Applying multiplication of signed numbers to motion: writing equations
About this video
A drone moves at a constant velocity of -4 meters per second for 5 seconds. The video explains that the negative sign shows direction and that velocity times time gives the change in position: -4 x 5 = -20 meters.
You'll be able to
- explain what a negative velocity means.
- write a multiplication equation for change in position using velocity times time.
- rule out equations that would need negative time or the wrong direction.
Key words
- velocity
- Speed with a direction; the sign tells which way something is moving.
- change in position
- How far and in which direction something has moved from where it started.
- constant
- Staying the same the whole time.
Transcript
- [Instructor] We're told that for the final five seconds of its flight, a drone travels at a constant velocity of negative four meters per second. Which multiplication equation could represent the drone's change in position? Pause this video. Have a go at that.
Okay, now let's just do this together. So the first thing you might be wondering is: What does it mean to have a constant velocity of negative four meters per second? Why do we have that negative there? Well, when we're talking about velocity, we care about its direction.
And usually someone might say if it's positive or negative, maybe negative means going to the left, positive means going to the right, or maybe it's a change in altitude, although here it looks like it's horizontal velocity and so maybe it's moving to the left; whatever left means in this situation.
And so if you wanna figure out the drone's change in position, what we wanna do is multiply its velocity, so negative four meters per second, and you wanna multiply it times the amount of time, so times five seconds is going to get us to negative 20, and the units work out.
Meters per second times seconds is meters. So negative 20 meters. So let's see which of these equations actually match that, and they didn't put the units here which is fair game. So essentially we just wanna see negative four times five is equal to negative 20 which is this first choice right over here.
This negative four times negative five, this doesn't make sense; that would mean negative time here. This one over here would imply negative time and a positive velocity which wasn't what we're talking about. And this one right over here would imply a positive velocity and a positive displacement or a positive position, which also would not be the case.
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