O Level Physics Chapter :02: Forces and Shape Prepared By: Shakil Raiman

2.1: Force Force is push or pull which changes or tries to change the state of an object or changes the shape of an object. Forces can make things start to move, accelerate or decelerate, or can change the direction of its motion. It is a vector quantity. It’s unit is Newton (N) A force of 1 N will make a mass of 1 kg accelerate at 1 m/s2.

2.2: Different Forces Weight or gravitational force – always acts vertically downwards air resistance (or drag) – always acts opposite to the direction of motion. (more speed means more air resistance) friction – opposing force due to contact tension – happens due to pull upthrust – acts on objects within fluids (liquid or gas) magnetic force – acts due to magnetic pole electrostatic force – acts between charged objects

2.3: Different forces on an object

2.4: Example of forces

2.5: Balanced and Unbalanced Forces Balanced Force: If the net force on an object is zero, it is said balanced force. Unbalanced Force: If the net force on an object is not zero, it is said unbalanced force.

2.6: Changing Shapes Forces can cause change in shape. Sometimes the change is temporary as in the suspension spring in the mountain bike. Sometimes the change is permanent as the crushed can.

2.7: Temporary Change of Shape All materials will stretch a little (or more) under tension or shorten under compression. Brittle: Brittle material go through little or no plastic deformation. Example: Glass Elastic or Resilient: Elastic material comes back to its original shape after the deforming force is removed. Example: Metal wire or metal spring Plastic: Change shape permanently. Example: putty, plasticine.

2.8: Springs and wires Springs or wires change length when a force acts on them and return to their original length when the force is removed.

2.9: Hooke’s Law Extension is directly proportional to the applied force within elastic limit.

2.11: Elastic bands or Rubber bands Rubber bands do not obey Hooke’s law – the extension is not directly proportional to the force causing it.

2.10: Spring going beyond Elastic Limit

2.11: Questions:

Thank You All Wish you all very good luck and excellent result.