# TBR Physics I #64 (Kinematics Question)

#### loveoforganic

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From the passage, there are two cannons. One is on a cliff, the other is on the ground. They're both pointing directly at each other. Cannon A is the one on the cliff, Cannon B is the one on the ground.

The question is

Using the setup shown in Figure 1, can a cannonball fired from Cannon A ever hit Cannon B?

A. Yes, if the speed of the ball upon exiting Cannon A is sufficiently large.

B. Yes, if the exit speed equals the speed the ball gains by falling 32 m.

C. Yes, regardless of the exit speed.

D. No.

The answer is D, with the explanation

Since the two cannons are aimed at each other, the cannonball would travel along a straight path and hit Cannon B if there were no gravity. But there is gravity. Gravity will change the path of the ball, specifically causing the ball to drop below this straight path. No matter how fast the cannonball travels, gravity will always cause it to drop below this line and therefore miss Cannon B.

I put choice A, and I still feel like it's the right answer. As the velocity of the ball approaches infinity, the time of travel approaches zero and thus the change in velocity and thus change in height due to gravity both approach zero as well. It doesn't state in the passage that the cannons are dimensionless (it even gives the barrels a length of 2.5 m).

What do you all think?

#### MCATBear

This is one of those questions with two answers you could argue for and only one is correct. There are a lot on the real mcat. I hate them. I usually choose the one that gives the most realistic answer (at normal velocities, the acceleration due to gravity will divert the course) as opposed to an extreme example or an example that requires qualification (like at extremely high velocity or deciding to define "cannon B" not as the specific point cannon a is pointed at but as that point and a reasonable amount of area below it as well based on the dimensions of the cannon). That said, sometimes the extreme or qualified answer is correct. Just one of the frustrations of MCAT practice and exam.

So the short response: A is not technically incorrect, but it's also not the answer they are looking for. You need to get used to what they are looking for and then make your best guess. Usually more so on verbal and bio, but sometimes in physical as well.

#### Seraph 84

It's most definitely no because as soon as that ball leaves the cannon, it will be affected by gravity and drop below that straight line they have depicted in the figure. I can see why someone would choose A, but for practical purposes, the cannon ball has a speed limit. I can't see it moving very fast relative to a bullet.

I'd like to point out that some of TBR physics answers I disagree with - mostly in the electrostatics/circuits sections. From my perspective these books need a little more polishing. It is definitely good practice, though.