projectile question (TBR Physics Q)

[angldrps]

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This question is Q.31 from passage 5 of TBR translational motion chapter

the passage discusses three cannon balls thrown up with different angles with initial velocity=50 m/s.
We are told that white cannon ball is launched at an angle of 30 degrees.

Question:
When the white cannon ball reached its apex, the:
A. acceleration is zero and velocity is not zero
B. acceleration is zero and its speed is zero
C. Acceleration is not zero and its velocity is zero
D. acceleration is not zero and its speed is not zero

I chose C becuase i remember learning that at maximun height, velocity=0

Correct answer is D. The explanation given is very confusing . can someone explain why C is wrong?

 

[milski]

What you remember is correct, but with a bit of clarification: the vertical component of velocity is 0 at the apex.

For projectile motion it's much easier to treat the motion as two separate components: vertical and horizontal. The horizontal acceleration is constant and stays 0, the horizontal velocity is also constant but usually some non-zero number, unless you shot the projectile straight up.

The vertical acceleration is a non-zero constant - g=9.81 in the case of Earth. That means that the vertical velocity will change through the flight of the projectile. It will start being upward, will decrease by magnitude until it reaches zero at the apex and will continue to grow after the apex in the down direction.

In contrast of velocity, speed is a single number which expresses only the 'amount' of motion but is not associated with direction. It will be zero only for non-moving object and will be positive for all other objects.

In the question the acceleration is not zero because there is an acceleration pointed down, towards the ground. The speed is also not zero because you still have some horizontal motion.

 

[osprey099]

acceleration is always g and since the ball is launched at 30 degrees, that means there will always be a horizontal velocity throughout the ball's trajectory

 

[MedPR]

This question is Q.31 from passage 5 of TBR translational motion chapter

the passage discusses three cannon balls thrown up with different angles with initial velocity=50 m/s.
We are told that white cannon ball is launched at an angle of 30 degrees.

Question:
When the white cannon ball reached its apex, the:
A. acceleration is zero and velocity is not zero
B. acceleration is zero and its speed is zero
C. Acceleration is not zero and its velocity is zero
D. acceleration is not zero and its speed is not zero

I chose C becuase i remember learning that at maximun height, velocity=0

Correct answer is D. The explanation given is very confusing . can someone explain why C is wrong?

Voy = 0 at max height. When you start with vox = 0 (e.g, shooting a bullet upwards perpendicular to the ground) you will have v = 0 at max height. However, since you launched the cannon ball at a 30 degree angle, initially you have a voy and a vox. You are correct that voy is 0 at the apex, which is due to gravity decelerating it on it's upward trajectory, but there is nothing opposing the horizontal velocity so the horizontal velocity is constant and non 0.