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A simple numerical on momentum

Car mass is 1200 kg moving velocity is 108 km/hr. On applying brakes velocity reduced to 36 km/hr. Find change in momentum?

(posted by Aditi)

m=1200 kg

u=108 x 5/18=30m/s


Change in momentum = m(v-u) = 1200 x (30-10)=1200 x 20= 24000 kg m/s

Why gravitational force of attraction is necessary for a satellite to revolve around earth?

The question was posted by Raj Kumar.

The satellite has a velocity. To revolve around, the direction of velocity must change. In the absence of a force it can move only in a straight line. It is the gravitational force which helps the satellite top change direction and revolve around the earth.

In other words, for a satellite to revolve in a circular path, a centripetal force is required. This centripetal force is provided by the force of gravitation only.

The mechanism of operation in a capacitor

The DC doesnt flow through capacitor. I thought it was because the electrons jus get saturated on one side and are lost on other side, such that it can’t take or give up anymore or plausibly due to voltage developed across it whereas in AC as directions keep changing, saturation doesnt occur. But it if it were so then AC would flow through two plates seperated by large distances or switches which can be considered capacitors.So please guide me as to what’s actually happening during current flow through capacitor and how it happens? (The question was posted by Thushar Misra)


Your logic is almost correct. A capacitor is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When there is a potential difference (voltage) across the conductors, a static electric field develops across the dielectric, causing positive charge to collect on one plate and negative charge on the other plate. Energy is stored in the electrostatic field.

A current is not “flowing” through a capacitor, the “current” that exists between the plates of a capacitor is called “Displacement current” which is actually caused by the time varying electric flux. This causes the circuit to be continuous though there is no electrical contact or flow of charges between the plates of the capacitor.

When a capacitor is connected to DC, there exists a change in electric flux when the switch is just turned on but soon it reaches steady state and there is no “current”. A capacitor can “conduct” only when there is a “change” in electric flux, which is continuously taking place in an AC.


Tested Physics Links to Learn the Laws of Motion online

Newton’s Laws of motion

First Law: Every body continues to be in its state of rest or state of uniform motion in a straight line unless an external force complels it to do so.

Second Law: The rate of change of momentum of a body is directly proportional to the net external force acting on it and the change in momentum takes place in the direction of motion

Third Law: To every action there is an equal and opposite reaction

The following links will help you learn the topic further and will provide an online resource for understanding the concepts related to it.

Change in Resistance due to stretching a wire

Sreshtha asked:

“A piece of wire is redrawn without any change in volume so that its radius become half the original. Compare the new resistance with the original value.”



When we redraw the wire, the volume remains constant and the resistivity also remains constant. SO, the variables are

  1. area of cross section and
  2. the length

When radius becomes half, the area of cross section increases in such a way that A1l1 = A2l2. This implies that when length is halved, area of cross section is doubled.

We know that

Therefore, the new resistance becomes (1/4) th the original value.

So, R2:R1 = 1:4

A problem from Transformation of Energy

Rosie asks:

“I can’t figure out how to solve this problem:

The total mass of a motorcycle and rider is 250kg. During braking, they are brought to rest from 16m/s in a time of 10s. What is the maximum amount of energy converted into heat by the brakes?

I don’t know whether to use WD = F x d or another set of formula?”


This question is based on conservation of energy. The heat energy liberated is equal to the change in kinetic energy. Since the bike is brought to rest, the change in KE is equal to its initial KE.

The answer assumes that the entire KE is converted into heat energy alone.
Heat =Change in KE =

Will the orbits of planets in the solar system change or will it be the same forever?

Sreeda asks:

Will the orbits of planets in the solar system change or will it be the same forever?


The orbits change over time.

More details To be posted soon

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