Projectile Energy Multiple Choice (1 Viewer)

[Jase]

A bullet is fired into the air.
If all friction and resistance is negligible, which of the following statements are true:

A) Energy of the bullet and acceleration remain constant
B) Energy varies and acceleration remains constant
C) Energy constant while acceleration varies
D) Evergy and acceleration both vary.


Dosn't the projectile gain potential energy as it moves up, and lose KE as it goes down, so its energy varies?

 

[JayWalker]

A bullet is fired into the air.
If all friction and resistance is negligible, which of the following statements are true:

A) Energy of the bullet and acceleration remain constant
B) Energy varies and acceleration remains constant
C) Energy constant while acceleration varies
D) Evergy and acceleration both vary.


Dosn't the projectile gain potential energy as it moves up, and lose KE as it goes down, so its energy varies?

To all those who are about to say: "Energy is constant" your wrong

Because the bullet is fired into the air, then the acceleration is ofcourse going tochange, but so is the energy,
Think about it
Gravity is slowing it down, And its Gravitation Potential Energy is changing, and its Kinetic energy.."

 

[JayWalker]

oh yeah forgot to answer..

Answers D) Energy and Acceleration Vary

 

[Jezzabelle]

A bullet is fired into the air.
If all friction and resistance is negligible, which of the following statements are true:

A) Energy of the bullet and acceleration remain constant
B) Energy varies and acceleration remains constant
C) Energy constant while acceleration varies
D) Evergy and acceleration both vary.


Dosn't the projectile gain potential energy as it moves up, and lose KE as it goes down, so its energy varies?

hmmm but,
the only force acting on a projectile is gravity, which is force of acceleration -9.8ms^-2. So its constant acceleration, its always down isnt it? As for energy.. hmmm its phrased dodgy, does it mean 'vary' as in changes states? from potential to kinetic etc or does it mean the magnitude of energy varies?

 

[JayWalker]

the only force acting on a projectile is gravity, which is force of acceleration -9.8ms^-2. So its constant acceleration. As for energy.. hmmm its phrased dodgy, does it mean 'vary' as in changes states? from potential to kinetic etc or does it mean the magnitude of energy varies?

Ohh yeah.. why didnt i relaise that..
This hit to my head made me stupid
But Energy is not constant, even if it does change from G_PE to E_Kinetic, The force applied on it by acceleration will change the bullets energy.

Where did you find this question? This sort of question will definitely not be asked in the HSC. Its too controversial.

P.S. Hi Jezza
Whats up good lookin

 

[CrashOveride]

u fire the bullet and work must be done on the bullet to move it away from the earths graviational field, so u give it energy so id say the energy was varying. Acceleration is constant.

 

[JayWalker]

And it does not specify a length of time, so my original answer was right, what about when it hits the ground [assuming your not firing at someone in particular].. When it hits ground, acceleration will change to 0 Due to Newtons First Law

Woohoo this question sux

 

[Xayma]

Actually energy will remain constant.

Conservation of energy means that, since no energy loss to sound/friction, its kinetic energy is transformed to potential energy. In fact that is how you can quickly do alot of projectile motion problems by transforming potential and kinetic energy.

Acceleration is always down at 9.8ms^-2

The answer is a.

 

[CrashOveride]

and as its going up, its the kinetic energy changing to GPE

 

[JayWalker]

Actually energy will remain constant.

Conservation of energy means that, since no energy loss to sound/friction, its kinetic energy is transformed to potential energy. In fact that is how you can quickly do alot of projectile motion problems by transforming potential and kinetic energy.

Acceleration is always down at 9.8ms^-2

The answer is a.

Umm ok Xayma, can you show me in the question "If all friction and resistance is negligible" Where it says that anything about loss due to sound? Or Kickback of the gun that fired the bullet??

 

[Jezzabelle]

Ohh yeah.. why didnt i relaise that..
This hit to my head made me stupid
But Energy is not constant, even if it does change from G_PE to E_Kinetic, The force applied on it by acceleration will change the bullets energy.

Where did you find this question? This sort of question will definitely not be asked in the HSC. Its too controversial.

P.S. Hi Jezza
Whats up good lookin

but isnt energy always the same, just changes states. like the stationary object would hold potential energy, when we lift an object from the ground to a particular height above the ground we must do work against the gravitational field of the earth. This work goes into increasing the gravitational potential energy of the body. The amount of work done is equal to the change in gravitational potential energy (GPE) of the body. work done on it to make it move would change this energy (Ep = mgh) but as it falls this just changes again to kinetic energy, then it hits the target and has potential energy again. The magnitude of energy required to lift the object and for it to fall back down would be constant wouldnt it?

 

[Jezzabelle]

P.S. Hi Jezza
Whats up good lookin

trying to cram medical physics damn MRI confusing me

 

[JayWalker]

but isnt energy always the same, just changes states. like the stationary object would hold potential energy, when we lift an object from the ground to a particular height above the ground we must do work against the gravitational field of the earth. This work goes into increasing the gravitational potential energy of the body. The amount of work done is equal to the change in gravitational potential energy (GPE) of the body. work done on it to make it move would change this energy (Ep = mgh) but as it falls this just changes again to kinetic energy, then it hits the target and has potential energy again. The magnitude of energy required to lift the object and for it to fall back down would be constant wouldnt it?

Yea h but the quesiton asks about the energy of the bullet, so conservation of energy does not apply as energy could be lost due to sound, kickback force of projectile...
Stupid question.

 

[tennille]

energy should be constant- conservation of energy. So energy can't vary. The type of energy can, but not the amount of energy.

 

[tennille]

What a crap question. It is too controversial. Couldn't be asked tomorrow.

But energy should still remain constant when the bullet is released, and just before it hits the target. I think you need to neglect the sound stuff, coz that happens as the bullet it shot.

 

[tennille]

So you imagine the bullet being shot, and then the energy it initially has should be conserved throughout the flight, since friction and resistance is neglected.

 

[mojako]

Yea h but the quesiton asks about the energy of the bullet, so conservation of energy does not apply as energy could be lost due to sound, kickback force of projectile...
Stupid question.

energy is constant

and we assume theres no lost of energy due to sound and heat and friction which are given off to the surrounding environment
we also assume there's no energy input due to.. ehm.. the hot air and the sun's light gives extra heat and energy to the ball which are retained there
so strictly speaking energy will vary

but in the context of this HSC exam energy is constant, which is contradictory to reality but anyway thats what you need to put if you want to do well

 

[Jezzabelle]

Yea h but the quesiton asks about the energy of the bullet, so conservation of energy does not apply as energy could be lost due to sound, kickback force of projectile...
Stupid question.

yeh my first comment was that it was dodgy... hmm i should log out and try for the fifth time this morning to get MRI into my head in a fashion thats, if not understandable, is at least regurgitatable...

 

[utopia_parkway]

i agreey that the energy is onstant. becuase it starts of with lots of K.E and no G.E. but as it goes up its K.E. is turned into G.E. but the amount of G.E. + K.E is always equal to the energy it had to start of with so its A)

 

[Jezzabelle]

i agreey that the energy is onstant. becuase it starts of with lots of K.E and no G.E. but as it goes up its K.E. is turned into G.E. but the amount of G.E. + K.E is always equal to the energy it had to start of with so its A)

so basicly, my first post was correct. energy and acceleration constant. goodo...
PS: Jaywalker.. i have webcam turned on /wink if your on msn