FORCE AND PRESSURE
When we have to move a box we push it. When we press clay between our hands, it changes its shape. When we walk we pull our feet against gravity. We open a door by pulling it in or pushing it out.
These are some of the examples in which effort is applied to push or pull or change the shape of the object. These pushes and pulls are called forces.
Force is a push or a pull that changes the state of rest or uniform motion; it can also change the direction or shape of the object. For example, when a ball at rest is hit, it starts to move.
The effort changed the state of rest of object into motion. Similarly, if a ball is going straight and is kicked in another direction, it will start to travel in the other direction. The uniform motion of ball is changed by a force. Similarly, when a page is crushed, the shape of page changes into a ball from a plain rectangular sheet.
CLASSIFICATION OF FORCE
The forces can be classified as:
- Contact forces
- Non-contact forces
- Consequential forces
The forces which act when objects are in contact with each other are called contact forces. It is the direct force. Some of the everyday examples are pushing a car, kicking a ball, pulling a cart up the hill. Muscular force, push and pull, and frictional forces are also the examples of contact forces.
A non-contact force is the force that is applied to an object by another object that is not in direct contact with it. The space in which the influence of non-contact force acts is called field. There are magnetic, electrostatic and gravitational field.
The effect of these forces decreases as the distance increases. Magnetic force, electrostatic force and gravitational force are some of the examples of the non-contact forces. When a ball is thrown upwards, it goes up and then comes down; this is due to the gravitational force.
These forces come into play when another force acts on an object. They do not exist on their own. For example, the elastic force, when a spring is stretched the elastic force tends to get back the spring in its original shape. The elastic force depends on the change of shape or length and the applied force.
VARIETY OF FORCES
Muscular Force: The force exerted by the muscles is called muscular force. Pushing, pulling and kicking are some of its examples.
Mechanical Force: The force exerted by machines is called mechanical force. Washing machine, engine of vehicles are some of its examples.
Elastic force: The force that acts on an object when another force acts on the body. They tend to bring the object back in its original shape when the force is removed. Examples are a rubber band stretched, a spring stretched.
Magnetic force: When the force is exerted by the magnet either attraction or repulsion it is called magnetic force. Example magnet attracts iron.
Electrostatic force: The force that exists between electric charges is called electrostatic force. When a charged comb is brought near small pieces of paper, it attracts them. This is so because the comb exerts acquires an electric charge and exerts electrostatic force.
Gravitational force: The force that is exerted by Earth on all the objects is called gravitational force. It is because of the gravitational force when anything is thrown upwards, it comes down.
Frictional force: The force acting against the motion of objects is called frictional force. When an object is slid on a sloppy floor it stops after certain distance. This is because the force of friction is acting against the motion of the object. The frictional force acts in the opposite direction of the motion of the object.
UNITS OF FORCE
The SI unit of force is newton (N) named after the scientist Issac Newton.
Kilogram-force (kg-f) is also used as a unit of force. 1 kg-f is the downward force exerted by earth on a mass of 1 kg. (1 kg-f= 9.8 N (approximately 10 N))
Weight of an object is the force with which it is pulled towards the center of Earth. It acts in downward direction.
MAGNITUDE AND DIRECTION OF FORCE
Every force has both magnitude and direction. This is so because the force is always applied in a direction. It will be applied upwards, downwards, towards east, or towards west. When we say that a force of 20 N is applied upwards, here 20 N is the magnitude and upwards is the direction.
It is represented by a line segment with an arrowhead. The arrow denotes the direction and length of the line segment denotes the line segment.
RESULTANT OF FORCES
When two or more forces act on any object then they can be replaced by a single force that will have the same effect as the combination of all the individual forces. This single force is called as resultant force.
- When forces act in the same direction then the resultant is the sum of the forces acting in the same direction.
- When forces act in the opposite direction then the resultant is the subtraction of the opposite forces acting in the direction of greater force.
- When two equal forces act in opposite direction then their resultant is zero. The object will remain in the same place.
The pressure is defined as force per unit area. The pressure depends on how much force is applied at a certain area. More is the force, more will be the pressure.
Pressure is dependent on force and area. Consider an example, when a wooden plank is stricken exactly in the middle with a hammer with a force of 4 N, nothing happens.
When it is stricken in the same area with force increased to 8 N, the plank breaks. It is because in the second case more pressure is exerted.
Also, lesser is the area more is the pressure. We all have noticed that some bulletin pins are pointed and some are blunt. The pointed pins easily pin notice on the notice board but if with the same force we try to pin notice with blunt pins, they don’t pin in. It is because the pressure in the first case is greater than the second.
UNIT OF PRESSURE
The SI unit of pressure is pascal (Pa). It is the pressure exerted by a force of 1 N acting over an area of 1 square metre. Thus, 1 Pa= 1N/m2.