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# Tag Archives: direction

## 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.

## Solving problems on Laws of Motion

Sir,in solving problems of laws of motion ,is it mandatory to take that direction as +ve in which acceleration acts?(i.e should i subtract other forces acting on a body from the forces that is acting towards the acceleration)

(**Pranay Talukdar** asks)

Answer: In general, we take the forces in the direction of acceleration (I.E, ENHANCING THE ACCELERATION) and the forces against the acceleration as negative.

The first step in solving such problems is to draw a free body diagram and represent all active forces and reactions to constraints in vector form with proper direction ad label. This will highly simplify the problem.

## Potential Difference – misconception

What actually it means when we say “potential difference across..” ? Does it indicate that the somehow the energy level of electrons across it is less than on the other side or is it just the amount of energy consumption across it. If latter is true then in AC we’ve voltages which dont algebraically add up together to give the applied voltage . reason stated is the phase difference. How can there be phases in something scalar like energy consumption ? (Posted by Thushar)

Answer: As per definition, “Potential difference is defined as the work done per unit charge in carrying a positive test charge between two points against the electric field.”

Just like water flows from a high altitude to low altitude, “current” flows from high positive potential to low positive potential. (And electrons flow in the reverse direction. Though electrons are the actually movable materials contained in a conductor, the direction of current was conventionally defined as the direction of motion of positive charges)

So, when there is a difference in potential the electrons will flow from lower “positive potential” (where the negative potential is high) to higher positive potential.

Phase refers to the angle of rotation corresponding to the present value of emf or current. One complete circle (or rotation) corresponds to 2(pi) phase. There is a difference between current and voltage refers to this.

In describing AC, we can imagine that the current and voltage in wave form is obtained by the projection of rotating vectors, I0 and E0. The difference in angle between these which represents the waveform obtained for E and I correctly is known as the phase difference.

(I feel that this won’t suffice. Please as more questions to clarify your point; preferable as comments to the same post)

## Speed Velocity and acceleration

Sir ir it true or false – “the speed of particle remains constant.this means that no unbalanced force acts on it.” (Purshkar asks)

Answer: If velocity remains constant, then we can say that no unbalanced force acts on it.

## Sign Convention in kinematics

Please tell me the sign convension for velocity & accleration for a body moving upwards ,downwards, right and left relative to another body? (Apoorv Katoch asked)

**Answer**: In motion, sign conventions are arbitrary. You take one direction as positive and the direction opposite to it as negative.

For example, if you consider a body projected vertically upwards, and if you take the initial velocity as positive, then all measurements taken in the upward direction will be taken positive and all measurements taken in the downward direction is taken negative.

You can ask further doubts related to this as comment to this post.

## Problem based on electromagnetic induction – motional emf

A horizontal straight wire 10 m long extending from east to west is falling with a speed of 5.0 m s^{–1}, at right angles to the horizontal component of the earth’s magnetic field, 0.30 × 10^{–4} Wb m^{–2}.

(a) What is the instantaneous value of the emf induced in the wire?

(b) What is the direction of the emf?

(c) Which end of the wire is at the higher electrical potential?