VCE Physics Question Thread!

[blasonduo]

Thanks for the responses but why doesnt mass affect this, like mg?

Hey!
I assume you're asking why different masses hit the ground at the same time?

When I drop a feather and a bowling ball at the same height (without air resistance) they will both accelerate at 9.8 metres per second, so if their accelerations are constant they must fall at the same velocity!

In regards to F = ma, because acceleration is constant but mass if very different, it means the force is different. Remember, the force isn't how fast it goes, but how hard it hits the ground, so back to the ball and feather, they would land at the same time, but one would hurt MUCH more as its force is different

[lzxnl]

Thanks for the responses but why doesnt mass affect this, like mg?

This is actually a very interesting question that led Einstein to formulate the theory of general relativity. Here's some background.

You may know/will learn that the centripetal force required to move in a circle of radius r at speed v is . If you measure your forces from your own rotating perspective, you will feel a centrifugal force of the same magnitude pushing you outwards. However, the centrifugal force isn't a real force; it comes about because you're accelerating and any accelerating person will measure 'incorrect' force values.

The force that you measure is proportional to your mass, like with gravity. It is what we call an inertial or fictitious force. No physical force, aside from gravity, is proportional to the mass. However, all inertial and fictitious forces are proportional to the mass. This led Einstein to believe that gravity isn't a force, but is just acceleration, like the centripetal acceleration above. In his model, free fall is motion with no acceleration. This may seem counterintuitive, but in a vacuum, if you're free falling and you let go of something, that object 'falls with you', because gravity accelerates it at the same rate. Therefore, ironically, when you're free falling, you can't detect the influence of gravity.

Tl;dr, gravity is special, it's the only force you'll study that has a constant acceleration for all masses.

Need help with some homework - Thermodynamics U1

1)Give two reasons why you feel cooler when the wind is blowing than you would in still air at the same temperature.

2) In humid weather, evaporation of perspiration takes place as it does in dry weather. However, the cooling effect is greatly reduced. Why?

3) In hot weather, sweat evaporates from the skin. Where does the energy required to evaporate the sweat come from?

1. Convection: the wind is cold, so more cold air hits your skin and draws heat away. Evaporation: the wind draws warm water vapour away from your skin.

2. In humid weather, you're surrounded by warm water vapour. The water from your body can't really escape because the vapour pressure is high.

3. The hot weather and your body both supply energy to evaporate the water on your skin.

[KiNSKi01]

Hey can I have some assistance with this question?

The question is "At the instant that m1 hits the ground the string breaks. Find the time taken for m2 to come to rest". (All relevant values are provided in the attachment)

I know you have to start by calculating the the acceleration of m2 however I can't even really visualise what would happen and consequently don't know what to do next
Thanks for the help!!

[Yertle the Turtle]

Hey can I have some assistance with this question?

The question is "At the instant that m1 hits the ground the string breaks. Find the time taken for m2 to come to rest". (All relevant values are provided in the attachment)

I know you have to start by calculating the the acceleration of m2 however I can't even really visualise what would happen and consequently don't know what to do next
Thanks for the help!!

I think that you would first calculate the net force on the system, which is coming from m1, due to gravity. That would be Fnet=2*9.8=19.6N
Then you find the total acceleration of the system, as a=Fnet/m. Therefore a=19.6/5= 3.92m s^-2
I don't know where you would go from here, beyond working out the time until m1 hits the ground. Is that all the information given in the question?

[KiNSKi01]

Yep thats all the info they give you. None of the other questions related to it give you any other extra information 

[Bri MT]

Hey can I have some assistance with this question?

The question is "At the instant that m1 hits the ground the string breaks. Find the time taken for m2 to come to rest". (All relevant values are provided in the attachment)

I know you have to start by calculating the the acceleration of m2 however I can't even really visualise what would happen and consequently don't know what to do next
Thanks for the help!!

