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Course: Physics library>Unit 1

Lesson 5: Old videos on projectile motion

Projectile motion (part 2)

A derivation of a new motion equation. Created by Sal Khan.

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• By solving in the end, we got 104 m/s. Yet, the penny is moving downwards so it should be -104 m/s. But, in the actual math, the negatives cancelled out and we were left with a positive integer. So to indicate downward motion, do we just put a negative in front of the 104, or should we have gotten a -104 at the end of the math? The math Sal did wasn't wrong, so I am confused.
• While solving for the final velocity, it is obvious that the penny is going downwards (conventionally represented as negative). Sal found out that Vf^2=10900. He took its square root to get approximately 104 m/s. The square root of a positive number can be both positive and negative. For example, 2^2=4 and (-2)^2=4. In this case Sal should have taken the negative square root of 10900, not the principal root. So, the final velocity is definitely -104 m/s or 104 m/s downwards.
Hope this helped.
• Should the velocity be -104 m/s?
• Sir,
Does the weight of the coin matter according to its mass?
• Can somebody explain what Vf and Vi really is?
I need a better understanding.
(1 vote)
• Vf is the final velocity after a time interval.
Vi is the initial velocity at the start of the time interval.
• by convention.... should't the downward direction be +ve since we're falling towards gravity(center of earth) and shouldn't upwards be -ve since we're moving against gravity ?
(1 vote)
• The usual convention is to assume the positive direction is the direction of motion not the direction of acceleration but the either way works.
• At , if the penny was thrown up at a velocity of 30 m/s, then it would go up, reach a velocity of 0m/s and by the time it comes down to the same point from which it was thrown, wouldn't the velocity at that point then be different than 30 m/s?
(1 vote)
• It would be -30 m/s, since velocity is a vector.
The speed at the bottom would be the same on the way down as on the way up.
It has to be, because the acceleration is constant the whole time, and the distance up is the same as the distance down.
(ignoring air resistance)
• doesn't weight matter when you fall?
• How high up will the penny have gone before it begins to fall? Is there another equation/formula for this?
(1 vote)
• well yes jacob you can figure it out because you know before that penny starts falling the penny will reach such a point where its velocity will be zero so u can calculate it by using this formula-
final velocity= initial velocity + acceleration x time taken
where your final velocity is equal to zero. I hope my answer and explanation was helpful and easy to understand for you