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### Course: Physics library>Unit 1

Lesson 5: Old videos on projectile motion

# Projectile motion (part 1)

Using the equations of motion to figure out things about falling objects. Created by Sal Khan.

## Want to join the conversation?

• how do you know when to use distance vs displacement when dealing with projectiles?
• You use distance when you want to find the length that was travelled by an object. You use displacement when you want to find out the distance from your original starting point to the final point of which you have travelled.
• I am confused as to why the acceleration due to gravity is -10 when it is acting downwards and so should be +9.81 as this is the regular direction for gravity....
• They were just asking why they used 10 m/s when it should be 9.8 m/s which is the correct acceleration due to gravity on the earth.
• I've been looking for a video about up and down motion in freefall, ie throwing a ball up in the air and then calculating its speed when it hits the ground, and I haven't been able to find one here. Do you have one? Is it named something else?
• I think projectile motion would be the one.

You could also try one on motion in a straight line
• In this video, the distance or the change in distance is negative. Well, isn't distance suppose to be positive in any given direction?
Like if i am in an elevator and i move from the 7th floor to the ground floor, it wont make sense to say that my "change in distance" is -7 floor! (plus distance is a scalar quantity - no direction - no negative signs)
i think it should be replaced with Displacement or change in 'Position', which is more appropriate, and is a vector quantity, and will make more sense.
• If you are in the 5th floor and you go up by minus 5 floors, you will end up in the ground floor. The negative just means that it is in the opposite direction of what you consider positive.
• How fast an object thrown at the sky that he should not come back anytime.
• What you are trying to ask is what is known as the escape velocity. Look it up on the net. It depends on the magnitude of gravity at any place.
• How come distance can be a negative number? I have been always told by my instructor that distance can never be a negative number.
• He is basically using the negative number to refer to going down the screen. As said earlier in the video, he defined acceleration due to gravity (which is pointing down the screen) as - 10 m / s. -500 basically means you are going down the screen another 500 metres.
• But isn't it supposed to be 'displacement' instead of 'distance' because distance is scalar and displacement is vector?
• You're right, once distance (scalar) is associated with direction its considered displacement which is indeed a vector quantity. In the video Sal was talking about the 'magnitude' of the height from which the object was dropped, which is essentially distance (Assuming the object fell in a perfectly straight vertical line) . When referring to a direction i.e up down (positive\negative) then it should be called displacement.
• How can the acceleration due to gravity be -10 m/s?
• The acceleration is -10 m/s because gravity is pulling down in the negative direction. If you were to throw something up it would be 10 m/s because its going up in the positive direction.