Fundamental Forces Sample Problem Answers

1. Calculate the force of gravity on a spacecraft 12,800km above the earth's surface if it has a mass of 700kg.

For this problem, we use the force of gravity equation.

FG = (g * 5.98e24kg * 700kg) / (6.37e6m + 12.8e6m)^2
FG = 760.063N

2. Each of two spheres 6.0e-2m apart carries a charge of 2.0e-6C.
a. Calculate the force between them.
b. Is the force attractive or repulsive?

a. 9.986N
b. repulsive

For part a, we need to use the electrical force equation, and since both charges are positive, the are repulsive.

FE = (k * 2.0e-6C * 2.0e-6C) / (6.0e-2m)^2
FE = 9.986N

3. What is the magnitude and direction of the force on an electron travelling at 100,000m/s east in a 1.5T magnetic field directed vertically upward?

2.403e-14N North

Just use the magnetic force equation to find the magnitude and then the right hand rule for the direction, remembering to reverse it because it is an electron.

FB = 1.602e-19C * 100,000m/s * 1.5T * sin 90°
FB = 2.403e-14N

Right hand rule = North