Everyday “work” and physical “work” have different meanings. In physics, work is defined using force and distance.
Basic Knowledge
Work W: The work done by a force on an object when a force is applied and the object moves in the direction of that force.Work W [J] = force F [N] × distance s [m]
The unit is the joule(J). 1J = 1N × 1m.
Important conventions:
・Lifting an object: Work is done
・Holding an object(keeping it still): Work is zero(physically)
・Moving perpendicular to the direction of the force: Work is zero(the supporting normal force does no work)
Power P: The amount of work done per second.Power P [W] = work W [J] / time t [s]
The unit is the watt(W). It is the same unit as electrical power because the rate of electrical work is also measured this way.
Work W（Force × distance. Unit J (joule)）
Joule (J)（Unit of work・energy. 1J = 1N・m）
Power P（Work per unit of time. Unit W(watt)）
Watt (W)（Unit of power・electric power. 1W = 1J/second）
Principle of work（The total amount of work does not change even when a tool is used）
Levers・pulleys・inclined planes（Tools that perform the same work with a smaller force(force ×, distance ○))
Further Study (Background・Significance)
Principle of work: Tools such as pulleys, levers, and inclined planes reduce the applied force, but the distance moved becomes longer, so the total amount of work does not change. This expresses the energy-conservation idea that “nothing becomes easier for free.”
Example: When lifting a 10kg object(weight about 100N) by 1m,
・Directly: 100N × 1m = 100J
・Inclined plane(length 2m): 50N × 2m = 100J(force is half, distance is twice, work is the same)
・One movable pulley: 50N × 2m = 100J(same as above)
Examples of power:
・Work of a person lifting 1500N by 2m in 1 minute → 3000J in 60 seconds → 50W
・Steady power of an adult about 70〜100W
・Power tools several hundred W〜several kW
Work and power are preliminary concepts for energy. They are studied in greater depth in the next lesson on mechanical energy.
- Work W = force × distance [J]
- 1 J = 1 N × 1 m
- Simply holding an object is physically zero work
- Power P = work / time [W]
- 1 W = 1 J/second
- Principle of work: Total work does not change when a tool is used
- Levers・pulleys・inclined planes = smaller force, greater distance
Points to Note (Common Confusions)
① Physical work means “force × distance moved in the direction of the force.” It differs from everyday “work.” ② Merely supporting an object(when it does not move) is zero work. Note the difference from feeling tired in real life. ③ Work rate W and electrical power W have the same unit(both are J/s). ④ Principle of work: Even if the task becomes easier, there is no gain in total work(related to energy conservation).
Practice
- Write the formula for finding work.
- Find the work done when an object is moved 5m by a force of 10N.
- Briefly explain the “principle of work.”