[Physics Class Notes] on Force, Motion and State of Motion Pdf for Exam

Force is the cause of change in the state of motion of a body or an object. It is a quantitative description of an interaction that causes a change in an object’s motion. Force can cause an object to move or accelerate, to slow down or decelerate, to stop, or to change its direction. The applied force can be a push, a pull, or dragging of an object.

 

Examples of Force in Everyday Life

1. Applying brakes to stop a vehicle

2. Lifting a load

3. Pushing and pulling a door 

4. Kneading and rolling the dough 

5. Kicking a football 

6. Stretching a spring or a rubber band 

7. Attracting paper bits with an electrostatically charged comb

8. Force exerted by our muscles while moving the limbs 

9. Throwing a stone in the air and it’s coming down.

10. A magnet attracting iron nails

 

The State of Motion and Causes of Change in Motion

Motion is the change in position of a body with respect to its surrounding environment, within a given interval of time. An object is said to be in motion if its position changes with time, with reference to a fixed frame.

A chair cannot move its own, what do you do to make a chair move?

We often say that a force has been applied to the chair when it is pushed.

The motion of an object is explained by its speed and direction of motion. If an object is at rest, the state is considered to be in the state of zero. By applying force, we can change the position of the object or can say that the object is in motion. 

While taking a penalty kick in football, before being hit, the ball was at rest. Then, its speed was zero. The player applied force on the ball. This applied force sets the ball in motion towards the goal.

Suppose, the ball hits the goal or the goalkeeper dives and saves the goal. In both conditions, the speed of the ball changes. Force can also cause an increase or decrease in motion if it is applied in the same direction or the opposite direction respectively.

Many times, an applied force may not result in any change in the state of motion.

What will happen when you push a wall with the maximum force that you can exert? 

No effect of force is observed.

 

How can Force Change the State of Motion?

1. The Applied Force can Cause Acceleration

The change in motion is equivalent to a change in velocity. A change in velocity applies that there will be an acceleration. The force causes a change in motion. So it produces acceleration too.

If an object is stationary in the beginning, it accelerates when it starts to move. Likewise, if an object is already moving and a force is applied in the same direction, the object will accelerate as long as the force is applied to the object. If the force is removed, the acceleration will also stop.

For example, James was walking towards the north at a speed of 10 metres per second. James speeds up and now begins running towards the west at 20 metres per second after 5 seconds. In this case, James has accelerated his velocity by 2 m/s² i.e his velocity has increased by 2m/s every second. 

Another example is of an apple falling down. It starts falling at zero metres per second. At the end of the first second, the apple is moving at 9.8 metres per second. The apple has accelerated. This acceleration here is caused by gravity.

2. The Applied Force can Cause Deceleration

If an object is moving and a force applied to it in the opposite direction of the motion, the object will decelerate or slow down.

Suppose, a cricketer hits the ball high up. It will slow down as it travels upwards due to the force of gravity. Likewise, a boat decelerates due to wind flowing opposite to the direction of motion of the boat.

Decelerating force can put a moving body to rest.

For example, when a car driver applies brakes, it begins to decelerate.

3. Force can Cause a Change in the Direction of Motion

A change in either the speed of a moving body or its direction or both are referred to as a change in its state of motion. Thus, the force can change the direction of motion.     

For example- In a cricket match, a bowler bowls the ball towards the batsman with some velocity(u). The batsman hits the ball and it travels in a different path with another velocity(v). This is because the batsman applies force on the ball and changes the direction of the ball.

 

Newton’s Laws of Motion

Newton the great physicist gave laws of motion that are useful in everyday lives. The three laws of motion are described below.

1. Law 1:- The first law of motion tells that an object will remain at the state of rest or continue to move at a certain speed unless an external force is applied on the object, which will violate the equilibrium of the system. The first law of motion by Newton is also called the “law of inertia” and it explains the concept of inertia, application of the force, and inertial frame of reference.

2. Law 2:- Newton’s second law is the quantitative description of the changes that force may induce in the object on which the force is applied.

This entails that when a force is applied on a given object of constant mass, the rate of change of the speed of the object will be directly proportional to the total force applied on the object. In simpler words, the acceleration produced in the object due to the application of the force will be directly proportional to the force applied.

i.e. Force applied ∝ acceleration produced

F ∝ a    F = Ka  and K here is the mass so 

F  = ma      or

Force applied = (mass of the object) (Acceleration produced in the object)

3. Law 3:- The third law of Newton states that for every action or force, there will be equal but opposite reactions or force. If you push the wall, the wall does not move as it exerts an equal and opposite reaction force. This law is commonly called the law of action and reaction in physics. 

Example:- If a book is lying on a table, which can be interpreted as the book is applying force equal to its weight on the table, according to this law, the table applies an equal and opposite force on the book which will counter the force of weight applied by the table.