# Homework Help: Finding the units in an algebraic equation for velocity

1. Dec 28, 2017

### Kennedy

1. The problem statement, all variables and given/known data
During a short interval of time the speed v in m/s of an automobile is given by v = at^3 + bt^4 , where the time t is in seconds. The units of a and b are respectively:
(a) m⋅s^2 ; m⋅s^4 (b) s^3/m; s^4/m (c) m/s^2 ; m/s^3 (d) m/s^3 ; m/s^4 (e) m/s^4 ; m/s^5

2. Relevant equations
None

3. The attempt at a solution
I have not a single clue, and I'm assuming that there is a trick to this that I'm not getting. I know that the velocity is measured in m/s, but I don't understand how finding the units of a and b is even possible.

2. Dec 28, 2017

### Staff: Mentor

What do you know about the relationship between the units of terms in an equation where you are summing the terms? Can you add apples to oranges?

3. Dec 28, 2017

### Kennedy

Nope, I know that you can only add apples to apples, or oranges to oranges, which means that both a and b need to be measured with the same units. Since this is an equation for velocity, I would assume that the units of the right hand side would also need to be m/s.

4. Dec 28, 2017

### Staff: Mentor

Think again. The terms also contain powers of t. What are the units associated with t?
Right. So you need to choose the units of a and b so that their terms (including their t's) both work out to m/s.

5. Dec 28, 2017

### Kennedy

So, I'm looking for units of a and b for which all of the s's in the numerator cancel, and there is only one s left in the denominator, and one m in the numerator?

6. Dec 28, 2017

### Staff: Mentor

Sounds like a plan!

7. Dec 28, 2017

Thank you!