Work–energy in SI units — meaning of one joule: Choose the statement that correctly defines one joule in the SI system.

Difficulty: Easy

Correct Answer: work is done by a force of 1 N when it displaces a body through 1 m

Explanation:


Introduction / Context:
Clarity on SI definitions avoids unit-conversion errors in energy, power, and torque problems. The joule (J) is the standard unit of work and energy in the International System of Units.


Given Data / Assumptions:

  • Work W = F * s when force and displacement are collinear.
  • SI base units: newton (N) for force, meter (m) for length.


Concept / Approach:
By definition, 1 joule equals the work done by a 1 newton force acting through 1 meter in the direction of the force. Hence, 1 J = 1 N·m. Non-SI definitions (dyne–cm) correspond to the CGS unit called the erg (1 erg = 1 dyne·cm = 1e−7 J).


Step-by-Step Solution:
Select the statement matching W = F * s with F = 1 N and s = 1 m.Compute W = 1 * 1 = 1 N·m = 1 J.Therefore, the correct choice is the SI definition with 1 N and 1 m.


Verification / Alternative check:
Dimensional check: J = kg·m^2/s^2. Since N = kg·m/s^2, multiplying by m yields kg·m^2/s^2, confirming consistency.


Why Other Options Are Wrong:

  • 1 kg and 1 g refer to mass, not force; SI force is the newton.
  • 1 dyne and 1 cm define the CGS unit (erg), not the SI joule.


Common Pitfalls:

  • Mixing energy (J) and torque (N·m). They share units but represent different physical quantities; context distinguishes them.


Final Answer:
work is done by a force of 1 N when it displaces a body through 1 m

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