| resultant force | the single force that has the same effect as all of the forces acting on an object |
| triangle of forces | a closed triangle drawn for an object in equilibrium. The sides of the triangle represent the forces in both magnitude and direction. |
| equilibrium | an object in equilibrium is either at rest or travelling with a constant velocity because the resultant force on it is zero |
| components (of a vector) | the magnitudes of a vector quantity in two perpendicular directions |
| resolving (the vector) | to split a vector (such as a force) into two vectors at right angles |
| free-body force diagram | a diagram showing all the forces acting on an object (but not the forces it exerts on other objects) |
| centre of gravity | the point where the entire weight of an object appears to act |
| moment of a force | the product of the force and the perpendicular distance from the line of action of the force to the point |
| principle of moments | the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about the same point provided the body is in equilibrium |
| couple | a pair of equal and opposite forces that act on an object at different points and produce rotation only |
| torque (of a couple) | the product of one of the forces and the perpendicular distance between the forces |
| energy | a calculated quantity that is conserved during any change; that which is transferred when a force does work |
| joule | the work done when a force of 1N moves a distance of 1m in the direction of the force |
| gravitational potential energy, Ep | the energy a body has due to its position in a gravitational field |
| principle of conservation of energy | energy cannot be created or destroyed. It can only be changed from one form to another |
| power | rate at which a device does work per unit of time |
| watt | one watt is one joule per second |
| work | when a force moves (displaces) the body in the direction of the force; energy is then transferred from one body to another |
| potential energy | the energy an object has because of its position or shape |
| linear momentum | the product of its mass and velocity |
| closed system | a system of interacting objects in which there are no external forces |
| conservation of momentum | the sum or total momentum of the bodies in a closed system is constant provided no resultant external force acts |
| perfectly elastic (collision) | the total kinetic energy of all the bodies remains constant |
| inelastic (collision) | kinetic energy is not conserved; some is transferred to other forms such as heat |
| Newton's first law of motion | an object remains at rest or travels at constant velocity unless it is acted on by a resultant force |
| Newton's second law of motion | the resultant force on a body is proportional (or equal) to the rate of change of momentum of the body |
| Newton's third law of motion | when body A produces a force on body B then body B produces an equal and opposite force on body A |
