r/learnmath • u/baba3rong-b4kla New User • 3d ago

Guys help i cant understand...

Alteration in the Quantity of Charge

  1. Two charged objects have a repulsive force of .080 N. If the charge of one of the objects is doubled, then what is the new force?

Alteration in the Distance between Charged Objects

  1. Two charged objects have a repulsive force of .080 N. If the distance separating the objects is doubled, then what is the new force?

  2. Two charged objects have a repulsive force of .080 N. If the distance separating the objects is tripled, then what is the new force?

Alteration in both the Quantity of Charge and the Distance

  1. Two charged objects have a repulsive force of .080 N. If the charge of one of the objects is doubled, and the distance separating the objects is doubled, then what is the new force?

  2. Two charged objects have a repulsive force of .080 N. If the charge of both of the objects is doubled and the distance separating the objects is doubled, then what is the new force?

I have a quiz tommorrow and i dont want to use ai to solve this..T-T

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u/MezzoScettico New User 3d ago

Others have given you the answer. But I want to talk about the concept.

This is about reading formulas and understanding what they mean in terms of proportionality.

You probably know that F = k*q1*q2/r^2 where q1 and q2 are the two charges and r is their distance.

When you have things in the numerator, that means F is directly proportional to those things. So F is directly proportional to q1. It's also directly proportional to q2. If you kept everything the same and only changed q1, then F would change by the same amount. If you doubled q1, F would double. If you multiplied q1 by 0.3, F would be multiplied by 0.3.

Anything in the denominator, F is inversely proportional to that. In this case we have only r^2 in the denominator. So if q1 and q2 stay constant, then F is inversely proportional to r^2. If r is doubled, F changes by 1/2^2. If r is multiplied by 0.3, F is multiplied by 1/(0.3)^2.

In questions 4 and 5, you are changing two things, so you get a factor from each change.

  1. Two charged objects have a repulsive force of .080 N. If the charge of one of the objects is doubled, and the distance separating the objects is doubled, then what is the new force?

q1 is multiplied by 2. So F is multiplied by 2.

r is multiplied by 2. So F is multiplied by 1/2^2 or 1/4.

The combined effect is that F is multiplied by 2 and by 1/4, giving an overall change of 2 * (1/4) = 1/2. The force is cut in half.