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A Complicated problem from force and motion

A particle is at rest. A force F is continuously pushing it forward at all velocities it attains till infinite time say till n seconds where n tends to infinity find the final relation for velocity, force, energy etc .”

Originally shared by Shatendra Sharma Professor at Jawaharlal Nehru University

Do you know the solution? Post them as comment to this post.

Angle between acceleration and velocity in projectile motion

What is the angle between the directions of  acceleration and velocity at the highest point of projectile?


At the highest point of a projectile, the vertical component of velocity is zero and the velocity is entirely horizontal. The direction of acceleration (due to gravity) is vertically downwards throughout the motion of the projectile.

Therefore, the angle between the direction of acceleration and velocity at the highest point of a projectile is 90 °

Motion of connected system – A numerical problem

What is the acceleration of the body m2 in the diagram below?

(The question was posted by Shuvam Shukla. Students and other visitors can post solution as comments to this post)

Newton’s Third Law of Motion and some doubts

Newton’s Third Law of Motion states that

“Every action has equal and opposite reaction”

In other words,

“Whenever a body exerts a force on another body, the second body exerts an equal and opposite force on the first body”

 Osho Garg asks:

“if a person is sitting on a chair both the chair and person exert force against it so we called that ” there is always an equal and opposite reaction ” but for example a elephant of mass 584 kg sit on a chair, the chair will broke.  So what we called for this . Please explain.



When an elephant “sits” on a chair, it breaks because it cannot give the “equal reaction” and  it “yields” to it.

Therefore, here the force is doing “work”.

More discussions are welcome

Passenger in a moving train

A passenger in a moving train tossed a coin which falls behind him .State the type of motion of the train ?


The train is in accelerated motion.



If the train were moving with uniform velocity, the coin would fall back to the passenger’s hand (This is due to the inertia of motion of the coin. When it was thrown ‘vertically’ up, it had the same horizontal velocity as that of the train. Therefore, it will move alongwith the train and fall back to the thrower’s hand)

If the train is accelerated, the horizontal velocity of the train would be more than that of the coin (which remains constant through out its flight) by the time the coin returns. Therefore, the train travels a greater horizontal distance than the coin and the coin falls behind the passenger.

Direction of velocity and acceleration

Is it possible that the velocity and the acceleration of a body point in opposite directions? if yes then why?
if no then why?


Yes. It is possible for the direction of acceleration to be opposite to the direction of velocity.

Acceleration is the rate of change of velocity. If velocity is increasing while moving in a straight line then the direction of acceleration is same as the direction of velocity.

Oscillating pendulum, showing acceleration and...
Oscillating pendulum, showing acceleration and velocity. Português: Pêndulo oscilante, com indicação da aceleração e da velocidade. Español: Pendulo oscilador, mostrando la aceleración y la velocidad. (Photo credit: Wikipedia)

It the velocity is decreasing whil travelling in a straight line then the direction of acceleration is opposite to the direction of velocity.

Example: When a stone is thrown vertically upwardsm its acceleration is downwards and velocity is upwards.

Problem from Kinematics

A particle moving with constant acceleration describes in the last second of its motion 9/25th of the total.if it starts from rest how long is the particle in motion and through what distance does it move if it describes 6 cm in first sec

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