Gravity.: Gravity is the force that acts at a right angle to the path of an orbiting object.
Radiant energy is the energy of electromagnetic and gravitational radiation
Answer:
a) Linear equation
Explanation:
Definition of acceleration

if a=constant and we integrate the last equation

So the relation between the time and the velocity is linear. If we plot the velocity in function of time, the plot is a line, and the acceleration is the slope of this line.
An arrow pointing from the bottom of the ramp to the top, I assume it would be friction.