# Homework Help: This is very confusing multiple choice question

1. Apr 19, 2004

### Evergreen

Which of the following statements are true? (Give the letters, i.e. ACD)

A) One can calculate nonrelativistically when the kinetic energy of an object is large compared with the rest energy of the object.
B) The proper time interval is the shortest time interval measured in an inertial reference frame.
C) The proper length of a meter stick is not always a meter.
D) The total kinetic energy of a body is limited, since its speed can never exceed `c'.
E) A moving clock runs slow.
F) The length along the direction of motion of moving objects are shorter than when they are at rest.
G) Relativistic effects are especially important when speeds are close to the speed of light.

if anyone could answer it, you are the man

2. Apr 19, 2004

### chroot

Staff Emeritus

- Warren

3. Apr 20, 2004

### Evergreen

for A) no clue
B) true,
C)true
D)false
E)true , on book
F) true, on book
G) tricky, because i dont know what he means by important,...no clue

i tried different arrangements, and they were all falst: ABEF,BCEF,BCDEF.ABCEFG.ABCEF.BEF,BEFG, these are all wrong, panicing...

4. Apr 20, 2004

### chroot

Staff Emeritus
Don't just say "true" or "false." Please tell us your reasoning as to why you think each is true or false.

We don't just give out answers here; we prefer to help students actually understand what they're doing.

- Warren

5. Apr 20, 2004

### Evergreen

ok,
for A) i really have no idea what it's talking about
but B) shortest time in frame were object and observer are at rest, the rest is proper time indicator, so it's true
C) proper lengh, it's mentioned in the book, that objects shrink if moving relative to an observer L0 => proper length
D)false because even though v is limites, m isnt as in formula KE = mC^2
E)this is in my physics book
F) also
G) no clue...but i think the denominator becomes zero when v approached to c, so it's not defined, and therefore, no relativistic effects? am i right?

6. Apr 20, 2004

### chroot

Staff Emeritus
A) The concept of 'rest mass' doesn't exist in Newtonian physics, so no, you cannot calculate it relativistically. false

B) You are correct, the shortest measured time between two events is always is the proper time, which is experienced by an observer at rest with the events. true

C) The "proper length" of an object is its length measured by an observer at rest with respect to it. The proper length of an object never changes. false

D) You are correct, the kinetic energy can go to infinity; it does so as v approaches c.

E) "Moving clocks run slow" is not a well-formed statement. It is true that a stationary observer will observe a moving clock as running slow, but that doesn't mean the clock is doing anything different in its own rest frame. To an observer moving along with the clock, the clock does not do anything out of the ordinary. false

F) false for the same reason as E. Nothing ever changes in the object's rest frame; changes are only seen by observers in relative motion to the object.

G) It's true -- the closer an object is to moving at light speed, the larger is gamma, and thus the more pronounced the deviation from Newtonian physics.

Please let me know if you did not understand any of these answers.

- Warren

7. Apr 20, 2004

### Evergreen

on my book,

"clocks moving relative to an observer are measured by that observer to run more slowly(as compared to clocks at rest) => time dilation" p.804

"the length of an object is measured to be shorter when it is moving relative to the observer than when it is at rest" p.809

my book is GIANCOLI

then, isn't E & F are both true then!?

8. Apr 20, 2004

### chroot

Staff Emeritus
Your quotes mention an observer. Questions E & F do not mention an observer. Questions E & F would only be correct if they were modified to agree with the book.

- Warren

9. Apr 20, 2004

### Evergreen

so the answer should be BG ? but...i tried it, still wrong?