Module 1 Review Questions

Which is a vector quantity: speed or velocity?

Velocity is the vector quantity.

What is the main difference between a scalar quantity and a vector quantity?

A vector quantity has information about direction, while a scalar quantity does not

What is the main difference between instantaneous and average velocity?

Average velocity is measured over a finite time interval. Instantaneous velocity uses an infinitesimally small time interval.

On a physics test, the first question asked the students to calculate the acceleration of an object under certain conditions. Two students answer this question with the same number, but the first student's answer is positive while the second student's answer is negative. The teacher says that the both got the problem 100% correct. How is this possible?

Both students got the right answer, but they defined their directions in the opposite way. Since their directions were defined differently, their answers had the opposite signs. The way to avoid this confusion is to explicitly state the direction in reference to a fixed point. For example, 2.3 m/sec^2 towards the building is better than simply -2.3 m/sec^2.

An object's velocity is zero. Does this mean its acceleration is zero? Why or why not?

No. Just because velocity is zero, acceleration does not have to be zero. Acceleration is the change in velocity.If the velocity is zero for a moment and then not zero the next moment, the velocity has changed, and therefore the acceleration cannot be zero.

What do physicists mean when they say that velocity is "relative"?

Physicists say that velocity is relative because the velocity an observer sees actually depends on the velocity of that observer.

What kinds of graphs (what-versus-what) do you study if you are interested in learning about acceleration?

Since acceleration tells us how velocity changes with time, you need to study velocity-versus-time graphs to learn about acceleration.

You are reading through someone else's laboratory notebook, and you notice a number written down: 12.3 m/sec^2. Even though it is not labeled, you should immediately be able to tell what physical quantity the experimenter measured. What is it?

Since the units attached to the number are m/sec^2, the only possible physical quantity it could be is acceleration, since that is the only physical quantity with those units.

Another experiment in the same laboratory notebook says that an object has a 1.4 m/sec^2 acceleration when it has a -12.6 m/sec velocity. At that instant in time, is the object speeding up of slowing down?

Since velocity and acceleration have opposite signs, the acceleration is opposing the velocity. Thus, the object is slowing down.

What physical quantity is represented by the slope of a position-versus-time graph?

The slope of a position-versus-time graph is the velocity.