Confused to the max Re-aranging equations.

In summary, the conversation discusses a student's confusion about manipulating equations from their physics exam. They substituted values into two different equations, E=hc/\lambda and \lambda=h/mv, and ended up with a seemingly incorrect answer. The person responding explains that just because two equations have similar symbols, they may not represent the same thing and cannot be substituted.
  • #1
slawted
1
0
Okay so today i sat a AS level physics paper and after finishing the exam i looked threw the equation booklet and decided to do some manipulation and i came up with this and was wondering what you guys think of it. I am confused! :)

E=hc/λ AND λ=h/mv
subbing in gives E=hcmv/h which is E=cmv
conservation of energy...
cmv=1/2mv^2
2c=v

how can velocity = 2*speed of light?

Thoughts?
 
Physics news on Phys.org
  • #2
slawted said:
Okay so today i sat a AS level physics paper and after finishing the exam i looked threw the equation booklet and decided to do some manipulation and i came up with this and was wondering what you guys think of it. I am confused! :)

E=hc/λ AND λ=h/mv
subbing in gives E=hcmv/h which is E=cmv
conservation of energy...
cmv=1/2mv^2
2c=v

how can velocity = 2*speed of light?

Thoughts?

Just because something has the same name or the same symbol, does not mean they are representing the SAME thing.

[itex]E=hc/\lambda[/itex] is the energy per photon with that particular wavelength, whereas [itex]\lambda=h/mv[/itex] is the deBroglie wavelength for a non-relativistic particle with mass m at velocity v!

You can't simply substitute apples into oranges. If you do that, you get ridiculous answer like what you got.

Zz.
 

Related to Confused to the max Re-aranging equations.

1. What is the purpose of re-arranging equations?

Re-arranging equations allows us to solve for a specific variable, making it easier to understand and apply in different situations. It also helps to simplify complex equations and make them more manageable.

2. How do I know which variable to solve for?

The variable you solve for depends on what information you are given and what information you are trying to find. Look for any variables that are already isolated on one side of the equation, or any variables that are known values. Use algebraic rules to manipulate the equation until you have the desired variable isolated on one side.

3. What are some common mistakes to avoid when re-arranging equations?

Some common mistakes when re-arranging equations include forgetting to perform the same operation on both sides of the equation, mixing up positive and negative signs, and incorrectly applying algebraic rules. It's important to carefully check your work and make sure all steps are correct.

4. Can I re-arrange any type of equation?

Yes, you can re-arrange any type of equation as long as it follows the rules of algebra. This includes linear equations, quadratic equations, and equations involving exponents, logarithms, and trigonometric functions.

5. Are there any shortcuts or tricks for re-arranging equations?

There are some common algebraic rules and tricks that can make re-arranging equations easier, such as using the distributive property, combining like terms, and cross-multiplying. It's important to practice and become familiar with these rules in order to efficiently solve equations.

Similar threads

  • Introductory Physics Homework Help
Replies
2
Views
790
  • Introductory Physics Homework Help
Replies
4
Views
967
  • Advanced Physics Homework Help
Replies
1
Views
3K
  • Introductory Physics Homework Help
Replies
30
Views
2K
  • Introductory Physics Homework Help
Replies
1
Views
1K
  • Quantum Physics
Replies
1
Views
3K
  • Introductory Physics Homework Help
Replies
1
Views
40K
  • Quantum Physics
Replies
1
Views
2K
  • Engineering and Comp Sci Homework Help
Replies
5
Views
3K
Replies
8
Views
3K
Back
Top