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Wave Motion/Momentum

When a pebble is thrown in a pond of still water, circular ripples called waves, move on the surface of the water. These waves are the form of disturbance that travels outward, and no portion of the medium (water in this case) is transported from one part to another part of the medium. The particles of the medium simply vibrate about their mean positions.

Wave Motion

Wave or wave motion is a form of disturbance which travels through a material medium due to the repeated periodic motion of the particles of the medium about their mean positions. In the wave motion, the disturbance is handed over from one particle to another particle of the medium without the actual movement of the particles of the medium. The velocity associated with the particle of the medium is called particle velocity, and the velocity associated with wave propagation is called wave velocity.

Characteristics of Wave Motion

1) Wave motion is the disturbance travelling through the medium.

2) When a wave travels through the medium, its particles execute simple harmonic motion about their mean position.

3) Particles of the medium hand over energy to their next neighbours, but their net displacement over one time period is zero.

4) As the disturbance reaches a particle, it starts vibrating. The disturbance is communicated to the next neighbour a little later. So, there is a phase difference in the vibrating motion of the consecutive particles.

5) The wave velocity is different from the particle velocity.

6) The particle velocity is a function of time. It is different at different displacements. The wave velocity is constant, depending upon the nature of the medium.

7) The vibrating particles of the medium possess both kinetic and potential energies.

Linear Momentum

The quantity of motion contained in a body is known as the linear momentum of the body.

Consider two balls A, and B. Consider that ball A is heavier than ball B. That is, the mass of ball A  is greater than the mass of ball B. Suppose both the balls are moving with the same velocity. It is observed that the force needed to stop the heavier ball (A) is more than the force needed to stop the lighter ball (B). This shows that the heavier ball has more quantity of motion than the lighter ball. Thus, the linear momentum of a body is directly proportional to the mass of the body.

Now consider a situation where two balls of the same mass are moving with different velocities. It is found that the force needed to stop a fast-moving ball is more than the force needed to stop a slow-moving ball. This shows that the quantity of motion of a body having high velocity is more than that of a body having low velocity. Thus, the linear momentum of a body is directly proportional to the velocity of the body. Therefore, the linear momentum of a body is the product of the mass and the velocity of the body.

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