SUVATs: V and U mixup Watch

WilliamSlim
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#1
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Hey guys a major issue I've always had with SUVATs is that I always get confused which one would be zero between V and U. Any helpful tips on deciphering which would be zero in question?
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Smug Life
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If it starts at rest, u=0. If it finishes at rest, v=0.
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WilliamSlim
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(Original post by Smug Life)
If it starts at rest, u=0. If it finishes at rest, v=0.
So if a ball is being dropped, U=0 because it is held at rest. And if a ball is thrown then U is also 0? Because the ball is at rest first because it is being held? But wouldn't U be zero on all of them then because every projectile or object like a runner or car starts off from a stationary position?
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lizard54142
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(Original post by WilliamSlim)
So if a ball is being dropped, U=0 because it is held at rest. And if a ball is thrown then U is also 0? Because the ball is at rest first because it is being held? But wouldn't U be zero on all of them then because every projectile or object like a runner or car starts off from a stationary position?
If the ball is simply released, then u = 0. But if it is thrown downwards, then u is not zero. The same with projectiles. There must be some external force acting on the projectile to launch it, which gives it an initial speed. The time t starts at the instant that the projectile is launched (t=0), when the force is applied to it. Before this time it has a speed of zero.

In summary, if an object is only accelerated by it's own means (i.e. no external forces act upon it in the direction of motion), u = 0.
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TomAyy
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(Original post by WilliamSlim)
So if a ball is being dropped, U=0 because it is held at rest. And if a ball is thrown then U is also 0? Because the ball is at rest first because it is being held? But wouldn't U be zero on all of them then because every projectile or object like a runner or car starts off from a stationary position?
Think about a car moving at 10ms-1 and accelerating for 2 seconds at 1ms-2.
From the question, u=10 not 0.
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uberteknik
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(Original post by WilliamSlim)
So if a ball is being dropped, U=0 because it is held at rest. And if a ball is thrown then U is also 0? Because the ball is at rest first because it is being held? But wouldn't U be zero on all of them then because every projectile or object like a runner or car starts off from a stationary position?
You are correct in observing that the ball in the throwers hand starts off at rest and must accelerate to it's initial speed by the time it leaves the throwers hand.

However........unless the question tells you to take account of this initial acceleration, then you must assume 'u' refers to the exact instant the ball leaves the throwers hand and not before.

i.e. u is not zero in throwing cases because the speed is not zero at the instant the ball leaves the hand.

u = 0 in dropped cases because the hand (and hence ball) is not moving at the instant the ball leaves the hand.
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Discipulus
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You also need to be clear in which direction the values of V and U are - vertical and horizontal components of velocity are independent.

For example, a bullet fired out of a gun horizontally, it would have a horizontal initial velocity (U) of e.g 500 ms^-1, however its initial vertical velocity will be 0 ms^-1.
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