Forces and Free-Body Diagrams
Introduced| English | Meaning |
|---|---|
| force/fɔːs/ | An interaction that can change an object’s motion or shape. |
| free-body diagram/friː ˈbɒdi ˈdaɪəɡræm/ | A diagram showing all external forces acting on a selected object. |
| newton/ˈnjuːtn/ | The unit of force equal to one kilogram metre per second squared. |
| weight/weɪt/ | The gravitational force acting on an object. |
| normal force/ˈnɔːml fɔːs/ | A contact force exerted perpendicular to the surfaces in contact. |
| tension/ˈtenʃn/ | A pulling force transmitted through a stretched string, cable or similar object. |
| friction/ˈfrɪkʃn/ | A force that opposes relative motion between contacting surfaces. |
| net force/net fɔːs/ | The vector sum of all forces acting on an object. |
Every push and pull, on one clean diagram
- A book on a table is doing nothing — yet several forces act on it at once.
- Gravity pulls down; the table pushes up; a hand might shove it sideways.
- To reason clearly, physicists strip away everything but the forces 力 on that one object.
- The result is a free-body diagram 受力图 — the single most useful sketch in mechanics.
What a force is
- A force is a push or a pull — a vector, measured in newtons 牛顿 ($\text{N}$).
- It has a size and a direction, so forces add like vectors, not plain numbers.
- A force always comes from an interaction between two objects.
- $1\ \text{N}$ is roughly the weight of a small apple.
What is the SI unit of force?
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Force is measured in newtons ($\text{N}$). Joules measure energy, watts power, pascals pressure.
The usual suspects
- Weight 重量 $mg$ — gravity's pull, always straight down.
- Normal force 法向力 $N$ — a surface pushing perpendicular to itself.
- Tension 张力 $T$ — a pull along a rope or string.
- Friction 摩擦力 $f$ — a surface resisting sliding, parallel to the surface.
Which force is always perpendicular to the surface an object rests on?
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The normal force is defined as the surface's push perpendicular to itself. Friction runs parallel to the surface.
Drawing a free-body diagram
- Shrink the object to a dot and draw every force as an arrow from that dot.
- Include only forces acting on the object — never forces it exerts on others.
- Point each arrow the right way; longer arrow = larger force.
- Then the net force 合力 is the vector sum of them all.
Build a free-body diagram
Change the applied force, friction and mass, and watch the force arrows and the net force respond.
A free-body diagram should include the forces the object exerts on other objects.
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A free-body diagram shows only the forces acting on the chosen object, never the forces it exerts on others.
A block is pushed along a rough table. Select all forces that belong on its free-body diagram.
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All four forces act on the block. The force the block exerts on the table acts on the table, so it is left out.
Adding the forces up
- Add forces component by component, just like any vectors.
- If they balance, the net force is zero and the object keeps constant velocity.
- If they don't, the leftover net force sets the acceleration (next lessons).
- Always ask: what is the net force, and which way does it point?
A crate feels $10\ \text{N}$ right and $4\ \text{N}$ left. What is the net horizontal force, in $\text{N}$?
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Take right as positive: $10 - 4 = 6\ \text{N}$ to the right.
The net force on an object is the ____ sum of all the forces acting on it.
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Forces have direction, so they combine as a vector sum, not a plain arithmetic sum.
The normal force is not always equal to the weight. On a slope, or when something presses down or lifts up on the object, $N$ changes. Never assume $N = mg$ — work it out from the forces perpendicular to the surface.
A crate feels $F = 10\ \text{N}$ to the right and friction $f = 4\ \text{N}$ to the left.
- Horizontal net force $= 10 - 4 = 6\ \text{N}$, to the right.
- Vertically, $N$ and $mg$ balance, so the crate stays on the floor.
A force (in newtons) is a push or pull from an interaction. A free-body diagram shows every force acting on one object as arrows from a dot. The net force is their vector sum — and the normal force is not always $mg$.
Write from a net force of 9 N
- A crate feels 14 N to the right and 5 N to the left.
- Find the net horizontal force.
A crate feels 14 N to the right and 5 N to the left. What is the net horizontal force?
14 minus 5 is 9.
What is the net horizontal force?
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Rightward 14 N minus leftward 5 N is 9 N. Other options — 6 N (the lesson net force); 19 N (the two forces added).