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## Physics library

### Course: Physics library>Unit 1

Lesson 5: Old videos on projectile motion

# Projectile motion (part 5)

How fast was the ball that you threw upwards? Created by Sal Khan.

## Want to join the conversation?

• what does projectile motion mean
• it is a form of motion where an object ( projectile) is thrown obliquely to the earths surface that is an angle not equal to 90 degrees . its 2D motions that is both x and y axis are taken into consideration. ( motion in a plane)
• In the original newer videos, Sal always uses S. Now he is using d. I know that d and S have different definitions, but is Sal using them interchangeably in these videos, or is he using the two different definitions? From my eyes, it seems like he uses them interchangeably, but I'd love some clarification.
• They both are correct, d is mostly used in the States, wheres S is used in Europe. I don't know why he changes them, maybe because of different sources he revises the information from.
• I've wondered if a time when the ball go to the highest point equal to the time when the ball goes back to where its started?
• What is the difference between Scalar and Vector?
• A scalar only has a magnitude, while a vector has a magnitude and direction.
• Why do we need to find the initial velocity? I thought since it is projectile the initial velocity would be 0 because it was yet to be thrown (I hasn't gained speed)
• When you drop an object, the initial velocity is zero. You must find the initial velocity because that is the velocity when the ball leaves your hand.
• Why is the acceleration negative?
• Because acceleration is in another direction. If you go upwards, it will be positive. If you go downwards, it will be negative. Ex: A ball lefted from the top of a building that has 50 meters of height. So, if we calculate the change in distance (delta S, or delta d, whatever), that 50m it's the final height and 0 m is the initial height. In that case, 50m is going to be negative.
• Hello. I am a little confused about the difference of distance and displacement. Can someone please explain in detail? Thanks in advance :-).
(1 vote)
• Suppose the distance between your school is 5 km by the route you follow. If you follow a different route, the distance may be less, say 4.5 km. But no matter what route you follow, your displacement will always be the same. Its something like you have two points A and B and there are a lots of ways of reaching B from A like curved paths, etc. this is the distance. But the displacement is like a line segment(straight path) joining A and B, which is obviously the shortest path. In brief, distance is the total path that you follow and displacement is the shortest path between the initial & final position.
• I got a question so if i have a ball from a cliff i throw it forward.will they be a change is speed?
(1 vote)
• just imagine that you are enjoying your ride on bicycle suddenly you get get a down then your speed [speed of your bicycle] changes it moves faster even though you are not pedling you will be moving fast
in the same way when you throw a ball from cliff speed of ball changes