Which of the following is an example in which you are traveling at constant speed but not at constant velocity?

a) rolling freely down a hill in a cart, traveling in a straight line.
b) driving backward at exactly 50 km/hr.
c) driving around in a circle at exactly 100 km/hr.
d) jumping up and down, with a period of exactly 60 hops per minute.
e) none of the above.

c) Driving around in a circle at exactly 100 km/hr.

This examples represents constant speed but not constant velocity.

Explanation:

To answer this question it is importan to know the difference between the concept of speed and velocity.

speed: Is the ratio of change in the displacement per unit of time. JUST  A MAGNITUDE

velocity: Is the ratio of change in the displacement per unit of time in a given direction. It is a composed value by magnitude and direction, this is know as a VECTOR.

So, in conclusion the speed is the magnitude or the scalar value, for the velocity vector.

In the example,

in a) Rolling freely down a hill in a cart, traveling in a straight line. There is a component of acceleration, from earth gravity, so the speed is changing.

in b) Driving backward at exactly 50 km/hr. The direction and speed are the same, so speed and velocity are constant.

In d) Jumping up and down, with a period of exactly 60 hops per minute. There are changes in the speed, due to the acceleration and decceleration between the changes in the direction. Actually you have to stop to change direction between the ups and downs in the jumps. Also there is the gravitational component, always changing the speed.

In c) Driving around in a circle at exactly 100 km/hr. In this case the speed remains constant, while the direction is changing all the time.

i think the answer is a, hope this is right, that would be my best guess.