work
Summary
Work is energy transferred to or from an object by a force acting through a displacement. Only the force component along the displacement counts, so W = Fd cosθ. Work is positive when the force has a component along the motion and negative when it opposes the motion.
Detail
Meaning: Work is energy transferred to or from an object by a force acting through a displacement. Only the force component parallel to the displacement counts, so a force perpendicular to motion, such as the normal force, does zero work. Net work equals the change in kinetic energy (work-energy theorem).
Equation: W = Fd cosθ, where W is work (J = N·m), F is force magnitude (N), d is displacement magnitude (m), and θ is the angle between force and displacement (rad). Net work: Wnet = ΔKE. For a gas at constant pressure, work done by the gas is W = PΔV and work done on the gas is −PΔV (P in Pa, ΔV in m³).
On the MCAT: Passages use a block on a ramp, a lifted load, or friction and ask for the sign or size of work by each force. Graphs use area under a force-displacement or P–V curve. Ideal simple machines reduce force but not work.
Common confusion: Holding a box still involves no work, since displacement is zero. Negative work is not zero work; friction removes energy. Physics often uses work by the gas (PΔV); chemistry's ΔU = q + w uses work on the system.
Sources
- Fundamentals of Physics
- College Physics
- Physics for Scientists and Engineers
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.