We have seen previously that speed is the amount of distance covered in a given amount of time. It’s nothing but distance covered over the time taken to cover that distance! Simply put, it tells us how fast an object moves. Also, if we know the speed and the time taken to cover a distance, we can easily find the distance covered. So if I tell you that I travelled at a speed of ‘50 meters per second’, for one minute, can you tell me how much distance I covered? I am sure you have come up with the answer. I have covered a total distance of 3000 meters. But if I covered 3000 meters from this point, am I here or here or over there? You had no clue as to which DIRECTION I headed off. This tells us something interesting! Speed offers ONLY the MAGNITUDE, making it what we call in physics a SCALAR quantity. It doesn’t specify the DIRECTION. It just tells how much ground is covered in unit time, regardless of the direction travelled. Scalar quantities have only magnitude and DO NOT specify direction. Do you remember the difference between distance and displacement? Distance just tells us the distance covered. But displacement is distance with direction. Say the distance between towns A and B is 20 kilometers. So if we take a 50 kilometer route from point A to B like this, then the distance covered is 50 kilometers and the displacement is 20 kilometers north of point A. So speed does not tell us the direction. Is there anything that tells us the speed along with direction? Yes, it’s velocity which can be written as lower case ‘v’! Let’s see how it differs from speed. What if I tell you that I travelled NORTH at a speed of ‘50meters per second’ for one minute. Would you be able to tell me where I landed up? Yes you would know I have covered a total distance of 3000 meters and landed up at this point. Velocity is SPEED with DIRECTION of motion. As such, Velocity is “Direction Aware”. So when you mention VELOCITY, you must keep track of direction. Something like the Car moved SOUTH at 55 miles per hour. If you do not mention the direction and just say the car moved at 55 miles per hour, you are not stating the velocity anymore. What you are stating is the SPEED. What do you think is the unit of measurement for velocity? The unit of measurement for velocity, just like speed is, ‘meters per second’. Now you might ask why ‘meters per second’! The Cars we drive indicate speed in miles per hour or kilometers per hour. So why is the unit for velocity meters per second? The established SI unit of measurement for velocity is meters per second. And SI stands for International system of units. This helps everyone in the world adhere to one common style of measurement. Imagine the confusion created with different measures like miles and kilometers, gallons and liters, US gallons and Imperial gallons, etc. Incidentally, speed and velocity are both measured in ‘meters per second’. But do you remember which one is a scalar quantity and which one is a vector quantity? Yes, speed is a scalar quantity, and the velocity is a vector quantity as the direction is also specified. Now let me ask you an interesting question. What is AVERAGE velocity? To make it a bit easier for you, let’s look at the example we looked at in our first video on motion. You have to travel 20 kilometers up north. But because of the road works, you took an alternative path which totaled 50 kilometers and you took 2 hours to reach the destination. This was your path of travel. Let’s make two columns here, one for ‘average speed’, and another for ‘average velocity’! Do you remember how we calculated average speed? Yes, it is the total distance covered, over the total time taken. So ‘50 over 2’ will give us ‘25 kilometers per hour’ as the average speed. Now what will be the average VELOCITY here? The average velocity depends only on the start and end points, regardless of path taken. Do you remember we had learned about Displacement? So the Average velocity equals Displacement over Time. What is the displacement here? It’s 20 kilometers up north. Distance with direction! So the average velocity will equal 20 kilometers over 2 hours, which will equal 10 kilometers per hour. That’s the concept of average velocity! So if you think you’ve really understood this concept, here’s a question for you. You are a formula 1 race car driver. You had a great race and finished the 300 kms race in one hour and forty minutes, at number one position. What was your average velocity? Okay, if you didn’t get it, let me give you a hint. This was your start point and finish line as well. They were both the same. You went around the track several times only to eventually finish at the same point where you started. So tell me now, what was your average velocity? First, can you tell me what your displacement is? You started and finished at the same point. So your displacement is ZERO. Zero divided by the time taken which is one hour forty minutes, equals zero. That puts your average velocity for the race today at zero meters per second. If however, you are trying to calculate the speed you drove at, that would be a different answer. Try and work it out. So now let me ask you another question. I was also in the race and you probably didn’t notice me because I finished last. I took 3 hours to finish. What was my average velocity? That’s correct. Zero again!