# AS Level help

AS Level help (Energy questions)

I didn't really know where to post this thread to so I'm sorry if it's in the wrong place.

I'm really stuck on some homework questions. I've been trying to figure them out all week and now they have to be handed in tomorrow.

I'd really appreciate it if someone could explain to me how to go about obtaining the answers. I'll post one question at a time.

An electric heater immersed in some water raises the temperature from 40ºC to 100ºC in 6 min. After another 25 min it is noticed that half the water has boiled away. What value does this experiment give for the specific latent heat of vaporisation of water?

Thank you very much.

Cathy

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homework &coursework questions.

Just read the thread about how I have to show my working before you guys decide to help.

I really have to idea how to do this question so all I wrote down was:

Change in energy = mass x the latent heat of vaporisation

Change in energy = mass x specific heat capacity x change in temperature

Mass = density x volume

Thanks.

Cathy

Motion questions

The other questions that I am stuck on are:

5. An aircraft on a famine relief mission flies horizontally at 50ms-1 at a height of 500m above level ground, and drops a sack of flour. Assuming negligible air resistance and taking g as 9.81ms-2, calculate:

(a) the time taken for the sack to fall to the ground

(b) the vertical component of its velocity as it hits the ground

(c) its resultant velocity as it hits the ground

(d) the horizontal distance it travels before landing

6. A rocket accelerates from rest for 20s with a constant upward acceleration of 10ms-2. At the end of 20s the fuel is used up and it completes its flight under gravity alone. Assuming that air resistance can be neglected and taking g = 9.8ms-2, calculate the:

(a) velocity reached after 20s

(b) height after 20s

(c) maximum height reached

(d) speed just before the rocket hits the ground

(e) total time taken for the flight

7. A rifle bullet, of mass 10g, is fired and takes 2.0ms to travel down the rifle barrel of length 0.50m with constant acceleration.

(a) Calculate the:
<i> muzzle velocity (bullet velocity on leaving the barrel)

<ii> acceleration in the barrel

(b) After travelling a short distance through air, the resistance of which can be neglected, the bullet hits a target and penetrates to a depth of 2.5 cm. Calculate the average force acting on the bullet while it is coming to rest. (work done = force x distance = kinetic energy in this case)

Thank you very much. I'd be really grateful for any help as I've tried to use suvat equations but have no idea which ones to pick.

Cathy

CathyLou said:
The other questions that I am stuck on are:

5. An aircraft on a famine relief mission flies horizontally at 50ms-1 at a height of 500m above level ground, and drops a sack of flour. Assuming negligible air resistance and taking g as 9.81ms-2, calculate:

(a) the time taken for the sack to fall to the ground

(b) the vertical component of its velocity as it hits the ground

(c) its resultant velocity as it hits the ground

(d) the horizontal distance it travels before landing

Thank you very much. I'd be really grateful for any help as I've tried to use suvat equations but have no idea which ones to pick.

Cathy
OK, one problem at a time.
For this problem:
what do you know?
what do you need to know?
which equation has only what you're looking for as a variable?

EDIT: and start by drawing a diagram!

s = 500m
u = ?
v = 50 ms-1
a = 9.81 ms-2
t = ?

I know s and a, but I was wondering whether 50 m/s is the value for u or for v, and whether I have to split a suvat equation up into horizontal and vertical components.

Cathy

I think that I've figured number five out now.

Cathy

There are a few kinematics equations that will help you finish these problems. Were you ever given them by you instructor or in a book? If you were rearrange the appropriate formula so that what you want = the rest of the formula.

I hope that wasnt confusing. Sorry if it was.

These are the only two questions that I'm still stuck on.

6. A rocket accelerates from rest for 20s with a constant upward acceleration of 10ms-2. At the end of 20s the fuel is used up and it completes its flight under gravity alone. Assuming that air resistance can be neglected and taking g = 9.8ms-2, calculate the:

(a) velocity reached after 20s

(b) height after 20s

(c) maximum height reached

(d) speed just before the rocket hits the ground

(e) total time taken for the flight

7. A rifle bullet, of mass 10g, is fired and takes 2.0ms to travel down the rifle barrel of length 0.50m with constant acceleration.

(a) Calculate the:
<i> muzzle velocity (bullet velocity on leaving the barrel)

<ii> acceleration in the barrel

(b) After travelling a short distance through air, the resistance of which can be neglected, the bullet hits a target and penetrates to a depth of 2.5 cm. Calculate the average force acting on the bullet while it is coming to rest. (work done = force x distance = kinetic energy in this case)

Could you please explain what you mean by "kinematics equations"?

Thank you.

Cathy

Hootenanny
Staff Emeritus
Gold Member
Kinematic equations describe the motion of an object without reference to forces. i.e. talking in terms of accelerations. These are the main kinematic equations you should know of A Level mechanics and/or Physics - https://www.physicsforums.com/showpost.php?p=905663&postcount=2 . Note that these equations only apply for uniform (constant) accelerations.

Thanks very much for your help! Cathy

Hootenanny
Staff Emeritus
Gold Member
CathyLou said:
Thanks very much for your help! Cathy
My pleasure.

I still really don't know how to complete these questions.

If someone could please suggest which forumulae I need to use then I would really appreciate it.

Thank you.

Cathy

I still really don't know how to complete these questions.

If someone could please suggest which forumulae I need to use then I would really appreciate it.

Thank you.

Cathy

Hootenanny
Staff Emeritus
Gold Member
Okay, lets start with question 6.(a); write down what you know. Then write down what you need to know. Which equation contains only the quantities you know and need to know?

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Okay. So, I know u = 10 ms-2, a = 9.81 ms-2 and t = 20 s but I don't know s and v.

Does this mean that I use v = u + at ?

Hootenanny
Staff Emeritus
Gold Member
CathyLou said:
Okay. So, I know u = 10 ms-2, a = 9.81 ms-2 and t = 20 s but I don't know s and v.

Does this mean that I use v = u + at ?
Sounds good to me.

I'm a little stuck now on part c. I assume that we add to the height reached after 20s the height reached after the fuel is used up, but I don't know how to calculate the latter. I got -40 but that obviously cannot be right!

Cathy

Hey, Cathy.
Personally, I'd appreciate a little more detail. How did you get that the height was -40? What equation did you use? What values did you plug in? Pretty hard to tell where you're going wrong with no clue what you did... ;)
If you think your equations are too ugly on the computer screen, check out latex (in the Science Education Forum, in the Math and Science Tutorial section).