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# AP Physics 1 review of 2D motion and vectors

In this video David quickly explains each 2D motion concept and does a quick example problem for each concept. Keep an eye on the scroll to the right to see where you are in the review. Created by David SantoPietro.

## Want to join the conversation?

• Hi all. Here's a dropbox link to download the 2D motion notes if you want them. Good luck on the exam! https://www.dropbox.com/s/c0n8o0v7ikp4d57/2D%20motion%20concept%20review%20sheets.pdf?dl=0
• Thank you very much, I really appreciate that ;)
(1 vote)
• why are you taking Vx as cos and sin why not tan ....?
(1 vote)
• you need to review trigonometry and vector components. Sal has vids on both.
• In a projectile, is the velocity when it strikes the ground equal to the velocity with which it was thrown? Also, are the angles the same?
(1 vote)
• The horizontal velocity in a projectile always remains constant since there is no acceleration, the vertical velocity will remain the same only if initial y and final y are the same. Likewise, the angles at launch and at the end of the projectiles' motion are the same only if the initial y and final y are the same, because projectile motion is parabolic.
• how would you find the volcity vector of a trajectory
(1 vote)
• In projectile motion, the magnitude of the velocity vector decreases until the projectile reaches its maximum height, and then increases while gravity is in the same direction as the motion (when it's traveling down). In reference to the horizontal, the angle of the velocity vector would decrease, and then become negative after the highest point of the projectile.
To look at why this happens, we can look at the individual components of the velocity vector. The x-component isn't affected during projectile motion, so it stays constant. However, the y-component is affected by gravitational acceleration. This causes it to go downwards by roughly 10 m/s per second. During the trip upwards, the y-component decreases in magnitude while the x-component stays constant. This means that the x component has a bigger "share" in the total magnitude of the vector, which means that the angle will get closer to the horizontal. When the projectile descends, the opposite happens, which turns the angle more and more to straight down.
To calculate the magnitude and direction of the velocity vector, you need information about the components. Using kinematics, you can find the velocity at an instant in both directions separately. Once you have that, doing the pythagorean theorem gives you the magnitude and evaluating tan(y / x) gives you the direction of the velocity vector. Hope this helps!
(1 vote)
• When creating the slope in the last problem, is it possible to create a slope that equals g by solving algebraically for it and labeling the numerator as the y-axis and the denominator as the x-axis (rise/run)? My only concern when suggesting this is that both varying variables (delta x and velocity) would be part of one axis.
(1 vote)
• If P=1/2 D^2
then doesn't differentiating give that P'=D meaning the slope changes relative to the already traveled distance and is not 1/2?
Please correct me if I'm wrong.