# Physics - Got Some Questions (1 Viewer)

#### Life'sHard

##### Well-Known Member
1)

Little confused about the worked solutions, can someone show their working? Thanks!

#### scoobertdoobert

##### New Member
Here's how I would approach this - more of an algebra q. Sorry if i'm completely wrong haha

#### Attachments

• 324.8 KB Views: 26

#### Life'sHard

##### Well-Known Member
Here's how I would approach this - more of an algebra q. Sorry if i'm completely wrong haha
You are correct.

Here's the sample answer provided. Can someone explain why 1.5*10^11 and not 1.5*10^8?

For part b) I've just realised I made a silly mistake tyvm.

#### scoobertdoobert

##### New Member
You are correct.
View attachment 31386
Here's the sample answer provided. Can someone explain why 1.5*10^11 and not 1.5*10^8?

For part b) I've just realised I made a silly mistake tyvm.
Probably conversion from km to m (into SI units)

#### Life'sHard

##### Well-Known Member
Probably conversion from km to m (into SI units)
-_-
ffs thank you for your help.

1)

2)

3)

4)

Last edited:

#### Life'sHard

##### Well-Known Member
@CM_Tutor Do you do physics as well? or just chemistry. Just curious when to ask you hahaha.

#### CM_Tutor

##### Moderator
Moderator
@Life'sHard, I haven't studied physics beyond first year uni level, and that was some while ago. However, I am a scientist and know the basics. For example, in this thread, I knew the Kepler's third law result (though I didn't recall the exact formula) and knew why it called for SI units. I recognise the others but am not certain enough to give any definitive answer. In short, I'll comment if I have something I see as helpful to add, but I won't routinely comment on physics questions.

#### CM_Tutor

##### Moderator
Moderator
But, since you asked, here are my thoughts...

Someone, anyone, please correct anything that is wrong.

I think 1 is C because the laminations need to be parallel to the magnetic field induced by the coil to suppress the eddy currents and minimise heating.

2, I think might be C. Time dilation under special relativity is in play here, 0.9c being more than enough speed for dilation to be significant. However, I think this is asking about reciprocity, where both the cook and the cabin boy each perceiving themselves to be stationary and the other to be moving and thus each seeing the other as slowed due to time dilation, thus each seeing the same time taken.

With 3, the only acceleration is gravity assuming the ball is moving at a constant 1.1 m/s and that the table is frictionless. However, only the component of gravity in the plane of the table is acting as the component perpendicular to the table is cancelled by the normal force... so, I'm thinking $\bg_white \left|\overrightarrow{a}\right| = g\sin{5^\circ} \approx 0.85\ \text{m s}^{-2}$.

As for 4, both objects have the same change in height and so convert the same amount of gravitational potential energy into kinetic energy (ignoring energy lost as heat due to friction in moving through the atmosphere). Crudely, this situation is
$\bg_white E_p = E_k \implies mg\Delta h = \cfrac{1}{2} mv^2 \implies v^2 = 2g\Delta h$
suggesting the same impact speed (I say crudely as $\bg_white g$ is not constant if starting sufficiently far above the surface of the Earth). However, $\bg_white m_1$ is moving in a decaying orbit and so starts with some speed and momentum whereas $\bg_white m_2$ is initially stationary. Thus, $\bg_white m_1$ has greater energy / momentum at impact than does $\bg_white m_2$, and so $\bg_white m_1$ should have a greater impact velocity.

These are my thoughts based on recollections but may not be correct.

#### Life'sHard

##### Well-Known Member
@Life'sHard, I haven't studied physics beyond first year uni level, and that was some while ago. However, I am a scientist and know the basics. For example, in this thread, I knew the Kepler's third law result (though I didn't recall the exact formula) and knew why it called for SI units. I recognise the others but am not certain enough to give any definitive answer. In short, I'll comment if I have something I see as helpful to add, but I won't routinely comment on physics questions.
Thanks! All correct I'm pretty sure. We need some physics mods hahahaha.

#### CM_Tutor

##### Moderator
Moderator
Thanks! All correct I'm pretty sure. We need some physics mods hahahaha.
Glad that my physics isn't totally rusted away!

#### notme123

##### Well-Known Member
But, since you asked, here are my thoughts...

Someone, anyone, please correct anything that is wrong.

I think 1 is C because the laminations need to be parallel to the magnetic field induced by the coil to suppress the eddy currents and minimise heating.

2, I think might be C. Time dilation under special relativity is in play here, 0.9c being more than enough speed for dilation to be significant. However, I think this is asking about reciprocity, where both the cook and the cabin boy each perceiving themselves to be stationary and the other to be moving and thus each seeing the other as slowed due to time dilation, thus each seeing the same time taken.

With 3, the only acceleration is gravity assuming the ball is moving at a constant 1.1 m/s and that the table is frictionless. However, only the component of gravity in the plane of the table is acting as the component perpendicular to the table is cancelled by the normal force... so, I'm thinking $\bg_white \left|\overrightarrow{a}\right| = g\sin{5^\circ} \approx 0.85\ \text{m s}^{-2}$.

As for 4, both objects have the same change in height and so convert the same amount of gravitational potential energy into kinetic energy (ignoring energy lost as heat due to friction in moving through the atmosphere). Crudely, this situation is
$\bg_white E_p = E_k \implies mg\Delta h = \cfrac{1}{2} mv^2 \implies v^2 = 2g\Delta h$
suggesting the same impact speed (I say crudely as $\bg_white g$ is not constant if starting sufficiently far above the surface of the Earth). However, $\bg_white m_1$ is moving in a decaying orbit and so starts with some speed and momentum whereas $\bg_white m_2$ is initially stationary. Thus, $\bg_white m_1$ has greater energy / momentum at impact than does $\bg_white m_2$, and so $\bg_white m_1$ should have a greater impact velocity.

These are my thoughts based on recollections but may not be correct.
I got the same for all the others and a very similar answer to 3. Strangely I got $\bg_white a=g\tan{5}$ and for small values of $\bg_white \theta$ : $\bg_white a=g\tan{5}\approx g\sin{5}$ so I think it checks out.

#### Life'sHard

##### Well-Known Member
Where are these questions from, they're excellent.
I cant remember which past paper, I think CSSA or maybe ruse?

#### notme123

##### Well-Known Member
I cant remember which past paper, I think CSSA or maybe ruse?
Out of the two, seems like CSSA to me because it has similar font to the 2019 one. I assume this is the 2020 one because the 2019 one didn't have good qs nor were the solutions correct.

#### jimmysmith560

##### Le Phénix Trilingue
Moderator
Where are these questions from, they're excellent.
All of those questions are from the North Sydney Girls High School 2020 Physics Trial Examination.

Last edited:

#### notme123

##### Well-Known Member
All of those questions are from the North Sydney Girls High School 2020 Physics Trial Examination.
Where did you find that?

#### jimmysmith560

##### Le Phénix Trilingue
Moderator
Where did you find that?
I've had a look at the paper, it's accessible online. Would you like to have the paper?

#### notme123

##### Well-Known Member
I've had a look at the paper, it's accessible online. Would you like to have the paper?
No worries, I found it on AceHSC. I guess using THSC solely is a bit limiting.

#### CM_Tutor

##### Moderator
Moderator
NSGHS has some really good questions in its maths exams so I am not surprised that the same is true of their physics exams. Definitely a source worth trying.