Rest Vs Motion A body is said to be at rest if it does not change its position w.r.t. its immediate fixed surroundings. A body is said to be in motion if it changes its position w.r.t. its immediate surroundings. An object may be in a state of motion w.r.t. a set of objects, but may be in a state of rest with respect to the other objects. (moving at the same speed.) Also, it is the observer and the surroundings that decide whether a given object is at rest or in motion. Rest and motion are relative terms.

Force is needed to change the state of rest to motion or state of motion to rest. Force as Push A force is considered to be pushed when we apply a force on it and the direction of motion is away from us. Force as Pull A force is said to be pulled when we apply a force and the body moves to us. Pg 31 TABLE

Force as a Stretch and Squeeze A body is considered to be stretched if a force applied on it increases its length i.e. increase in length is in the direction of the applied force An interaction of one object with another object results in a force between two objects. A body is considered to be squeezed if the force applied on it decreases its length or changes its dimension.

Force is a cause (push, pull, stretch, squeeze) which changes or tends to change the state of rest or of uniform motion, or the direction of the motion, or the shape and size of the body. When a body is kept on a surface, the body’s weight acts vertically downwards. As a result a normal reaction force acts vertically upwards. This is known as a NORMAL REACTION FORCE. Bothe these forces balance each other. While when an object is suspended by a string, the weight of the body acts downwards. As a result a force acts vertically upwards on the string. This is known as a TENSION FORCE. Both these forces balance each other.

Force cannot be seen but we can see the effects of force. A force can. Change the shape or size of the object. Make a stationary object move. Change the speed of a moving object. Change the direction of a moving object. Stop a moving object.

Types of Forces Contact force The object with which we apply force and the object to which force is applied are in direct contact. Non-contact force We apply a force on an object without any contact, it is Force acting at a distance. Mechanical force Frictional force Magnetic force Electrostatic force Gravitational force Muscular force Collision force

Contact forces

Muscular force The force applied by the muscles of a human or animal body. When two objects collide with each other. When a body moves with the help of a machine, the force exerted by the machine. The force which slows down the motion of a moving body in contact with the other body. Collision force Mechanical force Frictional force The Various types of Contact Forces are : Applied forceFrictional forceAir resistance force Spring forceTension forceNormal forceTension force

Non-contact forces

Non-contact forces. The force exerted by a magnet. The force exerted by a charged body on another charged or uncharged body. The earth has a property of its own to pull every object towards it. Weight is the force with which a body gets attracted towards the centre of the earth. Weight is measured in kgf. Magnetic force Electrostatic force Gravitational force Electrochemical force (electrolysis)

More information on some forces Frictional Force can be increased or reduced depending on your requirement. Eg. To move a truck stuck in mud. To move the door easily. Magnets have a north seeking pole at one end and a south seeking pole at the other end. Opposite poles attract. Similar poles repel. The force of attraction between two objects possessing mass is gravitational force. It exists everywhere in universe. Its strength depends on the distance between the two objects and their masses. Weight is the force with which a body gets attracted towards the centre of the earth.

Mass is measured in kg while weight is measured in kgf. Therefore mass and weight are not same. 1 kgf = 10 N Force of gravity only attracts, while magnetic and electrostatic for attract as well as repel.

Depending on the type of extension or compression of the spring we have Extension or Compression Spring balances. Spring Balance A special spring is enclosed in a metal case There is a slit cut at its centre along its length. Top end of spring is attached to the body of a case. Lower end of the spring is attached to a steel strip where a hook is fixed at its end. A small pointer which passes out of the slit is attached at the junction of the spring and the metal strip. On the side of the slit, there are markings in kgf or gf. The body whose weight is to be measured is attached to the hook. The spring gets stretched. The weight of the body is read, where the pointer finally stops on the scale.

Action and Reaction Sir Issac Newton discovered the fact that When you bang or throw a ball against the floor or wall (an action is performed by the ball on the wall). The wall will retaliate and exert a force on the ball the ball bounces back in the direction opposite to which it was thrown (as a result we see a reaction of the wall on the ball). The action and reaction forces are equal in magnitude but opposite in direction.

Net effect of force 1.If two or more forces act in the same direction, They add up to give a resultant force in the same direction. 2. If two forces act in opposite direction, The net force is equal to the difference of the two forces, in the direction of the greater force.