Forces in Physics: A Beginner's Guide, Schemes and Mind Maps of Physics

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FORCES

PHYSICS 101 by Lecturer Mosweu

FORCES

A force is a push or a pull. A force can cause an object to:  speed up(accelerate)  (^) slow down(decelerate)  change direction  change shape  Force is measured in: newtons (N). It is measured with an instrument called newton meter.

2. Normal reaction or contact: This is the repulsive force that stops two touching bodies moving into each other. The word ’ normal ’ means that this force acts at 90° to the surfaces of the bodies. It is caused by repulsive molecular forces.

3. Air resistance or drag

This is the force that opposes the movement of objects through air. Drag is a more general term used for the opposition force in any gas or liquid. Objects are often streamlined to reduce this force.

4. Upthrust (buoyancy): This is the force experienced by objects when they are placed into a fluid (liquid or gas). An object will float on a liquid if the upthrust force equals its weight.

 (^) Friction is needed for racing cars to grip the road, holding objects and for walking! Friction can cause:  (^) Wearing off of surface in contact  (^) Overheating of object in contact  (^) Slowing down of objects  Friction can be minimised by  (^) Oiling or lubricating moving parts  Streamlining  (^) Using rollers/wheels

EFFECTS OF FORCE ON AN OBJECT  (^) FORCE AND CHANGING SHAPE  (^) Force can change the shape of an object. A stretching force puts an object such as a wire or spring under tension.  (^) A squashing force puts an object under compression. Brittle materials such as glass do not change shape easily and break before noticeably stretching. Resilient materials do not break easily.  (^) Elastic materials return to their original shape when the forces on them are removed. Plastic materials retain their new shape.

ELASTIC LIMIT  (^) The right hand spring has been stretched beyond its elastic limit. Up to a certain extension if the force is removed the spring will return to its original length. The spring is behaving elastically. If this critical extension is exceeded, known as the elastic limit, the spring will be permanently stretched.   Hooke’s law is no longer obeyed by the spring if its elastic limit is exceeded.

 (^) Force against extension graph if the elastic limit is exceeded for a stretching elastic band or spring ELASTIC RUBBER BAND ELASTIC SPRING

IDENTICAL SPRINGS IN SERIES AND PARALLEL Series: Parallel: in series each spring experience the same load hang on them. The extension on each spring is as if it was alone in the arrangement. The total extension is given by: The total extension is given by: where n is the number of springs. Extension is for each spring. where n is the number of springs. Extension, e is the total for the springs.

Resultant force  A number of forces acting on a body may be replaced by a single force which has the same effect on the body as the original forces all acting together.  This overall force is called resultant force. It causes objects to speed up (accelerate) or down(decelerate).

NEWTON’S 2ND LAW OF

MOTION

states that acceleration of an object is directly proportional to resultant force for a fixed mass. The resultant force, mass and acceleration of an object are related by the equation: 𝑹𝒆𝒔𝒖𝒍𝒕𝒂𝒏𝒕 𝒇𝒐𝒓𝒄𝒆 = 𝒎𝒂𝒔𝒔 × 𝒂𝒄𝒄𝒆𝒍𝒆𝒓𝒂𝒕𝒊𝒐𝒏 𝑭 = 𝒎 𝒂  Where: f = force (N) m =mass (kg) a = acceleration (m/s 2 )

Example

1. Calculate the force required to cause a car of mass 1200 kg to accelerate by 5 m/s 2 . F = m a = 1200 kg x 5 m/s 2 = 6000 N
2. Calculate the acceleration produced by a force of 200N on a mass of 4kg. F = m a a = F/m = 200N/4kg acceleration = 50 m/s 2