C. Magnetism
It is a physical property, not a chemical property.
The centripetal force is force acting on a body in circular motion. In circular motion, velocity is always on tangent and if we took 2 different positions on a circle, the change on velocity is a vector pointing in the middle of circle. In circular motion velocity is constant, and acceleration lies on radius of circle pointing to te middle. This acceleration is called centripetal acceleration, and the force is centripetal.
C. Their strengths are added
Refer to the diagram shown below.
μ = the coefficient of dynamic friction between the crate and the ramp.
1. The applied force of F acts over a distance, d.
The work done is F*d.
2. The component of the weight of the crate acting down the ramp is
mg sin(30) = 0.5mg.
The work done by this force is 0.5mgd.
3. The normal force is N = mgcos(30) = 0.866mg.
This force is perpendicular to the ramp, therefore the work done is zero.
4. The frictional force is μN = μmgcos(30) = 0.866μmg.
The work done by the frictional force is 0.866μmgd.
5. The total force acting on the crate up the ramp is
F - mgsin(30) - μmgcos(30) = F - mg(0.5 - 0.866μ)
6. The work done on the crate by the total force is
d*(F - 0.5mg - 0.866μmg)