Mechanics: energy and momentum Q

[NOP90]

Homework Statement

Point [A] is 10m above ground and slopes down to the ground and then back up to point which is 4m above ground. Assuming that the object starts from rest at point [A] and that the slope is frictionless, calculate the ratio of the kinetic energy to the potential energy when the body reaches point

Homework Equations

I am not sure. That is the problem, I don't know what equation to use. It may one of the equations of motion. Also could be kei + pei = kef + pef or mvi^2/2 + mgyi = mvf^2/2 + mgyf (note that the i's and f' represent initial and final)

The Attempt at a Solution

it's the 3rd question of 8 and they seem to get more challenging as you continue so this should not be to hard after I find out which equation. I've just messed around with the heights and the Earth's gravitational pull 9.8m/s^2. It doesn't give an initial or final velocity or a mass.

 

[zorro]

Take the ground as your reference level for 0 potential energy.
1)Compute the Potential energy at A.
2)Find out its kinetic energy at the lowest point (on the ground).
3)This kinetic energy gets converted as the potential energy + kinetic energy at point B (some K.E. will also be there as the point B is lower than A). Find them

Now find the ratio.

EDIT:This question is no way related to momentum.

 

[tiny-tim]

Welcome to PF!

Hi NOP90! Welcome to PF!

(try using the X² icon just above the Reply box )

Point [A] is 10m above ground and slopes down to the ground and then back up to point which is 4m above ground. Assuming that the object starts from rest at point [A] and that the slope is frictionless, calculate the ratio of the kinetic energy to the potential energy when the body reaches point

You don't need to know m, it'll cancel.

Where is the PE is measured from?

If it's measured from A, you don't even need to know the heights.

 

[NOP90]

Take the ground as your reference level for 0 potential energy.
1)Compute the Potential energy at A.
2)Find out its kinetic energy at the lowest point (on the ground).
3)This kinetic energy gets converted as the potential energy + kinetic energy at point B (some K.E. will also be there as the point B is lower than A). Find them

Now find the ratio.

EDIT:This question is no way related to momentum.

Don't I need a mass to work out PE (PE=mgh)? Also, it's the equations that I am having trouble with, so when you say find them (how??). The booklet covers questions from energy and momentum. So I guess it's just energy, sorry.

Hi NOP90! Welcome to PF!

(try using the X² icon just above the Reply box )


You don't need to know m, it'll cancel.

Where is the PE is measured from?

If it's measured from A, you don't even need to know the heights.

Thank you for your welcoming the object slides from point [A] down the slope and then up another slope to point . There is a diagram in the book so I've tried to describe it for you in all its glory. I re-read my post and saw that I didn't mention it slides. The answer is 1.5 but it's the working I am interested in. I still don't know what equations to use You say m will cancel but I also don't know velocity intial or final. It doesn't give the distance between points [A] and just there heights.

 

[tiny-tim]

Hi NOP90!

… The answer is 1.5 …

ah, in that case, clearly the PE is being measured from the ground level.

ok, if PE at 0 m is 0, then what is the PE at 10 m and at 4 m?

(call the mass "m" and gravity "g")

and (since we know the KE at 10 m is zero) so what is the KE at 4 m ?

 

[NOP90]

Hi NOP90!


ah, in that case, clearly the PE is being measured from the ground level.

ok, if PE at 0 m is 0, then what is the PE at 10 m and at 4 m?

(call the mass "m" and gravity "g")

and (since we know the KE at 10 m is zero) so what is the KE at 4 m ?

hmm so 6/4 gives 1.5 but still not sure how this came about. Is it that kinetic energy is 0 at 10m and then when it reaches the ground it's kinetic energy is 10, it doesn't stop here however and continues sliding upwards of 4m. 10 - 4 then gives the kinetic energy at point B (6) seeming an upwards slope causes that body to lose kinetic energy. The potential energy corresponds to the height so at point B it's 4m high so its PE is 4? Ratio then becomes 6/4.

 

[zorro]

Don't I need a mass to work out PE (PE=mgh)? Also, it's the equations that I am having trouble with, so when you say find them (how??). The booklet covers questions from energy and momentum. So I guess it's just energy, sorry.

No you don't need to.
You have to find the ratio between K.E. and P.E. and in such problems there is no need of knowing mass, it gets cancelled.
Just keep the expressions in terms of 'm' and proceed.

 

[tiny-tim]

Hi NOP90!

(just got up :zzz: …)

hmm so 6/4 gives 1.5 but still not sure how this came about. Is it that kinetic energy is 0 at 10m and then when it reaches the ground it's kinetic energy is 10, it doesn't stop here however and continues sliding upwards of 4m. 10 - 4 then gives the kinetic energy at point B (6) seeming an upwards slope causes that body to lose kinetic energy. The potential energy corresponds to the height so at point B it's 4m high so its PE is 4? Ratio then becomes 6/4.

That's exactly right.

Though you didn't need to find the KE at the ground …

you could just have said that the PE at A is 10mg, the PE at B is 6mg, the KE at A is 0, and since KE + PE = constant, that means the KE at B is 4mg.