Force & Motion Vocabulary
Use balanced forces, unbalanced forces, resultant force, acceleration, deceleration, speed, moment, lever, fulcrum and mechanical advantage to explain and predict how forces affect an object's movement and turning.
What a learner can do afterwards
- Distinguish between balanced and unbalanced forces and describe the effect of each on an object's motion
- Use 'resultant force' correctly when describing the net effect of two or more forces acting on an object
- Apply 'moment', 'lever', and 'fulcrum' correctly when describing how simple machines work
1 · Read
A force is a push or pull with a size and a direction. Forces on one object are balanced when their resultant is zero. For straight-line movement, balanced forces keep motion unchanged: a resting object stays at rest, and a moving object keeps the same speed and direction. Forces can still be acting.
The resultant force combines all forces on one object, including their directions. It can be zero. A nonzero resultant means unbalanced forces. Force is measured in newtons (N). On one line, forces pointing the same way add: 6 N right + 2 N right = 8 N right. Opposite forces subtract: 6 N right and 2 N left leave 4 N right.
Speed tells how fast an object moves. A speed of 2 metres per second (2 m/s) means 2 metres each second. An object accelerates when its speed or direction changes. Slowing down is also acceleration, often called deceleration. A resultant along its movement speeds it up. One against its movement slows it while it keeps moving that way. A sideways resultant can change its direction.
A lever is a stiff bar turning around a pivot called the fulcrum. A moment, or torque, is a force's turning effect. A downward push on a level lever gives a larger moment with more force or more distance from the fulcrum.
A resting, level seesaw balances when opposite moments are equal. A heavier friend can balance a lighter friend by sitting closer to the fulcrum. Ignore the bar's weight and pivot friction here.
Effort is your applied force; output force is the lever's force on the load. Mechanical advantage compares these force sizes. Keep the load and fulcrum fixed on a level lever: pushing downward farther out needs less effort. Not every lever increases force.
Balanced forces have a zero resultant. Unbalanced forces change movement. A lever turns around its fulcrum. A moment is a force's turning effect.
This lesson, its questions and its flashcards were written for this learning dataset.
The readings, questions and flashcards were written for this learning dataset.
More to read
Keeping track of force directions
Choose one object before combining forces. A push on a cart and a push on a different cart do not belong in the same total.
For pushes along one line, record both size and direction. A 10 N push right and a 6 N push left leave 4 N right. Six newtons of each push cancel. Pushes pointing the same way add instead.
The total is called the resultant force. When it is zero, the forces are balanced. A cart already rolling along a straight track can keep its speed while forces are balanced. It does not have to stop.
Naming the parts of a lever
On a seesaw, the long bar is the lever. The support it turns around is the fulcrum. These words name different parts.
Your push or pull on a lever is the effort force. The force the lever applies to its load is the output force. Mechanical advantage compares output force with effort force.
A lever can increase force, but some levers do not. For a level lever with an unchanged load and fulcrum, moving a downward effort farther from the fulcrum reduces the effort needed to balance the load.
Why position matters on a seesaw
A person's weight pulls downward on a seesaw. That force has a turning effect around the fulcrum. The name for the turning effect is moment. Torque is another name for it.
For a level seesaw, the same weight makes a larger moment farther from the fulcrum. A heavier person can therefore balance a lighter person by sitting closer to the middle.
For the seesaw to stay level at rest, the moments turning it opposite ways must match. This explanation leaves out the bar's own weight and friction at the pivot. Comparing the two people's weights alone would miss the effect of their positions.
Changes in speed and direction
A speed tells how fast something moves. Three metres per second means covering three metres each second.
An object accelerates when its speed or direction changes. Speeding up is acceleration. Slowing down is also acceleration and is often called deceleration. Turning at an unchanged speed is acceleration too, because the direction changes.
To predict a change, compare the resultant force with the object's movement. A resultant in the same direction speeds it up. A resultant against its movement slows it while it continues moving that way. A sideways resultant can turn its path.
2 · Watch
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