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Work refers to an activity that involves movement of the object when force is applied on it. For example, work is done when a person pushes a car and the car moves.
The formula of work is:
Work = Force distance
It means that more work is done, if more force is applied to an object, or the object moves larger distance, or both.
If a force is applied on an object, but no motion takes place then no work is done. For Example, if a person pushes against a wall, then no work is done unless the wall moves in the direction in which it is pushed.
The unit of measurement of work is unit of force multiplied by the unit of distance i.e.
Newton metre (Nm) or Joule (J).
For example, if a stone is pushed with the force of 5 N and it travelled a distance of 3 meter, the work done is 15 Nm or 15 J.
A push or pull acting on an object is called a force.
A force can be used to:
Fig: Force exerted by two boys
Types of Force
There are different types of force:
Muscular force is applied when we push, pull or lift something with our hand.
Fig: Muscular Force
Gravitational force or gravity is the force that attracts objects to the centre of the earth
Because of gravitational force, we are able to stay on the ground.
Fig: Gravitational Force
Frictional force is a force of resistance that tries to stop the movement of objects across a surface. On smooth surfaces less friction is exerted and on rough surfaces more friction is exerted. Due to it we are able to walk.
Fig: Frictional Force
Elastic force arises when a body deforms. When we stretch a rubber band it regains its original position because of elastic force.
Fig: A picture showing stretching causes elasticity.
Mechanical force is used by most of the simple machines. A wedge uses mechanical force to separate two objects.
Fig: Mechanical Force
Buoyant force is the upward push of water on a floating object. When we push a mug or piece of wood in water, we can feel an upward thrust.
Fig: Buoyant Force
Energy is the capacity or ability to do work. Energy can exist in different forms such as mechanical energy, heat energy, chemical energy, sound energy, muscular energy, solar energy, wind energy, etc.
Forms of Energy
Mechanical energy: Mechanical energy of an object is its energy due to its position or its motion or both. Mechanical energy is classified into potential energy and kinetic energy based upon its source of origin.
Potential energy of the object is its energy due to its position. For example, a book on a table before it falls possesses potential energy.
Heal energy: A heated substance possesses heat energy. For example, burning wood possesses heat energy. Heat energy is used to move steam engines.
Fig: Heat Energy
Chemical energy: Chemical energy is the energy released or absorbed during a chemical reaction. For example, battery possesses chemical energy.
Fig: Chemical Energy
Sound energy: Energy possessed by sound wave is called sound energy. For example, sound from a music player possesses sound energy.
Fig: when someone plays guitar, the strings vibrate and transmits energy.
Muscular energy: Energy possessed by the muscles is called muscular energy. For example, a boy uses his muscular energy to lift dumbbell.
Fig: Muscular energy
Solar energy: Energy possessed by sunlight is called solar energy. For example, solar energy is used by plants to produce foods.
Fig: Solar energy
Wind energy: Energy possessed by wind is called wind energy. For example, wind energy is used for drying clothes or moving wind mill.
Fig: Wind Energy
Electrical energy: Energy possessed by electricity is called electrical energy. For example, a fan runs with electrical energy.
Fig: Electrical energy possessed by fan
Law of conservation of energy
The law of conservation of energy states that energy can only be transformed, it cannot be created or destroyed.
The international unit of energy JOULE is named in the honour of scientist James Prescott Joule.
Machines are simple tools which make our work easier and faster. They help us to do many works.
Some examples of simple machines are lever, inclined plane, wheel and axle, pulley and screw.
Levers are commonly used tools like scissors, hammers and screw drivers.
When we lift a rock with the help of a rod, the weight lifted by the person is the load and the the force with which it is lifted is the effort. The point of contact of rod and the stone is the fulcrum.
Fig: A Lever
Levers can be classified into the followings according to the position of the fulcrum, the load and the effort:
First class lever — when the fulcrum lies between the load and the effort, it is called first class lever. For example, see saw, scissors, etc.
Fig: First Class Lever
Second class lever — when the load lies between the fulcrum and the effort, it is called second class lever. For example, a wheel barrow, a bottle opener.
Fig: Second Class Lever
Third class lever - when the effort lies between the fulcrum and the load, it is called third class lever. For example, a stapler, a pair of tongs.
Fig: Third Class Lever
An inclined plane is a slope which makes work easier. When workers have to load or unload a truck they use a plane of wood as an inclined plane.
Fig: An inclined plane
A pulley is a small wheel with a groove around its outer edge. There are two types of pulleys: fixed pulley and movable pulley. The pulley used for drawing water from a well is a fixed pulley Movable pulleys along with fixed pulleys are used to lift loads.
Fig: A pulley pulled by a rope.
The Wheel and Axle
A wheel attached to a rod (called axle) is known as a wheel and axle arrangement. It is easier to push a load on wheels than carrying it. This arrangement is used in vehicles, sewing machines and cycles.
Fig: Wheel and Axle
Screw is a tool used to hold things tightly together. Bigger screws, called screw jacks, are used to lift cars and other heavy objects. When two things are joined by a screw, they are held together through a long distance while when we join things with a nail.
Fig: A Screw