TBR, Physics, Translational Motion, Passage 2 #11

[Sammy1024]

11. If a 200-gram ball is used in the slingshot with a launch angle of 45°, what will be observed when a heavier ball is used?

A. The lighter ball strikes the water sooner.
B. The heavier ball strikes the water sooner.
C. Both balls strike after the same amount of time, because g is constant.
D. Both balls strike after the same amount of time, if they are made of the same material.

Choice B is the best answer. It is easy to assume that mass makes no difference in projectile motion, because this concept has been presented so frequently in physics. In this case, however, there is more than just simple projectile motion to consider. There is also a transfer of momentum and kinetic energy. If the conditions are identical, then the elastic cable has the same potential energy at full displacement Each projectile leaves the slingshot with the same kinetic energy. However, if the mass of the projectile is greater, then the velocity of the projectile must be less. This means that the heavier projectile leaves with a lower initial velocity in the y-direction, so it remains in flight for a shorter time. The heavier object strikes the water sooner. This makes choice B the best answer. The best answer is choice B.

Just wanted to confirm. Normally, mass is excluded, because all of the other variables remain the same. However, since they only tell you that the mass is increased in one of the balls, and the angle remains the same we have to conclude that the heavier ball lands first? If they had said that mass 2 was heavier, same angle, same velocity, then C would have been correct but they would have just had different ranges?

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[DrknoSDN]

If they had said that mass 2 was heavier, same angle, same velocity, then C would have been correct but they would have just had different ranges?

If 2 was heavier with the same angle and same velocity then C would have been correct and they would have the same range (ignoring air resistance).

I might be missing important information but this is almost a bad question with the information presented. They are arguing that the elastic potential energy is a factor but there are also many other factors like air resistance etc that occur in "real world" examples. Any time the initial velocity is so great that air resistance becomes a major factor a heavier ball would actually go farther. (golf, bullets, etc)

Basically do the opposite test. Slingshot a golfball (45g) versus a pingpong (2.7g) ball and see what hits the ground first... (Real world answer Answer > pingpong)
Conclusion: Ambiguous question.

 

[Sammy1024]

Thank you so much! TBR seems to have a lot of bad questions. They should maybe update and work on that!

 

[DrknoSDN]

TBR overall very good. But in this example C is just as valid as B if you are not told the slingshot has a maximum energy and assume a constant launch velocity (which is not unreasonable).

To make it more fair it really should have said "given a slingshot pull displacement of X...what is observed?". Then you could know the 'spring coefficient' and inertia would be a factor with more mass.