Types of Forces

A force changes an object's shape or state of motion. It has magnitude and direction, drawn as an arrow, and is measured in newtons (N). Contact forces need touch (friction, elasticity); non-contact forces act at a distance (gravity, magnetic, electric). Slide the magnitude control to lengthen the arrow and compare contact with non-contact forces.

What is a Force?

Magnitude5 N

💡 Forces are Represented by Arrows

①A force has magnitude and direction

②Arrow length = force magnitude

③Arrow direction = force direction

④Multiple forces can act on one object simultaneously

Contact vs Non-contact Forces

🔍 Types of Forces

①Contact force: requires direct contact (friction, elasticity, tension)

②Non-contact force: acts at a distance (gravity, magnetic, electric)

③Gravity is Earth's pull on an object

④Opposite magnetic poles attract; like poles repel

Magnitude and Units

Unit of Force

1 N ≈ gravitational force on a 100 g object

Newton (N): the SI unit of force (named after Isaac Newton)

Representing Force

Force = magnitude + direction (arrow)

Vector quantity: has both magnitude and direction

📐 Measuring Force

①Spring scale measures force magnitude

②Unit: N (newton)

③1 N ≈ weight of a 100 g object

④Arrow start = point of application

Exam Key Points

Core Summary

Force → changes shape or motion state of an object

Forces deform objects or change their speed/direction

🎯 Exam Key Points

①Three elements: magnitude, direction, point of application

②Contact: friction, elasticity / Non-contact: gravity, magnetic, electric

③Unit: N (newton)

④Balanced forces → rest or uniform motion

⑤Longer arrow = greater force

Examples and Unit Test

Example 1

Among gravity, friction, and magnetic force, which are non-contact forces that act even without direct touch?

Contact forces act only when objects touch (friction, elasticity), while non-contact forces act from a distance (gravity, magnetic, electric force).

Gravity and magnetic force act from a distance and are non-contact forces, while friction is a contact force.

▸ Gravity and magnetic force (non-contact); friction is a contact force

Gravity, magnetic, and electric forces are non-contact; friction and elasticity are contact forces.

Example 2

About how many newtons is the gravity on an object of about 100 g, and what does the length of a force arrow represent?

The unit of force is the newton (N), and 1 N is about the gravity on a 100 g object.

So the gravity on a 100 g object is about 1 N. The length of a force arrow shows the size of the force.

▸ About 1 N / arrow length = size of the force

1 N is about the weight of 100 g, and the arrow length shows the size of the force.

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

Magnetic force acts even when magnets are apart, so it is a non-contact force; ② is correct.

Gravity is a non-contact force (①), the unit of force is N (③), a force is a vector with size and direction (④), and a longer arrow shows a larger force (⑤).

Gravity & Elasticity

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