Force, Work, Power and Energy

The One Thing

Force can produce motion or turning, work transfers energy, and power measures the rate of energy transfer. Machines provide mechanical advantage, but energy is transformed rather than created or destroyed, and real machines are less than perfectly efficient.

Reactions, Processes and Experiments

Key Terms

• Force: A push or pull that can change the state of rest or motion, direction, shape, or size of an object.
• Moment of force: The turning effect of a force about a pivot; it equals force multiplied by the perpendicular distance from the pivot to the line of action of the force.
• Torque: Another name for the turning effect of a force, represented by moment = force × perpendicular distance.
• Principle of moments: For a body in rotational equilibrium, the sum of clockwise moments about a point equals the sum of anticlockwise moments about that point.
• Centre of gravity: The point through which the entire weight of a body appears to act.
• Work: Work is done when a force produces displacement in its direction; for a constant force, W = F × s.
• Joule: The SI unit of work and energy; one joule is the work done when a force of one newton moves an object through one metre in the force's direction.
• Energy: The capacity of a body or system to do work.
• Kinetic energy: The energy possessed by a body because of its motion; KE = 1/2 mv².
• Potential energy: Stored energy due to position, shape, or configuration; gravitational potential energy near Earth's surface is PE = mgh.
• Power: The rate at which work is done or energy is transferred; P = W/t.
• Watt: The SI unit of power; one watt equals one joule per second.
• Machine: A device that helps perform work by multiplying force, changing its direction, or increasing the speed or distance of motion.
• Load: The resistance or force that a machine is used to overcome.
• Effort: The force applied to a machine to overcome the load.
• Mechanical advantage: The ratio of load to effort; M.A. = L/E.
• Velocity ratio: The ratio of the distance moved by the effort to the distance moved by the load; V.R. = distance moved by effort/distance moved by load.
• Efficiency: The ratio of useful output work to input work, expressed as a percentage; efficiency = (M.A./V.R.) × 100%.
• Lever: A rigid bar that turns about a fixed point called the fulcrum.
• Fulcrum: The fixed support or pivot about which a lever rotates.
• First-class lever: A lever in which the fulcrum lies between the effort and the load, such as scissors or a see-saw.
• Second-class lever: A lever in which the load lies between the fulcrum and the effort, such as a wheelbarrow.
• Third-class lever: A lever in which the effort lies between the fulcrum and the load, such as the human forearm.
• Conservation of energy: Energy cannot be created or destroyed; it can only be transformed from one form into another.

Easily Confused

• Moment of force and torque: These are two names for the same turning effect, calculated using force × perpendicular distance.
• Force and work: A force alone does not necessarily produce work; work requires displacement in the direction of the force.
• Work and power: Work measures energy transferred, whereas power measures how quickly work is done or energy is transferred.
• Kinetic energy and potential energy: Kinetic energy results from motion, whereas potential energy is stored because of position, shape, or configuration.
• Load and effort: The load is the resistance being overcome, whereas the effort is the force applied to overcome it.
• Mechanical advantage and velocity ratio: Mechanical advantage compares load with effort, whereas velocity ratio compares the distance moved by effort with the distance moved by load.
• Ideal and real machines: An ideal machine has 100% efficiency and M.A. equal to V.R.; real machines have efficiency below 100% because of friction, deformation, and other energy losses.
• First-, second- and third-class levers: The classes differ according to whether the fulcrum, load, or effort lies between the other two components.
• Centre of gravity and fulcrum: The centre of gravity is the point through which the weight appears to act, whereas the fulcrum is the fixed support about which a lever rotates.

What Gets Asked

• Define and calculate a moment of force using moment of force = Force × perpendicular distance; marks are lost by using a non-perpendicular distance or omitting the unit newton metre (N m).
• Apply the principle of moments to a balanced body or lever; marks are lost by failing to equate clockwise and anticlockwise moments.
• Determine whether work is done in a stated situation; marks are lost by assuming that any applied force does work, even when the force is perpendicular to displacement.
• Calculate kinetic energy, gravitational potential energy, work, or power using KE = 1/2 mv², PE = mgh, W = F × s, and P = W/t; marks are lost by confusing energy transferred with the rate of transfer.
• Calculate mechanical advantage, velocity ratio, or efficiency; marks are lost by interchanging load and effort or by using the wrong distance in the velocity-ratio expression.
• Identify a lever as first-class, second-class, or third-class from the positions of the fulcrum, load, and effort; marks are lost by relying on the example rather than identifying the arrangement.
• Explain energy transformations, including free fall; marks are lost by stating that gravitational potential energy is destroyed instead of explaining that it decreases as kinetic energy increases while total energy remains constant.
• Compare ideal and real machines; marks are lost by stating that machines reduce the total work required, rather than explaining that real machines lose energy through friction, deformation, heat, or sound.