### All GRE Subject Test: Physics Resources

## Example Questions

### Example Question #1 : Energy And Momentum

A relativistic particle of mass *m *has a total energy 37 times its rest energy. What is the momentum of the particle, in units of *mc*?

**Possible Answers:**

21

37

144

98

52

**Correct answer:**

37

The total energy *E *of a relativistic particle is related to its rest mass energy *E _{o}* by:

Where gamma is related to the momentum by:

Combining the equations and solving for p, we get:

Which, in the units specified, is 37.

### Example Question #2 : Energy And Momentum

A particle of mass *m* traveling at a relativistic speed has a momentum of 50 *mc*. What is the total energy *E *of the particle, expressed in units of the rest mass energy *E _{o}*?

**Possible Answers:**

**Correct answer:**

For a relativistic particle, momentum is given by:

from which we can solve for gamma:

Total energy of a relativistic particle is given by:

### Example Question #3 : Energy And Momentum

The rest mass energy * *of a particle with mass is one quarter of its total energy . What is the of the particle's momentum, in units of ?

**Possible Answers:**

**Correct answer:**

The question tells us:

Where the rest mass energy of a particle is:

Using the equation for the total energy of a particle, we can substitute:

Solving this for , we find:

Which, in units of *mc, *gives us the correct answer.

### Example Question #4 : Energy And Momentum

The relativistic momentum of a particle with mass * *is *. *What is the total energy * *of the particle, given in units of the rest mass energy *?*

**Possible Answers:**

**Correct answer:**

The total energy of a relativistic particle is given by:

Substituting the momentum, we get:

Because the rest mass energy of a particle is given by:

The total energy is:

### Example Question #5 : Energy And Momentum

A scientist measures the spectrum of relativistic jet emitted from a black hole. He finds that the a particular spectral line, which has a stationary wavelength of 212.5 nm, has a Doppler shifted wavelength of 643.7 nm. What is the radial velocity of the relativistic jet?

**Possible Answers:**

**Correct answer:**

The relativistic Doppler shift equation is given by:

Where is defined as:

Because the stationary wavelength is shorter than the moving wavelength, the object must be receding from the Earth, eliminating two answers.

The speed of light is approximately , so is not a possible answer.

Making the approximation that

Combining this with the first equation:

From beta, we can find the velocity:

### Example Question #6 : Energy And Momentum

A particle in an accelerator has a total energy and a relativistic momentum* *. What is the rest mass of the particle, in units of ^{ }?

**Possible Answers:**

**Correct answer:**

The total energy of a relativistic particle is given by:

Solving for the rest mass :

Which, in the units specified, gives the answer 3.

### All GRE Subject Test: Physics Resources

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