AS physics work energy

NEED HELP!!!

A car is on the top of a hill and moves down starting from rest.
Tell the energy conversions from the top of the hill to the bottom.

NO INFORMATION ABOUT AIR RESISTANCE OR ROAD FRICTION IS GIVEN. WIL WE INCLUDE IT IN THE ANSWER OR NOT?

Reply 1

ODES_PDES

Original post by Insia786

I trink this is the wrong forum
:smile:

(someone to move it please?)

Reply 2

Insia786

I am new here. Where should i post it?

Reply 3

pri12321

Original post by Insia786

Gains Kinetic Energy because it starts from rest then accelerates under gravity. Loses gravitational potential energy because it's height decreases.

Reply 4

Insia786

No information about friction or air resistance is given
Do we include it orr nottt?

Reply 5

samb1234

Original post by Insia786

No information about friction or air resistance is given
Do we include it orr nottt?

I would yes. The most logical way of modelling it, especially in a qualitative question, is as a real object rather than as a perfectly smooth surface with no friction. If you word it carefully you will be able to get the marks regardless of whether they want it or not anyway

Reply 6

pri12321

Original post by Insia786

No information about friction or air resistance is given
Do we include it orr nottt?

If you include it then the car loses energy as Thermal Energy too.

Reply 7

Insia786

@samb1234

But is it possible they give no information about friction or air resistance and just ask the energy conversions?
If it is how should i form my answer?

Reply 8

samb1234

Original post by Insia786

@samb1234

But is it possible they give no information about friction or air resistance and just ask the energy conversions?
If it is how should i form my answer?

Well firstly it's more that they haven't mentioned them that kind of gives it away. It is more artificial not to include friction or air resistance, so in most cases if they want you to do so they will tell you in the question. You can easily answer it anyway, e.g. you could say kinetic ----> heat energy (assuming friction is not assumed to be negligible) and then you're fine either way

Reply 9

Insia786

@samb1234
Lastly if its not a qualitative answer
But if they want you to work out something like the kinetic energy at the bottom.
No information about friction is given nor asked in any part.
So we have no other option then to ignore it in our workings

As either they will ask about friction in some part or give its info.
If its neither we just assume it to be 0?

(edited 8 years ago)

Reply 10

samb1234

Original post by Insia786

Correct. It's usually fairly easy to tell what you need to do based on what is and isn't given. You aren't given mu or a work done against friction so you obviously can't use them

Reply 11

Insia786

@samb1234

Just one moree thinggiee.
In a qualitative answer.
If they state its a SMOOTH plane with the car moving down on a slope and ask the energy conversions.

Smooth means road friction is negligible. Not AIR RESISTANCE.

So how would you answer this?
Loss in gravitational energy
Gain in kinetic
No energy loss in road friction ( smooth plane).
What about air resistance?

Reply 12

samb1234

Original post by Insia786

You can mention it. It isn't wrong so they will not deduct marks for saying it, especially if you were to say that the amount of energy converted into heat through air resistance is probably negligible

Reply 13

Insia786

@samb1234

right. But what if they are expecting us to write that there is energy loss through air resistance but we write that probably enegy loss through air resistance is negligible.
Wont marks be deducted?

Reply 14

samb1234

Original post by Insia786

You can say that the energy lost to it will be very small compared to the total energy of the system (which is true for a car) which is essentially what negligible means, so you are acknowledging that yes some will be lost to air resistance but contextualising it as being relatively insignificant. Maybe replace neglible with a small amount but it means the same thing

Reply 15

Insia786

So from your understanding heres what ill write @samb1234

The gravitiational potential energy of the car will ALL be converted to the kinetic energy of the car as there is no loss to road friction as its smooth but there maybe negligible loss as thermal energy due to air resistance.

Right?

(edited 8 years ago)

Reply 16

Insia786

@samb1234 is my answer correct?

Reply 17

samb1234

Original post by Insia786

Maybe say the vast majority instead of all, and a small amount instead of negligible. Sorry been working

Reply 18

1+5t

Original post by Insia786

Thats easy! The answer is 6 cars-3 buns and 3 x-rays squared. I hope this helped

Reply 19

Insia786

@samb1234
Vast majority or all Gpe will be converted to kinetic energy of the car. Little loss might occur due to loss as heat due to air resistance.

Sometimes in the marking scheme the word ALL is underlined means you have to write it to get the marks
So ill write both all or vast majority
??