Problem 21-40a:
the electric field near the surface of earth points downward and has a magnitude of 153 n/c. what is the ratio of the magnitude of the upward electric force on an electron to the magnitude of gravitational force on the electron?
2.74×1012
problem 21-40b:
what charge should be placed on a plastic ball of mass 5.25 g so that the electric force balances the weight of the ball near earth's surface?

As part of your daily workout, you lie on your back and push with your feet against a platform attached to two stiff springs arranged side by side so that they are parallel to each other. when you push the platform, you compress the springs. you do an amount of work of 79.0 j when you compress the springs a distance of 0.230 m from their uncompressed length. (a) what magnitude of force must you apply to hold the platform in this position? (b)how much additional work must you do to move the platform a distance 0.230 m farther? (c) what maximum force must you apply to move the platform a distance 0.230 m farther?

It is reasonable to assume that the bulk modulus of blood is about the same as that of water (2.2 gpa). as one goes deeper and deeper in the ocean, the pressure increases by 10000 pa for every meter below the surface. if a diver goes down 80.0 m in the ocean, by how much does each cubic centimeter of her blood change in volume? give the answer in cubic centimeters (actually one cubic centimeter equals one milliliter).

Is the vertical component of velocity ever zero?