating vertically or several other vertical forces act, the support force may differ.
Do not assume equality without considering the motion and all forces.
Friction needs a contact interaction
Friction acts between interacting surfaces.
Its direction depends on the relative motion or tendency to slide.
Do not automatically add friction to every diagram.
First ask whether a relevant surface interaction exists.
Applied force
An applied force is a useful general description for a push or pull exerted by a person or object.
For example:
applied push from the hand on the box
This makes both the interaction and chosen object clear.
Tension in a rope
If a rope pulls an object, the force from the rope acts along the rope.
For a hanging object, tension may point upward.
For an object being pulled across a floor, the rope may pull horizontally or at an angle.
Magnetic force
A magnet can exert a force on another magnet or suitable magnetic material without touching it.
If magnetism is important in a situation, include the magnetic force in the diagram.
The arrow should show the direction of the force acting on your chosen object.
Force diagram example: Magnet attracting a metal object
Choose the metal object.
Possible forces include:
- gravity downward
- support force upward if it rests on a surface
- magnetic force toward the magnet
- friction if the surface resists movement
The horizontal net force depends on the relative strengths of magnetic force and friction.
Force diagram example: Pulling a sled or cart
Choose the sled or cart.
Possible forces include:
- gravity downward
- support force upward
- tension or applied pull in the rope direction
- friction or resistance opposing motion
If the pull is angled upward, part of the force is upward as well as forward.
At this level, correctly identifying the direction is more important than calculating components.
Interaction pairs need two objects
Remember Newton's third law.
If object A exerts a force on object B, object B exerts an equal strength force in the opposite direction on object A.
But those forces act on different objects.
Only include the force acting on your chosen object in that object's diagram.
Example: Hand and box
Your hand pushes the box to the right.
The box pushes your hand to the left.
If you are drawing the box:
show the force from the hand on the box
If you are drawing the hand:
show the force from the box on the hand
Do not place both forces on the box diagram.
A useful force identification checklist
For your chosen object, ask:
- Does gravity act on it?
- Is a surface supporting it?
- Is anything pushing or pulling it?
- Is a rope, string or strap pulling it?
- Is friction important?
- Is air resistance important?
- Is a magnetic interaction important?
Not every situation includes every force.
Common mistake 1: Drawing forces that act on another object
If your diagram is for a book, every force arrow should represent a force on the book.
The book's downward push on the table belongs on a diagram of the table, not on the book.
Common mistake 2: Drawing velocity as a force
Velocity is not a force.
A right pointing velocity does not mean a right pointing force must exist.
An object can move right while the net force is zero or even while the net force points left.
Common mistake 3: Assuming every moving object has an applied push
After a push ends, an object can continue moving because of inertia.
Only draw an applied push while the interaction actually exists.
Common mistake 4: Forgetting the support force
An object resting on a surface usually experiences a contact force from that surface.
For a horizontal table supporting a book, that force points upward.
Common mistake 5: Forgetting gravity
Near Earth's surface, gravity acts on ordinary objects whether they are moving or stationary.
Do not omit gravity simply because the object is supported.
Common mistake 6: Drawing friction in the wrong direction automatically
Friction does not simply mean left or backward.
Its direction depends on relative slipping or the tendency to slip between surfaces.
Think about the actual interaction.
Common mistake 7: Making all arrows equal
Equal arrow lengths should represent equal force strengths only when your drawing uses arrow length for strength.
If the object is accelerating, opposing forces should not necessarily be shown as equal.
Practice challenge 1
A lamp hangs motionless from the ceiling by a cord.
Choose the lamp.
Which two main forces act?
Gravity downward and tension upward.
Are they balanced?
Yes, if the lamp remains motionless.
Practice challenge 2
A child pushes a box to the right and the box speeds up.
Choose the box.
Which horizontal force is greater?
The total rightward force must be greater than the opposing horizontal force because the box accelerates to the right.
Practice challenge 3
A ball has been thrown upward and is no longer touching the hand.
What force should not appear in the diagram?
The force from the hand should not appear because contact has ended.
Gravity continues acting downward.
Practice challenge 4
A toy car moves right but slows down.
Can the net force point left?
Yes.
A leftward net force can reduce the car's rightward velocity.
Practice challenge 5
A person stands still on the floor.
Is the upward force from the floor an interaction pair with gravity?
No.
Those two forces both act on the person and can balance.
The interaction partner of the floor's upward force on the person is the person's downward force on the floor.
Practice challenge 6
A bicycle moves straight at constant speed.
What does this tell you about its net force?
The net force is zero.
Forward and backward effects balance, and vertical forces balance.
Build your own force diagram
Choose one ordinary situation:
- a book on a desk
- a bag hanging from a strap
- a ball falling
- a box being pushed
- a toy car slowing
- a person standing
Then complete these steps.
- Name the object you are analysing.
- List the interactions involving that object.
- Identify the forces acting on it.
- Decide the direction of every force.
- Draw and label one arrow for each force.
- Compare forces in opposite directions.
- Decide whether the net force is zero or nonzero.
- Use the net force to describe whether the object's velocity should change.
Explain your diagram in words
After drawing the diagram, write a short explanation.
For example:
The box experiences gravity downward and a support force upward. These vertical forces balance. A hand pushes the box to the right while friction acts to the left. The push is greater than friction, so the box has a net force to the right and accelerates to the right.
A written explanation helps show that you understand what the arrows mean.
Final review checklist
Before finishing a force diagram, check:
- Did I choose one object?
- Does every arrow represent a real force on that object?
- Did I include gravity where relevant?
- Did I include important contact forces?
- Do the arrows point in the correct directions?
- Are the labels clear?
- Did I avoid drawing motion as a force?
- Did I remove forces from interactions that have ended?
- Did I compare opposite forces correctly?
- Can I explain whether the net force is zero or nonzero?
The main idea
A useful force diagram begins by choosing one object.
Identify the real interactions involving that object and draw arrows for the forces acting on it.
Gravity, support, friction, tension, applied forces, air resistance and magnetic forces are examples that may appear depending on the situation.
Do not confuse motion with force, and do not mix forces acting on different objects.
Once the forces are identified, compare their directions and strengths to decide whether they balance.
A zero net force means the object's velocity does not change.
A nonzero net force means the object accelerates.
Force diagrams turn everyday motion into a clear scientific picture of the interactions that cause changes in motion.
Continue learning
Review Forces and Motion in Everyday Life whenever you need to revisit net force, friction, gravity, inertia or Newton's laws.