Struggling to Understand 'Simple Physics' Could anyone please help me

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Sam00
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#1
Report Thread starter 6 years ago
#1
I am revising for a physics exam and I really don't understand the concept.

Here are the 1st 3 questions I am practicing but don't get:

Q: Using the information below find the units for the following quantities
1: frequency (f) f=1/t
2: Speed of light (c) c= λf
3: Energy of an object due to its mass (E) E=mc^2

Where f, t, c, E and m represent frequency, time, speed of light, energy and mass

I really don't understand this supposedly simple physics concept, could anyone please help?
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jonnypdot
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#2
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#2
(Original post by Sam00)
I am revising for a physics exam and I really don't understand the concept.

Here are the 1st 3 questions I am practicing but don't get:

Q: Using the information below find the units for the following quantities
1: frequency (f) f=1/t
2: Speed of light (c) c= λf
3: Energy of an object due to its mass (E) E=mc^2

Where f, t, c, E and m represent frequency, time, speed of light, energy and mass

I really don't understand this supposedly simple physics concept, could anyone please help?
period/Time is measured in seconds, s so 1 over s will give you s^-1, which is units for frequency
λ, wavelength is measure in meters, m so m x s^-1 gives you units for speed
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ombtom
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(Original post by Sam00)
I am revising for a physics exam and I really don't understand the concept.

Here are the 1st 3 questions I am practicing but don't get:

Q: Using the information below find the units for the following quantities
1: frequency (f) f=1/t
2: Speed of light (c) c= λf
3: Energy of an object due to its mass (E) E=mc^2

Where f, t, c, E and m represent frequency, time, speed of light, energy and mass

I really don't understand this supposedly simple physics concept, could anyone please help?
You can replace the symbols with the units.

1: f = 1/t. Unit for t = s (seconds), so unit for f = 1/s = s^-1. This is the same as Hz (hertz).

2: c = λf. Unit for λ = m (metres), unit for f = Hz = s^-1, so unit for c = ms^-1 = m/s (metres per second).

I'll leave the last one for you to try. If you have any problems I'll be happy to help.
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Sam00
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#4
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(Original post by ombtom)
You can replace the symbols with the units.

1: f = 1/t. Unit for t = s (seconds), so unit for f = 1/s = s^-1. This is the same as Hz (hertz).

2: c = λf. Unit for λ = m (metres), unit for f = Hz = s^-1, so unit for c = ms^-1 = m/s (metres per second).

I'll leave the last one for you to try. If you have any problems I'll be happy to help.
I have the answer in my book, I just don't understand why the answer is what it is. My book says it is km^2 / s^2? I also don't understand your explanations as I don't fathom the concept.
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ombtom
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(Original post by Sam00)
I have the answer in my book, I just don't understand why the answer is what it is. My book says it is km^2 / s^2? I also don't understand your explanations as I don't fathom the concept.
Unit for E = kg * (ms^-1)^2 = kg*m^2*s^-2
(I assume you meant that the book says kg*m^2/s^2).

Do you understand how to convert 1/x to x^-1?

If you have an equation, say speed = distance / time, and you know the units for all but one quantity, you can find the unknown units, by substituting in the known units and rearranging. So if you know the units for speed = m/s (=ms^-1), and the units for time = s, then since distance = speed*time, the units for distance = (m/s)*s = m.

Hope this helps.
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Sam00
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#6
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(Original post by ombtom)
Unit for E = kg * (ms^-1)^2 = kg*m^2*s^-2
(I assume you meant that the book says kg*m^2/s^2).

Do you understand how to convert 1/x to x^-1?

If you have an equation, say speed = distance / time, and you know the units for all but one quantity, you can find the unknown units, by substituting in the known units and rearranging. So if you know the units for speed = m/s (=ms^-1), and the units for time = s, then since distance = speed*time, the units for distance = (m/s)*s = m.

Hope this helps.
I know from GCSE the simple equation
D
S x T

But this is different for example I have

A hamster in its ball starts at rest and accelerates to 3m/s in 6 seconds.

a) What was the acceleration?- The answer is 0.5ms^2 (Acceleration = v - u / t)b)

b) What was the distance the hamster traveled? I assumed it would be (3 x 6 = 18) but the answer is actually 9m.

This goes against the formula I was taught at GCSE
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ombtom
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#7
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#7
(Original post by Sam00)
I know from GCSE the simple equation
D
S x T

But this is different for example I have

Ahamster in its ball starts at rest and accelerates to 3m/s in 6 seconds.
a) What was the acceleration?- The answer is 0.5ms^2 (Acceleration = v - u / t)b) What was the distance the hamster traveled. I assumed it would be (3 x 6 = 18) but the answer is actually 9m. This goes against the formula I was taught at GCSE
Distance = speed * time only works when acceleration = 0.

Suvat equations only work when acceleration = constant (so they work when a = 0).

You probably won't come across variable acceleration at AS level, except for in graph form (where calculations will involve area and gradient).

So for your hamster question, you need to use suvat.

s = ?
u = 0
v = 3
a = 0.5
t = 6

v^2 = u^2 + 2as
9 = 2*0.5*s
s = 9/1 = 9m.
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