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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 6

Lesson 2: Average velocity and average speed

# Displacement from time and velocity example

AP.PHYS:
INT‑3.A (EU)
,
INT‑3.A.1 (EK)
,
INT‑3.A.1.1 (LO)
,
INT‑3.A.1.2 (LO)
,
INT‑3.A.1.3 (LO)
Worked example of calculating displacement from time and velocity. Created by Sal Khan.

## Want to join the conversation?

• i still dont understand the displacement
can you help •  The simplest way to remember what is the meaning of displacement is to just remember this phrase- "Displacement is the shortest distance between two points in space."
To illustrate, imagine a body which travels 4m north and turns right at 90 degrees to the east, and travels 3m. So the shortest distance or DISPLACEMENT is actually the hypotenuse, or 5m. Best of Luck!
• Why is s used for displacement, and not d with a half-arrow over it? How did it come about that we used s instead? •  The answer to this question is calculus. In calculus, Godfrey Wilhelm Leibniz used the notation d/dx to indicate when he was taking a derivative of an equation, as opposed to Isaac Newton who simply used a hash mark. (As it turns out, Leibniz's notation is much simpler to use in calculus when dealing with derivatives and integrals and the like.)
So, the reason s is used is so that when you start doing calculus in physics, you don't make a confuse the displacement of an object with taking a derivative of an object and get a really bad answer.

I hope that helped.
• I was taught that since the answer is "south" it should have a negative and a y-hat symbol. Is that wrong? • What's the difference between displacement and distance? • The first answer was correct, displacement does need a quantity and direction. Displacement can be calculated by measuring the final distance away from a point, and then subtracting the initial distance. Displacement is key when determining velocity (which is also a vector). Velocity = displacement/time whereas speed is distance/time. If I walked to school, then i realized that I forgot my homework and ran back home (all of which took me 20 min. and I live 500 meters away from school), then my average velocity would be 0meters/20min. My average speed on the other-hand would be 1,000meters/20min.
• Why is it not -300 m? Or is it just 300 m because Sal includes that the direction is to the south? If he didn't mention that the direction is south would it be -300 m? • Yes, it is just 300m because he used 'south' when describing her displacement. If there is no direction then only will you put a sign (this is to keep it a vector). Plus, there is no way of knowing which direction '-' is. Is it south, or is it west? Maybe it is forward or backward. Unless it states it in the problem, the safe bet would be to simply put the direction.
• At , Sal cancels out the seconds... How does that work? How come we can cancel out the units? What about the corresponding numbers? • if we denote west as (-) and east as (+)
then what do we denote north and south with .
the above infor mation was from the video before this one
solving of time. • Normally in a problem, you will be given only the forward and backward directions which are opposite to each other. This is One dimensional motion (motion in a straight line). Usually, you will be given only opposite directions.(like east and west, north and south, up and down, etc.) When you are given east and west, you won't have north and south in your problem and vice-versa.
North, east are normally denoted as positive.
South, west are normally denoted as negative.
You usually won't get any problems having north, east or south, west mixed up. Ignore signs in such cases.
• my teacher talked about m/s/s or m/s2 what does that mean? • Hello,
Can I get an example of when "time" is not actually "change in time"? In every problem I can think of, "time" pretty much means "change in time".
Thanks a lot!  