Mass and acceleration

[premedicine555]

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It seems that I keep getting these types of questions wrong, so something's not right in my thinking process.

For TBR, page 46, Passage 1, question 2 the mass does not affect the range. I get that. Because "m" isn't in the kinematic equations, nor in the "R" (range equation). Question 17 reaffirms this.

However, out of no where, Passage 5, Q #30 says that mass does affect the range, because a change of mass---> changes KE (1/2mv^2)---> and changes the velocity, which in turn changes the range...

So why wasn't this same explanation was given for Passage 1, question 2?? Isn't there a KE change if you throw the stone from the top of the rocket? Then range (x=Vox * t) would also change because a change in mass, changes KE, velocity, range.. etc.

edit
nvm. i think i get it.. it has to do with keeping velocity fixed or not.

 

[MedPR]

It seems that I keep getting these types of questions wrong, so something's not right in my thinking process.

For TBR, page 46, Passage 1, question 2 the mass does not affect the range. I get that. Because "m" isn't in the kinematic equations, nor in the "R" (range equation). Question 17 reaffirms this.

However, out of no where, Passage 5, Q #30 says that mass does affect the range, because a change of mass---> changes KE (1/2mv^2)---> and changes the velocity, which in turn changes the range...

So why wasn't this same explanation was given for Passage 1, question 2?? Isn't there a KE change if you throw the stone from the top of the rocket? Then range (x=Vox * t) would also change because a change in mass, changes KE, velocity, range.. etc.

edit
nvm. i think i get it.. it has to do with keeping velocity fixed or not.

In free fall mass does not affect the range. Remember that an object with initial velocity only in the x direction is in free fall. For instance, if you shoot a gun directly horizontal off a 200m high cliff, the bullet is in free fall. Ignoring air resistance, every mass bullet will have the same range.

 

[ljc]

In free fall mass does not affect the range. Remember that an object with initial velocity only in the x direction is in free fall. For instance, if you shoot a gun directly horizontal off a 200m high cliff, the bullet is in free fall. Ignoring air resistance, every mass bullet will have the same range.

To add on --
However, if you are firing a bullet from a gun with a set amount of energy, and you increase the mass of the bullet, now your range is effected. The gun can only produce so much power, so increasing the mass decreases the velocity of the bullet. Think about trying to throw a baseball versus a bowling ball. If you threw them horizontally off a roof, they would both stay in the air the same time, but the baseball would go much further.

The time of flight would still be the same, though.

 

[MedPR]

To add on --
However, if you are firing a bullet from a gun with a set amount of energy, and you increase the mass of the bullet, now your range is effected. The gun can only produce so much power, so increasing the mass decreases the velocity of the bullet. Think about trying to throw a baseball versus a bowling ball. If you threw them horizontally off a roof, they would both stay in the air the same time, but the baseball would go much further.

The time of flight would still be the same, though.

Yes. So, OP, you're kind of asking two different questions. Both answers are pointing to the same concepts, just in different context.