Net force refers to the (vector) sum of all the forces acting on something. It's a mathematical construction, so it's not a single identifiable force. The forces themselves are real, but the net force is not an actual force.
Hope this helps you out.
Answer:
a) 5 N b) 225 N c) 5 N
Explanation:
a) Per Coulomb's Law the repulsive force between 2 equal sign charges, is directly proportional to the product of the charges, and inversely proportional to the square of the distance between them, acting along the line that joins the charges, as follows:
F₁₂ = K Q₁ Q₂ / r₁₂²
So, if we make Q1 = Q1/5, the net effect will be to reduce the force in the same factor, i.e. F₁₂ = 25 N / 5 = 5 N
b) If we reduce the distance, from r, to r/3, as the factor is squared, the net effect will be to increase the force in a factor equal to 3² = 9.
So, we will have F₁₂ = 9. 25 N = 225 N
c) If we make Q2 = 5Q2, the force would be increased 5 times, but if at the same , we increase the distance 5 times, as the factor is squared, the net factor will be 5/25 = 1/5, so we will have:
F₁₂ = 25 N .1/5 = 5 N
This is EXACTLY the same scenario as the skydiver jumping
out of the airplane, except the whole thing is turned on its side.
==> The skydiver leaves the airplane.
The force of gravity on him (his weight) makes him accelerate down.
But the air resists his downward motion.
The faster he falls, the more UPWARD force the air exerts on him.
The more upward force the air exerts, the less he accelerates down.
When his falling speed is great enough, he stops accelerating, and
falls with a constant speed. He calls that speed his 'terminal velocity'.
==> The submarine turns on its engines, at maximum power.
The force of the engines makes the sub accelerate forward.
But the water resists its forward motion.
The faster it moves, the more BACKWARD force the water exerts on it.
The more backward force the water exerts, the less it accelerates forward.
When the forward speed is great enough, it stops accelerating, and moves
with a constant speed. I don't know if they use the same term in submarines,
but you might say that speed is the 'terminal velocity' in water.
Take 10m/s^2 for the gravitational acceleration, as we know this is a free fall, we can use the equation: d=1/2*g*t^2
Substitute g=10m/s^2, t=5s, d=125m