# Physics-Light/Energy questions

Status
Not open for further replies.

#### Xe54

7+ Year Member
Q1:

As a spacecraft passes directly over Cape Canaveral, radar pulses are transmitter toward the craft and are then reflected back toward the ground. If that total time interval was 3.00x10^-3 s, how far above the ground was the spacecraft when it passed over Cape Canaveral?

I know v=d/t and d=v/t. Using my calculator, I've punched in different numbers and continuously end up with an incorrect answer. This is what I've done:

d=343/.003.
---------------------------------------------
Q2:

Find the illumination 8.0m below a 405lm lamp. The answer I received was: .504lx. Is this answer correct?
-----------------------------------------------
Q3:

Two lamps illuminate a screen equally. The first lamp has an intensity of 12.5cd and is 3.0m from the screen. The second lamp is 9.0m from the screen. What is its intensity?

I know I need to use P=4*pi*(I), but there is no variable included for distance (d). Another formula given is: Eill=P/(4*pi*d^2). How should I go about solving this problem now? Lastly, I am cognizant that 1lm=1cd.
-----------------------------------------------
Q4: A 15cd point source lamp provide equal illuminations on a wall. if the 45cd lamp is 12m away from the wall, how far from the wall is the 15cd lamp.

Again, there is no variable for distance in the equation: P=4*pi*I.

#### BerkReviewTeach

##### Company Rep & Bad Singer
Vendor
10+ Year Member
Q1:

As a spacecraft passes directly over Cape Canaveral, radar pulses are transmitter toward the craft and are then reflected back toward the ground. If that total time interval was 3.00x10^-3 s, how far above the ground was the spacecraft when it passed over Cape Canaveral?

I know v=d/t and d=v/t. Using my calculator, I've punched in different numbers and continuously end up with an incorrect answer. This is what I've done:

d=343/.003.
---------------------------------------------
Q2:

Find the illumination 8.0m below a 405lm lamp. The answer I received was: .504lx. Is this answer correct?
-----------------------------------------------
Q3:

Two lamps illuminate a screen equally. The first lamp has an intensity of 12.5cd and is 3.0m from the screen. The second lamp is 9.0m from the screen. What is its intensity?

I know I need to use P=4*pi*(I), but there is no variable included for distance (d). Another formula given is: Eill=P/(4*pi*d^2). How should I go about solving this problem now? Lastly, I am cognizant that 1lm=1cd.
-----------------------------------------------
Q4: A 15cd point source lamp provide equal illuminations on a wall. if the 45cd lamp is 12m away from the wall, how far from the wall is the 15cd lamp.

Again, there is no variable for distance in the equation: P=4*pi*I.
SDN is not site for homework solutions, so it is highly unlikely you'll get many takers here. That said, on question #1, are you off by a factor of 2 by any chance when you use the speed of light rather than the speed of sound?

Let's make that question more of an MCAT question and see if that helps.

Qu As a spacecraft passes directly over Cape Canaveral, radar pulses are transmitted toward the craft and are then reflected back toward the ground. If that total time interval was 3.00x10^-3 s, how far above the ground was the spacecraft when it passed over Cape Canaveral?

A. 3x10^8 x 0.003
B. 343 x 0.003
C. 3x10^8 x 0.0015
D. 3x10^8/0.0015

#### ksmi117

##### GEAUX TIGERS!!!
Moderator Emeritus