What is Work in Physics?
In physics, work is done when a force moves an object through a distance. The formula is W = F?d?cos(?), where F is force, d is distance, and ? is the angle between force and displacement. Work is measured in joules (J). Importantly, if force is perpendicular to motion (? = 90 degrees), no work is done. For example, carrying a book horizontally does no work on the book because the upward force doesn't move it upward.
Positive, Negative, and Zero Work
Work can be positive, negative, or zero depending on the angle. When force and displacement are in the same direction (? = 0 degrees), work is positive. When they're opposite (? = 180 degrees), work is negative, meaning energy is removed from the object. Friction always does negative work. When force is perpendicular to displacement (? = 90 degrees), work is zero. The centripetal force in circular motion does no work because it's always perpendicular to motion.
The Work-Energy Theorem
The work-energy theorem states that net work done on an object equals its change in kinetic energy: W_net = ?KE. This powerful principle connects force, distance, and energy. When you push a car to get it moving, the work you do increases its kinetic energy. When brakes apply friction, negative work decreases kinetic energy. Understanding work is essential for analyzing machines, engines, and any system where forces cause motion.
Quick Tips
- Always verify units are consistent
- Use scientific notation for very large/small numbers
- Results are approximations — real conditions may vary
Frequently Asked Questions
Work is the dot product of force and displacement vectors, which results in a scalar. The dot product accounts for the angle between the vectors using cosine.
In physics, no. If there's no displacement, no work is done, even if you exert great force. Holding a heavy object stationary requires effort but does no work in the physics sense.
Energy is the capacity to do work. Work is the transfer of energy from one system to another through the application of force over a distance. They're measured in the same units (joules).
Power is the rate of doing work: P = W/t. If you do the same work faster, you're using more power. A powerful engine does more work per unit time than a weak one.
The upward force you apply is perpendicular to the horizontal displacement. Since cos(90 degrees) = 0, W = F?d?cos(90 degrees) = 0. Your muscles expend energy, but no physics work is done on the suitcase.