Kick a ball and it flies off; pull a rubber band and it stretches. Whatever changes an object's motion or shape like this is called a force.
A force has a size and a direction, so it is drawn as an arrow: the starting point is where the force acts (the point of application), the length is the size, the direction is the direction of the force, and size is measured in newtons (N).
Some forces, like friction or the push of the floor, need two objects to touch, while others, like gravity and magnetic force, act even across a gap.
All four of these forces act on the iron box in the picture at once, so as you change how hard the magnet pulls you can compare which arrows change and which stay the same.
Drawing Forces as Arrows
Even a single box sitting on the floor has several forces acting on it at once. The Earth pulls it down, the floor pushes it up, a nearby magnet pulls it sideways, and the floor resists that pull with friction. Drawing all of them as arrows in one picture shows at a glance which force balances which, and why the box does not move.
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Four forces on an iron box on the floor: arrow length is size, the dot is the point of application
3 N
In the picture every newton is drawn with the same length. The red gravity arrow and the blue normal force arrow are both 10 N, so they are equally long and point opposite ways. Raise the magnet's pull from 0 N to 5 N and the green magnetic arrow and the orange friction arrow grow together; while the box stays still they are always the same length. The dot at the start of each arrow is the point of application: gravity and magnetic force act at the middle of the box, while the normal force and friction from the floor act at the face that touches the floor.
💡 Three Things an Arrow Tells You
①The length of the arrow is the size of the force; within one picture 1 N must always have the same length, or sizes cannot be compared.
②The way the arrow points is the direction of the force.
③The point where the arrow starts is the point of application. Size, direction and point of application are the three elements of a force.
④Several forces can act on one object at the same time, and if two opposite forces are also equal in size, the object's motion does not change.
Forces That Need Contact and Forces That Don't
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Contact forces (friction, elastic) and non-contact forces (gravity, magnetic, electric)
It is easy to think gravity needs contact, but a ball thrown into the air still falls, so gravity acts without touching the Earth. Many people also think friction appears only when something is moving, yet as the picture shows, a box that stays still feels friction equal to the pull on it. When the pull is 0, the friction is 0 too.
🔍 How Forces Are Sorted
①Contact forces arise only when two objects touch: friction, elastic force and the push of the floor.
②Non-contact forces act across a gap: gravity, magnetic force and electric force.
③Gravity is the Earth pulling an object toward its center, so it always points down.
④Magnets pull opposite poles together and push like poles apart, and iron is attracted to a magnet.
Size and Unit of Force
Unit of force
1 N ≈ the gravity on an object of mass 100 g
The newton (N) is the unit of force, named after Isaac Newton, who set out the laws of motion.
Describing a force
force = size + direction + point of application (arrow)
Size alone cannot describe a force, so the direction and point of application are drawn as well.
📐 Measuring a Force
①Force is measured with a spring scale; the stretch of the spring is proportional to the force.
②The unit is the newton (N).
③An object of mass 100 g hung on a spring scale reads about 1 N.
④When drawing arrows, first decide how many centimeters stand for 1 N and keep that ratio throughout.
Exam Key Points
The up-down pair and the left-right pair matching so the box stays still is called balanced forces. The next chapter looks more closely at gravity and elastic force and shows how a spring's stretch measures the size of a force. How far friction holds before it lets the box go is taken up in the chapter on friction.
Summary
force → changes an object's shape or its motion (speed, direction)
A stretched rubber band has changed shape; a rolling ball that stops or turns has changed its motion.
🎯 Exam Points
①The three elements of a force are size, direction and point of application.
②Contact forces include friction and elastic force; non-contact forces are gravity, magnetic force and electric force.
③The unit of force is the newton, and the gravity on an object of mass 100 g is about 1 N.
④If two equal forces act in opposite directions, the object's motion does not change.
⑤A longer arrow means a larger force.
Examples and Unit Test
For a force problem, first choose one object and list the forces acting on it one by one. For each force, check what exerts it, whether it needs contact or works at a distance, and which way it points; then draw arrows with lengths matched to the sizes, and you can handle both sorting contact from non-contact forces and comparing sizes.
Example 1
Of gravity, friction and magnetic force, which act on an object without direct contact?
1
Contact forces arise only when objects touch (friction, elastic force); non-contact forces act across a gap (gravity, magnetic force, electric force).
2
Gravity acts on a falling ball and magnetic force pulls iron from a distance, so both are non-contact forces. Friction comes from surfaces pressing together, so it is a contact force.
▸ Gravity and magnetic force (non-contact) / friction is a contact force
Imagine opening a gap between the two objects; if the force is still there, it is a non-contact force.
Example 2
About how many newtons of gravity act on an object of mass about 100 g, and what does the length of a force arrow mean?
1
The gravity on an object of mass 100 g is about 1 N; hung on a spring scale it reads close to 1 N.
2
The length of a force arrow stands for the size of the force; you fix the length for 1 N and draw multiples of it.
▸ About 1 N / arrow length = size of the force
A mass of 1 kg (1000 g) weighs about 10 N; the 10 N iron box in the picture is a 1 kg box.
Unit-test style
Which statement about types of forces is correct?
①Gravity is a contact force that acts only on direct touch
②Magnetic force is a non-contact force that acts from a distance
③The unit of force is cm
④A force has only size and no direction
⑤The shorter the arrow, the larger the force
▸ ② Magnetic force is a non-contact force that acts from a distance
1
Magnetic force acts even when the magnet is some distance away, so it is a non-contact force; ② is correct.
2
Gravity is a non-contact force (①), the unit of force is the newton (③), a force has both size and direction (④), and a longer arrow means a larger force (⑤).