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### Course: Physics library>Unit 1

Lesson 5: Old videos on projectile motion

# Projectile motion (part 4)

Solving for time when you are given the change in distance, acceleration, and initial velocity. Created by Sal Khan.

## Want to join the conversation?

• In one of the questions you have to solve for Vi using the sqrt route of Vf^2+2AD, which of course means the answer could be the negative or positive version, I typed in the positive version and got it wrong. I was wondering if there is a quick way to determine which answer fits? for time is simple as it can't have a negative value whereas acceleration can
See screenshot for question and calculator
• I solved this question by first solving for the final velocity using "v2 = u2 + 2ad" and then solved for time using "t = d / average velocity". How do I decide whether I have to use the positive final velocity or the negative final velocity for the time calculation?
• If it is movement in the direction you use + for then use +. If it is in the direction you use for - use -.
(1 vote)
• Is the chance in distance the displacement or is it how much the object moved? For example, an object moved 5m to the right and 2m to the left. Would the change in distance be 7m or 3m?
• The net "change in distance" is 3m.
Displacement is basically the shortest distance from the initial position to the final position. So if you see the Object moving 5 steps forward and 2 backward the distance from the Object's initial position is 3 steps.
Distance on the other hand is "total" distance that the object has traveled. So the object has first walked 5 steps then 2 steps (here the direction does not matter since you are calculating the magnitude) So, totally the object has taken 7 steps.
• I have a question: If a scientist invents the time machine and tests the prototype he successfully goes back in time by 10seconds(excluding the confusion of his co-workers and his past-self☺) would in this case time be negative?
• Yeahh, Leo! Time can be called Negative in this scenario. Great Creativity:)
• I got a bit confused in the last part.Where did the 10 s come from?
• It came from the first video (or part 1, this is part 4), where Sal threw a penny of a cliff 500 meters high and it took 10 seconds for the penny to reach the ground.
• why does a negative time (t) makes no sense?
(1 vote)
• Because it would mean that the thing landed before it launched (assuming t=0 at launch)
• When sal put change in distance, he put Ad, would it be the same equation if you put a delta symbol in front of the d?
(1 vote)
• I think you are mistaken. Sal never put Ad for change in distance. He had put `delta d` only. May be you didn't understand the handwriting. It's absolutely alright!
Hope this helps:)