For 9.2 see Topic 2 and 9.10, see Topic 3
A force is a push or pull on an object that is caused by interaction. Interaction by contact means that both objects need to touch by friction or normal contact force. Interaction without contact is caused by interacting fields; the gravitational attraction between the Earth and the Moon is caused by the interaction of their gravitational fields. Magnetic or electrostatic fields cause attraction or repulsion. When two objects interact, both objects experience an equal but opposite force. The pair of forces is called an interaction pair and can be represented by a pair of vectors.
Free-body force diagrams show an isolated body and all the forces acting on it. It should include every force acting on the body but none of the forces it exerts on external bodies. The size of the arrows should represent the relative magnitude of the forces and the directions should show the direction of the force
The force which can replace all the forces upon a body to have the same effect is the resultant force. If the resultant force is zero, the object is in equilibrium, moving at constant velocity or is at rest
To find a resultant force, draw a scale diagram using arrows at correct angles and lengths to represent forces. Draw lines to make a parallelogram. The resultant force is the diagonal of the parallelogram. Another method uses a tip-to-tail diagram where the resultant force is the product of the two forces
If all the forces acting on an object combine to give a resultant force of zero, the object is in equilibrium. A tip-to-tail diagram of all the forces should join up perfectly in a triangle
A force can be resolved into components which are at right angles to each other. Acting together, these components have the same effect as the single force. Draw the force on a scale grid then add the horizonal and vertical components along the gridlines
A force can cause an object to rotate. The principal of moments says that for can object in equilibrium the sum of the clockwise moments is equal to the sum of the anticlockwise moments. Levers transfer thee turning effect of a force, increasing moments as the distance increases. Pushing one end of a lever down causes the other end to rise due to the rotation around a fulcrum. Gears are circular cogs with teeth around their edge. The teeth interlock so that turning one causes the other to turn in the opposite direction. The rotational effect of the force is transmitted. Interlocking gears rotate at different speeds