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).
Change the magnitude and the arrow lengthens; contact and non-contact sit side by side.
What is a Force?
When several actions overlap on one box, memorizing names does not decide which arrow to attach to which face. A downward pull and an upward brace through a surface can sit together; you have to sort first whether each action needs a shared face before you can compare their sizes on one drawing. Matching the unit to the newton (N) is for that comparison, not for decorating a label. Hold onto arrow-reading and the shared-face question first, and later you can judge balance with the same symbols.
seegongsik.com
Forces on an object — magnitude and direction shown by arrows
5 N
The first picture has a box in the middle, with arrows leaving the bottom, top, left, and right faces. The slider runs from 1 N to 10 N; raising it lengthens all four arrows in the same ratio and updates only the number at the lower left. The second picture is split by a dashed line: a box on a bar with an arrow to the left, and on the right a small circle above a globe with a dashed line between them. That second picture has no slider, so the lengths and the captions stay put.
💡 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
⑤Rest or uniform motion under balanced forces is in the next chapter
Contact vs Non-contact Forces
seegongsik.com
Contact (friction, elasticity) vs non-contact (gravity, magnetic, electric)
A common slip is to write a downward pull as the same kind of touch as a hand on a string. Empty space in between does not cancel that pull, so a touch-only split leaves no place for the downward arrow. If you treat force as having size only, the upward and downward arrows become the same number and you cannot judge balance. Reading a shorter arrow as a stronger force also reverses the length-for-size rule used here.
🔍 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
The arrow seats and the shared-face question you learn here are what you will use when Earth's pull and a spring are drawn on one object next. If those two arrows match in length and opposite direction, rest or steady motion is checked on that later drawing. Measuring size with a spring scale continues on the same unit, N. Next comes stacking a support through a face with a pull that remains even when the objects are apart, and if you drop the point of application you will have to hunt again for which face the spring sits on.
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
When a list of forces appears, first write one line for each: do the two objects have to share a face? If the question is only about size, match the unit to N and check that the arrow length is that number, not the class. If you skip which face the arrow starts from, the direction can be right and the attachment still wrong. Before you commit an answer, check that you did not put class and size in the same cell, and that the number at the lower left is the same as the arrow length.
Example 1
Among gravity, friction, and magnetic force, which are non-contact forces that act even without direct touch?
1
Contact forces act only when objects touch (friction, elasticity), while non-contact forces act from a distance (gravity, magnetic, electric force).
2
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?
1
The unit of force is the newton (N), and 1 N is about the gravity on a 100 g object.
2
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
1
Magnetic force acts even when magnets are apart, so it is a non-contact force; ② is correct.
2
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).