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# Plotting projectile displacement, acceleration, and velocity

Plotting projectile displacement, acceleration, and velocity as a function of time. Created by Sal Khan.

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• If acceleration is always constant, when the ball is in its peak, would the acceleration be zero?
• The velocity is zero at the peak, but not the acceleration. If the acceleration were also zero, the ball would not fall back down and stay in the air. The acceleration from gravity causes the velocity to change in the downward direction.
• I didn't understand how a ball thrown in the air could have an initial velocity. Can you please give an example?
• Initial velocity in this case refers to the velocity of the ball with which it is thrown in the air .A ball thrown in the air not having initial velocity would mean holding the ball in the hand and then removing our hand just leaving it to the mercy of the earth's gravity. In order for the ball to move up a force has to be applied which is against the gravitational pull and the applied force results in velocity to the ball.
• It may be a silly question but can somebody help me here? When the ball hits the ground, I just can't figure out how come the velocity of the ball is not zero (even for a short while - same as velocity of ball at its peak in air). I understand it would hit the ground hard but couldn't it have to stop shortly before bouncing back? Thanks
• No question is a silly question, Pham. So, the final velocity is the velocity of the object right before it touches the ground. It is not the velocity after hits the ground.
• In the displacement formula shown in the video (S = Vi*t+1/2a(t)^2), Kahn only applies the 1/2 to the acceleration, not time squared. In other words, when time is 2 seconds, he takes half of the -9.8m/s, but then uses the full 4 seconds. Is that correct, or does the 1/2 not get distributed to the full value (including time)?
• Its actually 1/2*a*t*t (=(at^2)/2) so its the same thing.
So when you simplify 1/2*9.8*t*t you get -4.9*t*t
• Are we missing a video? I’m kinda confused 😂
• I agree, it feels like there was supposed to be a video before, but it wasn't included
• if a ball is thrown upwards from the top of a tower and then falls on the ground ,is acceleration g or -g
• You can always define direction however you want, as long as you are consistent throughout your problem. If you define down as positive, then acceleration is 9.8 m/s^2. If you define up as positive, then acceleration is -9.8 m/s^2.
• I learned that acceleration due to gravity is 9.81 m/s^2. How much of a difference would it from his answers using just 9.8?
• Gravity acceleration on Earth's surface varies anywhere from 9.78 to 9.83 m/s^2 depending on where you are, so it doesn't make much sense to go beyond 9.8 m/s^2.
• i don't understand why didn't Sal calculate the mas of the object? wouldn't it mean that if we drop 2 objects, the object with a greater mass and the object with the smaller mas will fall at the same time if they fell at the same time and same distance?
(1 vote)
• On earth, the mass of the object does actually make a difference to the rate of acceleration. This is due to air resistance. However, in mathematics, calculations are usually made without factoring in air resistance. Scientists once conducted an experiment on the moon (there is no air on the moon) where an astronaut dropped a feather and a hammer, from the same height and at the same time. Both the feather and the hammer landed on the ground at exactly the same time.