Class 9 Science

Forces and Laws of Motion

Table of Contents


It is the force that enables us to do any work.

To do anything, either we pull or push the object. Therefore, pull or push is called force.

Example: to open a door, either we push or pull it. A drawer is pulled to open and pushed to close.

Effect 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.

Types of Force

  1. Balanced Forces
  2. Unbalanced Forces

Balanced Forces

If the resultant of applied forces is equal to zero, it is called balanced forces.

Example: In the tug of war if both the teams apply similar magnitude of forces in oppoisite directions, rope does not move in either side. This happens becasue of balanced forces in which resultant of applied forces become zero.

Balanced forces do not cause any change of state of an object. Balanced forces 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.

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:

Laws of Motion:

Galileo Galilei: Galileo first of all said that object move with a constant speed when no forces act on them. This means if an object is moving on a frictionless path and no other force is acting upon it, the object would be moving forever. That is there is no unbalanced force working on the object.

But practically it is not possible for any object. Because to attain the condition of zero unbalanced force is impossible. Force of friction, force air and many other forces always acting upon an object.

Newton’s Laws of Motion:

Newton studied the ideas of Galileo and gave the three laws of motion. These laws are known as Newton’s Laws of Motion.

Newton’s First Law of Motion:

Any object remains in the state of rest or in uniform motion along a straight line, until it is compelled to change the state by applying external force.

Explanation: If any object is in the state of rest, then it will remain in rest untill a exernal force is applied to change its state. Similarly an object will remain in motion untill any exeternal force is applied over it to change its state. This means all objects resist to in changing their state. The state of any object can be changed by applying external forces only.

Newton’s First Law of Motion in Everyday Life:

  1. A person standing in a bus falls backward when bus is start moving suddenly. This happens because the person and bus both are in rest while bus is not moving, but as the bus starts moving the legs of the person start moving along with bus but rest portion of his body has tendency to remain in rest. Because of this person falls backward; if he is not alert.
  2. A person standing in a moving bus falls forward if driver applies brakes suddenly. This happens because when bus is moving, the person standing in it is also in motion along with bus. But when driver applies brakes the speed of bus decreases suddenly or bus comes in the state of rest suddenly, in this condition the legs of the person which are in the contact with bus come in rest while the rest parts of his body have tendency to remain in motion. Because of this person falls forward if he is not alert.
  3. Before hanging the wet clothes over laundry line, usually many jerks are given to the cloths to get them dried quickly. Because of jerks droplets of water from the pores of the cloth falls on the ground and reduced amount of water in clothes dried them quickly. This happens because, when suddenly cloth are made in motion by giving jerks, the water droplets in it have tendency to remain in rest and they are separated from cloths and fall on the ground.
  4. When the pile of coin on the carom-board hit by a striker; coin only at the bottom moves away leaving rest of the pile of coin at same place. This happens because when the pile is struck with a striker, the coin at the bottom comes in motion while rest of the coin in the pile has tendency to remain in the rest and they vertically falls the carom board and remain at same place.
  5. Seat belts are used in car and other vehicles, to prevent the passengers being thrown in the condition of sudden braking or other emergency. In the condition of sudden braking of the vehicles or any other emergency such as accident, the speed of vehicle would decrease or vehicle may stop suddenly, in that condition passengers may be thrown in the direction of the motion of vehicle because of the tendency to remain in the state of motion.
  6. The head of hammer is tightened on a wooden handle by banging the handle against a hard surface. When handle of the hammer is struck against a surface, handle comes in rest while hammer over it's head has tendency to remain in motion and thus after some jerks it tightens over the handle.

Mass and Inertia

The property of an object because of which it resists to get disturbed its state is called Inertia. Inertia of an object is measured by its mass. Intertia is directly proportional to the mass. This means inertia increases with increase in mass and decreases with decrease in mass. A heavy object will have more inertia than lighter one.

In other words, the natural tendecny of an object that resists the change in state of motion or rest of the boject is called intertia.

Since a heavy object has more intertia, thus it is difficult to push or pul a heavy box over the ground than lighter one.

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