1. An object will remain in a state of rest or continue to move with a constant velocity unless acted upon by a resultant external force. If the forces acting on an object are balanced, that is they all add up to zero, the object cannot accelerate. That is it cannot speed up, slow down or change direction. An object moving with constant velocity is in equilibrium. Newton’s First LawGlossary © Hodder & Stoughton 2015

2. This is a force diagram in which all the forces acting on an object have been identified and drawn. The body is imagined isolated from its surroundings so that only the forces acting on the object are drawn. The task involves identifying all the forces acting on the object and then drawing them with arrows whose length is proportional to the size of the force pointing in the direction of the force. Free Body DiagramGlossary © Hodder & Stoughton 2015

3. The point where the weight of an object appears to act from. An object suspended from its centre of gravity will not rotate. If an object is suspended from several different points the verticals from the points of suspension will all intersect at one point, this is the centre of gravity sometimes called the centre of mass. Centre of GravityGlossary © Hodder & Stoughton 2015

4. When two skew forces acting on an object are drawn they form two adjacent sides of a parallelogram. The diagonal of the parallelogram is the resultant of the two forces. Obviously if the forces are parallel the resultant is simply the arithmetic sum. The resultant in green of the two red forces is the diagonal of the parallelogram formed by the two red forces. Parallelogram of ForcesGlossary © Hodder & Stoughton 2015

5. The acceleration of a body of constant mass is proportional to the resultant force applied to it and in the direction of the resultant force. The effect of a resultant force acting on an object is to change its motion. That is the object will speed up, slow down or change direction. It is important to understand that speeding slowing down and changing direction are all examples of acceleration. Newton’s Second LawGlossary © Hodder & Stoughton 2015

6. When all the forces acting on an object are summed using vector algebra what is left is the resultant force. If the resultant force is zero then the object is in equilibrium, otherwise it is accelerating in the direction of the resultant force. If the brakes are applied to a car a resultant force acts in the opposite direction to its motion. It will accelerate in this direction, that is it must slow down. It is important to realise that the direction of the acceleration gives no indication of the direction of motion. Resultant ForceGlossary © Hodder & Stoughton 2015

7. If body A exerts a force on body B, body B will exert an equal and opposite force on body A. Problems arise when forces are not described correctly. For example as I stand on the floor I push the floor with my feet. The floor equally pushes my feet with an equal and opposite force. The Earth pulls me with a force called my weight. I equally pull the Earth with the same gravitational force of attraction. Newton’s Third LawGlossary © Hodder & Stoughton 2015

8. A force which arises between two electrically charged objects. It can be repulsive or attractive. The electric force can be felt between a charged object and a neutral one because as the charged object is moved close to the neutral one opposite charges are induced in the region close to the charged object. Electrostatic ForceGlossary © Hodder & Stoughton 2015

9. When an electric current flows a magnetic field is set up. This magnetic field will interact with other magnetic fields and is known as the magnetic force. The field lines run from North to South. Not to be confused with electric forces despite the fact that electric and magnetic forces have a common origin. Stationary charges are not affected by a magnetic field, only moving charges are affected and then only if they cut across the field. As for the electric force the magnetic force can be repulsive or attractive. Magnetic ForceGlossary © Hodder & Stoughton 2015