In this post we are going to show the detailed solution for class 9th science chapter 9. All the solution are prepared by our esteemed who are very well experienced in the teaching.
Summary of the chapter :
Force and Newton’s Laws of Motion:
A push or pull on a body is called force. Forces are used in our everyday actions like pushing, lifting, pulling, twisting, hitting, and pressing. A force cannot be seen. A force can be judged only by the effects which it can produce in several bodies (or objects) around us.
Force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of Newton and represented by the symbol of F.
Effects of Force :
- Force can make a stationary body in motion. For example a football can be set to move by kicking it, i.e. by applying a force.
- Force can stop a moving body – For example by applying brakes, a running cycle or a running vehicle can be stopped.
- Force can change the direction of a moving object. For example: by applying force i.e. by moving handle the direction of a running bicycle can be changed. Similarly by moving steering the direction of a running vehicle is changed.
- Force can change the speed of a moving body – By accelerating, the speed of a running vehicle can be increased or by applying brakes the speed of a running vehicle can be decreased.
- Force can change the shape and size of an object. For example: by hammering, a block of metal can be turned into a thin sheet. By hammering a stone can be broken into pieces.
Balanced and Unbalanced Forces
1. Balanced Forces
If the resultant of applied force is equal to zero, it is called balanced forces. Balanced forces do not cause any change of state of an object. Balanced force are equal in magnitude and opposite in direction. Balanced forces can change the shape and size of an object. For example – When forces are applied from both sides over a balloon, the size and shape of balloon is changed.
2. Unbalanced Forces
If the resultant of applied forces are greater than zero the forces are called unbalanced forces. An object in rest can be moved because of applying balanced forces.
Unbalanced forces can do the following :
- Move a stationary object.
- Increases the speed of a moving object.
- Decreases the speed of a moving object.
- Stop a moving object.
- Change the shape and size of an object.
Some common Forces:
- Muscular Force: The force obtained by the working of human body muscles is called muscular force. Example: Lifting objects, doing exercise etc.
- Gravitational Force: The force applied by earth on an object in downward direction is called gravitational force.
- Frictional Force: The force which opposes motion is known as frictional force. It acts in the direction opposite to the velocity of body.
- Air Resistance: When an object moves through air, air applies a small force in direction opposite to velocity. This force is called air resistance.
Newton’s Laws of Motion
Newton’s has given three laws to define the motion of bodies. These laws are known as Newton’s laws of motion.
Newton’s 1 st law of motion
A body at rest will remain in rest, and a body in motion will continue in motion in a straight line with uniform speed, unless it is compelled by an external force to change its state of rest or of uniform motion.
Inertia and Mass: Inertia is that property of body due to which it resists a change in its state of rest or of uniform motion.
SI unit of mass is kilogram (kg).
Momentum: The momentum of a body is defined as the product of its mass and velocity.
Thus, momentum = mass x velocity
Or, p = m x v
Where, p = momentum
m = mass of the body
v = velocity of the body
The SI unit of momentum is kilogram meter per second (kg.m/s)
Note: The force required to stop a moving body is directly proportional to its mass and velocity.
Change in momentum: It is defined as the difference between final momentum and initial momentum. Suppose initial momentum is mu, and final momentum is mv, then
Change in momentum = mv – mu
Rate of change of momentum: The rate at which momentum is changing is known as rate of change of momentum.
Rate of change of momentum = mv – mu / t
Or, = m(v – u) / t
Newton’s 2nd Law of motion
The rate of change of momentum of a body is directly proportional to the applied force, and takes place in the direction in which the force acts.
Force = change in momentum / time taken
F = mv – mu / t
But we know that,
F = m x a
Or Force = Mass x Acceleration
Its SI unit is Newton (N)
Thus, one unit of force is defined as the amount that produces an acceleration of 1 ms-2 in an object of mass 1 kg.
Newton’s 3rd Law of Motion
Whenever one body exerts a force on another body, the second body exerts an equal and opposite force to the first body. Or, to every action there is an equal and opposite reaction.
Action and reaction forces are equal and opposite.
Conservation of Momentum
If two or more objects apply force on each other with no external force, their final momentum remains same as intial momentum.
Total momentum before collision = Total momentum after collision
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