Physics: Post your doubts here!

[Coolio_777]

W16 p11

correct answer is B but how?

 

[Daniel Jeng]

View attachment 62735

W16 p11

correct answer is B but how?

Firstly you would want to know what is the time taken for the cyclist to land at the bottom of the track.

You know that the acceleration is 9.81ms^-2 downwards, the initial vertical velocity, u, as 0, and the vertical displacement, s, as 2.0m. To find the time taken just use the equation s=ut+(1/2)at^2, and you can find time easily.

Now, as there is no acceleration in the horizontal direction, just use the formula v=s/t to find the horizontal velocity of the cyclist, and that will be the initial horizontal velocity as that velocity stays constant. The same time can be used because it takes the same time for the cyclist to fall down 2m as well as travel 6m across.

 

[Daniel Jeng]

Btw, can someone explain to do the graph question on paper 5 in general terms.

Hmmm, some of the tips might be as follows:

- Make sure, when plotting points, that the diameter of the crosses do not exceed one small square on the graph paper
- Both the line of best fit and worst fit needs to cover more than 50% of the graph both vertically and horizontally
- The worst fit line MUST cross every single error bar, therefore sometimes you cannot draw the worst fit graph in one particular way, but can in the other direction.
- Make sure you put the small "hats" on the error bars so the examiners can see clearly the boundaries of your error bars

That's the general things to keep an eye out for in the graphs. However for the tables and calculations:

- When you are writing the table, if there are no uncertainties associated with the value, express the values to 1 more significant figure
- All absolute uncertainties at 1 significant figure, and all values associated needs to be at the same place value
- Show one of your uncertainty calculations beneath the table, even if there's no space for it
- Log values do have units, for example if you log a length value, write the units in the table as e.g. log(d/m). Put the units within the log expression
- When asked for the uncertainty of the gradient, the absolute uncertainty will be the absolute difference between the gradient of the best fit and worst fit line
- When asked for the uncertainty of the y-intercept value, the absolute uncertainty will be the absolute difference between the y-intercept of the best fit and worst fit line

I think that's all I can remember. Are you sitting both AS and A2 level this year?

 

[Daniel Jeng]

Can u share the question if u remember?.

Here's a sloppy drawing I had drawn for the physics teacher while asking about it. The question asks for the change in the potential difference across AB and CD when the switch turns from X to Y.

 

[Astralgod]

Hmmm, some of the tips might be as follows:

- Make sure, when plotting points, that the diameter of the crosses do not exceed one small square on the graph paper
- Both the line of best fit and worst fit needs to cover more than 50% of the graph both vertically and horizontally
- The worst fit line MUST cross every single error bar, therefore sometimes you cannot draw the worst fit graph in one particular way, but can in the other direction.
- Make sure you put the small "hats" on the error bars so the examiners can see clearly the boundaries of your error bars

That's the general things to keep an eye out for in the graphs. However for the tables and calculations:

- When you are writing the table, if there are no uncertainties associated with the value, express the values to 1 more significant figure
- All absolute uncertainties at 1 significant figure, and all values associated needs to be at the same place value
- Show one of your uncertainty calculations beneath the table, even if there's no space for it
- Log values do have units, for example if you log a length value, write the units in the table as e.g. log(d/m). Put the units within the log expression
- When asked for the uncertainty of the gradient, the absolute uncertainty will be the absolute difference between the gradient of the best fit and worst fit line
- When asked for the uncertainty of the y-intercept value, the absolute uncertainty will be the absolute difference between the y-intercept of the best fit and worst fit line

I think that's all I can remember. Are you sitting both AS and A2 level this year?

Yep daniel. Also can you possibly run through how to drawn error bars and a worst acceptable line? Thanks in advance

 

[Daniel Jeng]

Yep daniel. Also can you possibly run through how to drawn error bars and a worst acceptable line? Thanks in advance

Error bars are just the absolute uncertainty of the values you have entered in the tables. e.g. if it's +/- 0.2 you would draw the error bar as having a length of 0.4, 0.2 above and below the plot point. However there are some cases where you draw horizontal error bars but that's determined by the table that you had been given and which variable's uncertainty you need to calculate.

For the worst acceptable line, draw from the bottom of the error bar of the first plot point to the top of the error bar of the last plot point. Conversely you can draw the line from the top of the first plot point error bar to the bottom of the last error bar. That'll give you the worst acceptable line. But like I said, make sure the worst acceptable line passes through every single error bar otherwise you will lose a mark. That's why you have two choices as to how to draw it.

