Forces - Equations and Definitions

force (N) = mass (kg) × acceleration (m/s2)

momentum (kgm/s) = mass (kg) × velocity (m/s)

work done (J) = force (N) × distance (m) (along the line of action of the force)

power (W) = work done (J) / time(s)


force exerted by a spring (N) = extension (m) × spring constant (N/m)

energy transferred in stretching (J) = 0.5 × spring constant (N/m)

× (extension (m))2



gravity force (N) = mass (kg) × gravitational field strength, g (N/kg)

(in a gravity field) potential energy (J) = mass (kg) × height (m) × gravitational field strength, g (N/kg)

pressure (Pa) = force normal to a surface (N) / area of that surface (m2)

moment of a force (Nm) = force (N) × distance (m) (normal to direction of the force)

Average speed - the average rate of change of an object's position with respect to time.


Speed - the rate of change of an object's position with respect to time.


Uniform motion - motion where speed is a constant.


Uniformly accelerated motion - motion where speed increases or decreases at a constant rate with respect to time.


Final speed - if an object is uniformly accelerated, this will be twice the average speed.


SI Units - these are the standard international units that all quantities are measured in.  They are all derived from the SI Base units which are metres, seconds, kilograms, degrees Kelvin, Amperes, (and the mole and the candela, but we rarely use them!)  Be sure to get rid of any prefixes before you do any calculations!


Scalar Quantity - A quantity which only has size, for example distance, energy or time.


Vector Quantity - A quantity which has both size and direction, for example displacement, force and current.


Displacement - a vector quantity which means distance in a given direction.


Velocity - a vector quantity which means speed in a given direction.


Acceleration - a rate of change of velocity with respect to time.


Displacement-time graph - a graph showing the displacement of an object over a period of time.  Gradient at any point is equal to the velocity.


Velocity-time graph - a graph showing the velocity of an object over a period of time.  The gradient is equal to the acceleration and the area under the graph is equal to the total distance travelled.


Gradient - also called slope.  The gradient of a line is equal to the vertical height divided by the horizontal length.


Area under a graph - calculate by splitting into rectangular and triangular areas, or trapezium rule.


Kinetic energy - energy stored in an object because it is moving, proportional to its mass and to the square of its velocity.