*Forces:*

**Push or Pull is called a force**

*A force is a push or a pull which can affect the motion of an object by changing its speed or direction. If two equal and opposite forces act on an object it may be squashed or stretched. Both the magnitude and direction of force acting on an object must be stated, because both affect the way in which the object moves. If the direction in which a force is acting is known, it is possible to predict the way the object it affects may move. Forces are represented by arrowed lines whose length corresponds to the magnitude of the force. The arrow indicates the direction in which it is acting. Force is measured in Newtons (N).*

*Magnetic and Electrical Forces:*

*Magnets can exert a force of attraction and repulsion*

*There are two types of forces which act at a distance: magnetic and electrical forces. Objects which extra a magnetic or an electrical force can attract or repel objects which are brought near them. The region in which the forces act is called a field. The magnitude of the forces depends on the distance between the objects. The closer they are together, the stronger the forces they exist.*

*Frictional Forces:*

**Friction between the tyres and the road**

*Friction is the force which resists the motion of two materials rubbing together. Sometimes it is useful force – for example, it enables us to grip the ground as we walk. A vehicle is able to grip the road due to the friction between its tyres and the road surface. But friction also has unwanted effects. The friction between the moving parts of a machine produces heat which wastes energy. The friction between a cyclist and the air resist his or her forward movement.*

*Gravitation and Weight:*

*Gravitation is another force which acts at a distance. It is the force which exists between any two masses, attracting them towards each other. Usually it is a weak force, but if one object is massive, such as a planet, the force becomes noticeable. Gravitational force depends on the distance between objects. The closer the objects are together, the stronger the force they exert on each other.*

*Weight is a measure of a planet’s gravitational pull on an object. Like all forces, it is measured in Newton’s (N). The weight of an object depends on its distance from a planet and the planet’s mass. On the Earth’s surface, the force of gravity acting on a mass of 1 Kg is approximately 9.8 N. The magnitude of the force diminishes as the mass moves further away from the Earth’s surface. An object’s mass, however, remains the same wherever it.*

*Elasticity:*

**Force is Proportional to Elastic**

**Graph between Force and Extension**

*When a force is applied to an object which cannot move, the object stretches. Its molecules are pulled slightly apart and it becomes distorted. If the object remains distorted when the force is removed, its distortion is called plastic. If its molecules return to their original position, the distortion is called elastic. Elasticity is, therefore, a material’s ability to return to its original shape. To study the elasticity of a material, such as a strip of copper, rubber or nylon, weights of increasing size are suspended from the material. The amount by which the material is stretched is found by subtracting its original length from its extended length. The size of the force is then increased and the results are used to make a graph.*

*Hooke’s Law:*

*Hooke’s law states that the extension of a material is proportional to the force which is stretching it.*

*There is a point, however, beyond which Hooke’s law is no longer obeyed. This is called the limit of proportionality. If the substance is stretched further than this point, it reaches its elastic limit. The substance stops being elastic and remains distorted even when the stretching force is removed.*

*Provided a material’s elastic limit is not exceeded, the principle of Hooke’s law can be used in calculations to determine an unknown force or extension.*

*For example, if a force of 10N stretches a spring by 60mm, the force which would produce an extension of 42mm is calculated as follows:*

*60mm extension produced by 10N*

*1mm extension is produced by 10/60*

*Therefore, the force which would produce a 42mm extension is calculated as follows: 10X42/60 = 7N*

*A Spring Balance:*

*The easiest way of measuring forces is to use a spring balance, after called a Newton Balance. This is a device containing a spring. The spring obeys Hooke’s Law. This means that it stretches in direct proportion to the force applied to it. For example, if the forces applied to the spring are doubled, its extension doubles. The spring balance will measure forces accurately until it is stretched beyond its elasticity limit and it becomes permanently distorted.*

*Scalar and Vector Quantities:*

*Quantities in physics are described as either scalar or vector quantities.*

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*A scalar quantity is one which has magnitude only. For example, Mass and Temperature are scalar quantities.*

*A vector quantity is one which has both direction and magnitude. Force is a vector quantity. The magnitude and direction of a vector must always be stated. Vectors can be represented with arrowed lines.*