 

[Astralgod]

Thank you man, much appreciated.

 

[Daniel Jeng]

Thank you man, much appreciated.

You're welcome currently not faring well with literature -.-

 

[Unknown202]

Water of depth 10m exerts a pressure equal to atmospheric pressure.
An air bubble rises to the surface of a lake which is 20m deep. When the bubble reaches the
surface, its volume is 6.0cm3
.

What is the volume of the air bubble at the bottom of the lake?
help plzzz

 

[Unknown202]

and
Microwaves are used to transmit television signals to and from a satellite.
Which statement about microwaves is correct?
A They have a longer wavelength than radio waves.
B They penetrate the atmosphere without significant loss of energy.
C They travel much faster than radio waves in a vacuum.
D They warm the satellite and stop it freezing.
why is it B

 

[A*****]

Water of depth 10m exerts a pressure equal to atmospheric pressure.
An air bubble rises to the surface of a lake which is 20m deep. When the bubble reaches the
surface, its volume is 6.0cm3
.

What is the volume of the air bubble at the bottom of the lake?
help plzzz

Use the equation P1V1=P2V2
At the surface, the pressure is 1 atm and the volume of bubble is 6cm3 and 20m below, the pressure is 2 atm.
So applying the equation, 1x6=2 x V2
V2= 1x6/2 =3 cm3

 

[A*****]

Btw if I'm not mistaken, this question is from O level Physics, right?

 

[Unknown202]

an

Use the equation P1V1=P2V2
At the surface, the pressure is 1 atm and the volume of bubble is 6cm3 and 20m below, the pressure is 2 atm.
So applying the equation, 1x6=2 x V2
V2= 1x6/2 =3 cm3

answer is 2???not 3

 

[A*****]

an

answer is 2???not 3

Ohh I'm sorry...pressure at the bottom will be 3 atm, not 2 bqz 2 is due to the water above and 1 of the atmosphere itself so yes it would be 1x6/3=2 cm3

 

[A*****]

Can someone plz explain this question...

 

[Unknown202]

http://physics-ref.blogspot.com/2014/11/physics-9702-doubts-help-page-13.html answer here

Can someone plz explain this question...
View attachment 62738

 

[Daniel Jeng]

and
Microwaves are used to transmit television signals to and from a satellite.
Which statement about microwaves is correct?
A They have a longer wavelength than radio waves.
B They penetrate the atmosphere without significant loss of energy.
C They travel much faster than radio waves in a vacuum.
D They warm the satellite and stop it freezing.
why is it B

A is incorrect because microwaves have more energy than radio waves. This means that the microwaves have a higher frequency and therefore lower wavelength.
C is incorrect as all electromagnetic waves travel at the same speed in space i.e. 3x10^8 ms^-1.
And D is just absolutely ridiculous.
By way of elimination, B is the answer which makes the most sense.

 

[Daniel Jeng]

A is incorrect because microwaves have more energy than radio waves. This means that the microwaves have a higher frequency and therefore lower wavelength.
C is incorrect as all electromagnetic waves travel at the same speed in space i.e. 3x10^8 ms^-1.
And D is just absolutely ridiculous.
By way of elimination, B is the answer which makes the most sense.

Thought I'd just make D a little clearer: the main problem is the sun overheating the satellite, rather than freezing, ironically.

 

[Amirtha]

How to do this??
-------------------------

The diagram shows a junction in a circuit where three wires
P, Q and R meet. The currents in P and Q are 1 A and 3 A
respectively, in the directions shown.

How many coulombs of charge pass a given point in wire R in
5s?
A) 0.4 B) 0.8 C) 2 D) 10

 

[Daniel Jeng]

How to do this??
-------------------------

The diagram shows a junction in a circuit where three wires
P, Q and R meet. The currents in P and Q are 1 A and 3 A
respectively, in the directions shown.

How many coulombs of charge pass a given point in wire R in
5s?
A) 0.4 B) 0.8 C) 2 D) 10

By Kirchhoff's law of the conservation of charge, the current in wire R must be 2A into the junction, as current in=current out.
To figure out the charge passing through the wire Q the equation Q=It can be used, so the charge passing through Q would be 2x5=10C.
Therefore the answer will be D, 10C.