How I would approach this is by splitting it into 2 stages.
1. M1 is falling
2.M2 is falling

1.
- consider the force acting to make m1 fall and the force resisting this motion
- use F = ma
- find the time taken for M1 to hit the ground
- find the displacement of M2

2.
- find the acceleration of M2 in the relevant direction
- find the time taken for M2 to reach the end of the ramp


I haven't stepped this through,  but I hope this helps

I'd draw a seperate diagram for each stage to assist

[KiNSKi01]

Thank you for taking the time to reply!! 

[skrt skrt]

Hey people just need help with this VCAA problem.

A stone is thrown from the top of a 15m high cliff above the sea at an angle of 30degrees.
Calculate the time taken for the stone to reach the sea. (answer is 3 seconds)

I first found the initial vertical velocity which was 10m/s and was hoping to find the find final velocity using v^2=u^2+2as then substitute my values into v=u+at. However I keep getting a negative number to find the final velocity which I cant square root.

I substituted u=10
                    a=-9.8
                    s=15

Please tell me what I'm doing wrong.
Thank you in advance

[blasonduo]

Hey people just need help with this VCAA problem.

A stone is thrown from the top of a 15m high cliff above the sea at an angle of 30degrees.
Calculate the time taken for the stone to reach the sea. (answer is 3 seconds)

I first found the initial vertical velocity which was 10m/s and was hoping to find the find final velocity using v^2=u^2+2as then substitute my values into v=u+at. However I keep getting a negative number to find the final velocity which I cant square root.

I substituted u=10
                    a=-9.8
                    s=15

Please tell me what I'm doing wrong.
Thank you in advance

Hey!

Remember that s is the change in y, and you have set the positive direction to be upwards, so your final displacement (in the y-direction) should be negative! (-15) This gives a positive number

EDIT How did you figure out the initial velocity was 10? From the values, it is impossible to figure this out with just acceleration and the height of the cliff only. (the angle would help, but there have been no given values of its velocity, it could take as long as it wants to hit the sea depending on its initial velocity.

[skrt skrt]

Sorry forgot to mention velocity was 20m/s. 

Thanks for the reply, makes things clear

[Khadijah]

Could someone help in this question-

Question 13
In the fusion process, a proton of rest mass 1.673 × 10–27 kg and a neutron of rest mass 1.675 × 10–27 kg combine
to form a deuterium nucleus of rest mass 3.344 × 10–27 kg, with a release of energy.
According to Einstein’s postulate of the equivalence of mass and energy, which one of the following is the best
estimate of the energy released in this interaction?
A. 1.2 × 10–21 J
B. 3.6 × 10–13J
C. 4.0 × 10–3 J
D. 3.6 × 1014 J

[Bri MT]

Could someone help in this question-

Question 13
In the fusion process, a proton of rest mass 1.673 × 10–27 kg and a neutron of rest mass 1.675 × 10–27 kg combine
to form a deuterium nucleus of rest mass 3.344 × 10–27 kg, with a release of energy.
According to Einstein’s postulate of the equivalence of mass and energy, which one of the following is the best
estimate of the energy released in this interaction?
A. 1.2 × 10–21 J
B. 3.6 × 10–13J
C. 4.0 × 10–3 J
D. 3.6 × 1014 J

All you need to do here is find the change in mass and plug that into E=mc^2.
(Proton rest mass) + (neutron rest mass) - (deuterium rest mass) = m
c= 3*10^8

Let me know if you're still struggling after this

[skrt skrt]

Hey guys, I seem to be struggling be with conical pendulum question.
A ball of mass of 2.9kg swings on a string length of 2.2m and moves at a speed of 4.4m/s in a horizontal circular path.
Find the angle.

Thanks for the help.
(Btw you can find this question on http://www.physicseducation.com.au, really good website)

[KiNSKi01]

Sup skrt skrt!

All you have to do is substitute the values into this equation and rearrange for theta:

tan(theta)= v^2 / rg . In this question the mass is irrelevant 

You get an answer of 41.92 degrees! (please correct me if the answer is wrong because I couldn't find the solution in the website)

[skrt skrt]

*face palm* I completely forgot you could use that equation for this problem, normally I just do trig but thanks for the help.