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### Course: AP®︎/College Physics 1>Unit 1

Lesson 3: Average velocity and average speed

# Average velocity and speed worked example

Sal solves a word problem to find average velocity and speed of an object in one-dimension.

## Want to join the conversation?

• After Sal calculated the average velocity and rounded it off to 2 significant digits, shouldn't the calculated value be negative 0.033 instead of simply 0.033?
• There should be a note on the bottom right-hand corner that states "Sal rounds the -1/30 to 0.033, but forgets the negative sign."
• I'm still confused about the difference between average velocity and average speed. Can anyone help me, please?
• Although it doesn’t help you understand the meaning behind the concept, one way I have of remembering which one applies to the one that’s a vector, displacement, and which one applies to the one that’s a scalar, distance traveled, is: Vector and velocity go together, and both start with “v”, while scalar and speed go together, and both start with “s”.
• At , I thought it was suppose to be Xf - Xi. Why did you subtract all three numbers?
• he was trying to get the distance the pig traveled(displacement), which obtained by Xf-Xi as you said but this equation only works regarding position on a number line. He is trying to get the displacement by using the length the pig traveled, and he got the displacement correctly.
• in case of calculating average velocity , shouldn't the final position be -30m ,hence the displacement should be -30 - 20m = -50m? please explain me if i am wrong
• In the case you are talking about, you have taken the 20m part as the pig running in the negative direction while the question states that the pig ran to the right and rightwards has to be taken as positive. Hence the 20m will be positive and the answer will be negative 10m.
• What year in school do we learn this? What grade?
• in highschool (9th-12th) depends on what school district you may be in, just as an example the district I'm in does it 11th-12th grade
• How is rate different from speed?
• I'm not a physics student or anything so don't quote me on this but, I think rate is the amount of something per unit of time, while speed has to be distance per unit of time.
• Shouldn't the 0.033 be a negative 0.033
• He said that to one significant digit, 50/300 is approx .1. Should that not be rounded up to .2?
• Its because he said Two significant figures that are 1 and 7 in 0.17
• Wait but didn’t the pig have to take a few seconds to eat? Or does that not count? 👉🏻👈🏻😐
At
• Good point there! I guess he just scooped it up on the run. 😂
• Could someone explain this to me? This video was pretty short and didn't explain it very well. (The stuff in the video in general)
• Sure, I’ll explain. So basically he is saying how to get the average velocity from the amount of seconds the pig traveled, and the displacement of the pig. So he took the displacement and divided it by the number of total seconds it took for the pig to do all of that and then got the average velocity from that calculation. Then he got the Average rate of speed in meters per second using distance NOT displacement. Hope this helps!

## Video transcript

- [Instructor] We are told a pig runs rightward 20 meters to eat a juicy apple. It then walks leftward five meters to eat a nut. Finally it walks leftward another 25 meters to eat another nut. The total time taken by the pig was 300 seconds. What was the pig's average velocity and average speed over this time? And assume rightwards is positive and leftwards is negative and round your answer to two significant digits. So, pause this video and try to work it out on your own. All right, now let's do this together and first let's just draw a diagram of what is going on. So, this is our pig. It first runs rightward 20 meters, so we could say that's a positive 20-meter displacement, so it goes plus 20 meters, ends up right over there. Then it walks leftwards five meters, so then from there it's gonna walk leftwards five meters, so we could call that a negative five-meter displacement and then finally, it walks leftward another 25 meters. So, then it walks leftward another 25 meters, so it gets right over there, so that would be a displacement of negative 25 meters to eat another nut, so it ends up right over there. Now, to figure out our average velocity, let me write it down, so our velocity average and even this is one dimensional, it is a vector, it has direction to it. We specify the direction with the sign positive being rightwards being positive and leftwards being negative. You oftentimes for one-dimensional vectors might not see an arrow there or might not see it bolded and just written like this. But our average velocity is going to be equal to, you could view it as our displacement or our change in X divided by how much time has actually lapsed and so, what is our displacement going to be? What's it? We have plus 20 meters and then we have minus five meters and then we go to the left another 25 meters, minus 25 meters and then all of that is going to over the elapsed change or change in time, all of that is over 300 seconds. So, what is this numerator going to be? This is 20 minus 30, so that's gonna be equal to negative 10, so this is equal to negative 10 meters over 300 seconds. So, the average velocity is going to be equal to negative 1/30 meters per second, the negative specifies that on average the velocity is towards the left. If you wanna specify this as a decimal with two significant digits, this is going to be, so this will approximately equal to 0.033. That would be 1/30. Now let's try to tackle average speed. So, our speed, R sometimes is used for speed, R for rate, our average speed is not going to be our displacement divided by our lapsed time, it is going to be our distance divided by our elapsed time and we'll see that these are not going to be the same thing. That's one of the points of this problem, so our distance divided by our lapsed time. So, what's our distance traveled? Well, it's gonna be the absolute value of each of these numbers, so it's gonna be 20 meters plus five meters plus 25 meters. Notice, there's a difference here. We're not subtracting the five and the 25, we're just adding all of that. We just care about the magnitudes. Divided by 300 seconds and so, this is going to be equal to 50 meters, over 300 seconds which is equal to five over 30 which is equal to 1/6 of a meter per second and if we want to write it as a decimal, let's see, six goes into one, let's put some zeros here, six goes into 10 one time, one times six is six, I can scroll down a little bit and then we subtract, we get a four, six goes into 40 six times, six times six is 36 and then we get, let me just scroll down a little bit more again and then we get another four and then we're just gonna keep getting sixes over here, so this is gonna be approximately equal to 0.1, if we want two significant digits, 17 meters per second and we are done. We figured out the average velocity and the average speed.