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- When we push, pull, kick, lift, throw, flick, hit, pick, squeeze, press, inflate, open, close an object we say a force is applied on the object. These actions are nothing but application of force.
- Whatever may be the method of application of the force, they are of only two types a push or a pull.
- An agency which produces motion in a body or alters the existing state of the motion in a body is called a force.
Examples of Push:
We push door to shut it or open it.A baby walks by pushing a baby walker. In petrol engine, the gas expands and push piston.
Examples of Pull:
A bucket full of water lifted up by applying pull. A load lifted using a pulley. car towed by a towing crane. A body falling under gravity.
Point of Application of Force:
The point on the body where a force acts is called the point of application of force.
Line of Action of Force:
A line drawn through the point of application of the force in the direction of force is called the line of action of force.
Necessity of Force:
- to move a stationary object i.e. to move a body which is at rest.
- to change the direction of motion of an object.
- to change the magnitude of the velocity (speed) of motion of an object.
- To change the shape of an object.
Effects of Force:
It may set a body into motion.
e.g. A football when kicked begins to move. An object released from a height starts moving vertically downward under influence of gravitational force. When a golf ball is pushed by golf stick starts rolling. Due to wind, branches and leaves move. Pull of railway engine makes the compartments to move with it.
It may bring a body to rest.
e.g. A moving ball can be stopped by hand. Rolling body stops after sometime due to friction. A car can be stopped by applying brakes. A rotating top can be stopped by hand. When bullet hits the target it stops due to resistance to penetration by the target.
It may change the magnitude of motion.
e.g. When a force is applied in the direction of motion, magnitude of velocity increases. e.g. an accelerator of a car is pushed to increase the speed of the car, due to which driving force in the direction of motion increases. When a force is applied in the opposite direction of motion, magnitude of velocity decreases. Brake of a car is applied to decrease the speed of the car, due to which retarding force in the opposite direction of motion increases.
It may change the direction of motion.
e.g. When a ball is hit by a bat its direction changes. A ball strikes on a wall and rebounds due to reaction given by the wall on the ball.
It may change the magnitude and direction of motion.
e.g. When a ball is hit by a bat its speed and direction change. A body projected making acute angle with the ground changes its magnitude and direction of velocity continuously.
It may change the shape of an object.
e.g. length of the spring can be changed by stretching or compressing. The shape of a sponge can be changed by applying force. By applying force on plaster of paris statues and models can be made. When a balloon is filled with air its shape changes.
Characteristics of Force:
- Forces are due to an interaction of at least two objects.
- It may change the state of motion of an object.
- It may change the shape of an object.
- Forces applied on an object in the same direction add to one another and resultant is in the same direction.
- When forces are applied on an object in the opposite direction then their resultant or net force is the difference between these opposing forces and its resulting direction is the same as that of larger force.
- If the two forces acting on object are equal in magnitude but opposite in direction, then the net force acting on the body is zero.
- It is a vector quantity hence they should be specified by giving its magnitude and the direction.
- If the magnitude or the direction or both changes, then the effect of force also changes.
State of a Motion of an Object:
- The state of motion of an object can be described completely by its speed and the direction of motion.
- If the body is stationary with respect to an observer, then the body is said to be at the rest with respect to the observer.The state of rest is considered to be state of zero speed.
- If the body changes its position with respect to an observer, then the body is said to be in motion with respect to the observer.
- A change of state of motion means a change in either the speed of an object or its direction of motion or both.
- A force is necessary for bringing a change in the state of motion of an object.
Types of Forces:
- Depending upon the interaction between a force and an object, forces are classified asa) Contact forces b) Non contact forces
A force which can be applied only when it is in contact with an object is called a contact force. All mechanical forces are contact forces. e,g, muscular force, force of friction.
Non Contact Forces:
A force which can be applied without any contact with two bodies is called a non contact force. e.g. magnetic force, electrostatic force, gravitational force
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