Writing multiplication equations with rational numbers to represent situations
About this video
A window washer's platform descends at 3/4 meter per second and its total change in height after t seconds is -9 meters. The video writes rate times time equals change: -3/4 x t = -9, also written -3/4 t = -9.
You'll be able to
- use a negative rate to show something moving down.
- write a multiplication equation of the form rate x time = change for a situation.
Key words
- rate
- How much something changes each second; its sign shows the direction.
- descends
- Moves downward, so the change in height is negative.
- change in height
- How much higher or lower something ends up than where it started.
Transcript
- [Instructor] We're told that a window washer's platform descends at a rate of 3/4 of a meter per second. After t seconds, the platform's total change in height is -9 meters. Write an equation using t to represent this situation. So why don't you have a go at that?
All right, now let's do this together. So we essentially just want to multiply the rate times the time to figure out the change in height. So our rate, we're descending at a rate of 3/4 meters per second. So our rate is negative. It's giving us our direction. So our rate is -3/4.
If we wanted to write units, we could write meters per second. And then we would want to multiply that times the number of seconds, so times t seconds. Maybe that's confusing to put the units, so let me just put times t seconds is going to be equal to our total change in height, which is -9 meters.
So we could write it this way. We could also write -3/4 times t this way is equal to -9. We could also write -3/4 t, this is a four right over here, is equal to -9. And so this means our rate is going down after some number of seconds. We have gone down by nine meters.
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