The sun keeps all of the planets orbiting it because
a)the law of universal gravitation states that an increase in mass causes an increase in gravitational force
b)the law of ellipses states that the planets orbit the sun in circular patterns
c) the law of harmonies states that the motion of the planets will not stop unless an outside force is applied
d) the law of equal areas states that planets sweep equal areas in equal periods of time around the sun

Aparticle's position is given by x = 3.00 - 9.00t + 3t2, in which x is in meters and t is in seconds. (a) what is its velocity at t = 1 s? (b) is it moving in the positive or negative direction of x just then? (c) what is its speed just then? (d) is the speed increasing or decreasing just then? (try answering the next two questions without further calculation.) (e) is there ever an instant when the velocity is zero? if so, give the time t; if not, answer "0". (f) is there a time after t = 3 s when the particle is moving in the negative direction of x? if so, give the time t; if not, answer "0".