Susan asks…
true or false.speed is velocity with direction?
true or false.speed is velocity with direction
Nagesh answers:
FALSE
Velocity = speed with direction.
Think of speed and direction like rockets and missiles. Rockets are stupid. Missiles are smart. Rockets go in one direction. Missiles can track their targets, they have a specific destination, a specific direction.
Velocity is often used in physics, because its almost useless to know how fast an object is going if you don’t know which direction it is going.
Think of it like this. If the Weather man told you a hurricane was traveling at 30 miles an hour, but didn’t tell you which direction it was going, you would have no idea where to run, or if it was going to hit you at all. However, if he told you it was going 30 miles an hour to the North, and you were West of it, you would be fine, and wouldn’t have to worry.
Lizzie asks…
Direction of E field?
Image: http://tinyurl.com/ll6vsts
Consider an electromagnetic wave traveling in the positive y direction. The magnetic field associated with the wave at some location at some instant points in the negative x direction as shown in the figure below. What is the direction of the electric field at this position and at this instant?
the positive x direction
the positive y direction
the positive z direction
the negative z direction
the negative y direction
I assume this is right hand rule, but when i do it i get positive z, i don’t know if im even using RHR correctly, so any help would be appreciated
Thanks in advance
But wait, isn’t positive z the direction of the magnetic force? not the direction of the electric field? How are they in the same direction?
Nagesh answers:
The positive z direction is correct.
Using your right hand, point your thumb in the direction of the magnetic field, point your middle finger in the direction of propagation. Your index finger will point in the direction of the electric field.
Sandra asks…
Physics: Direction of the Magnetic Field Vector?
An EM wave is travelling in the i + j direction. The electric field vector of this wave is in the i – j direction. In which direction does the magnetic field vector point at this instant?
Nagesh answers:
The direction of the EM wave is in the direction of the vector formed from the cross product of the E and B fields, or from EXB
if the E field is in the i-j, it must cross with a vector to produce a field in the i+j
a B field oriented in the -k direction will combine with this E field, since
EXB -> (i-j)x(-k) = -ixk +jxk = kxi +jxk = j+i or i+j QED
Mary asks…
Sailing and Direction?
I always wondered how years ago when all the ships of the world were powered by sails, how was the ship able to sail in the desired direction if the wind was blowing in the opposite or in another direction than in the direction you wanted to go?
Nagesh answers:
By a practise called “Tacking”. Where by you change your direction and sail configurations to optimize your sailing angle potential in the desired direction. By this method you can work your way upwind or into the wind. Sailing into the wind is less favorable than sailing with the wind or downwind.
As well currents, tides, times of year, all play an important part in manuvering your boat in the direction you wish to go. Generally speaking the currents of the world move in a East to West direction in the more hospitable climates. About 35 degrees N. To 35 degrees S. Naturally local currents, land masses and storms can effect your directions as well.
Ruth asks…
direction of given vector?
this question is of vector concept-
a car travelling at 36km/h north turns west in 5 seconds and maintains the same speed.what is the magnitude and direction of acceleration of the car?
Answer is 2√2 m/s^2 along south west direction .Here I have a problem that how it can be along south west direction as south direction is not even in question.please help
Nagesh answers:
A bit tricky to explain without diagrams, but here goes.
Note:
– we are working out the average acceleration; the question should really state ‘average’;
– 36km/h = 36 x 1000/3600 m/s = 10m/s.
The direction of an acceleration is the same as the direction of the force producing it.
It you imagine making the car change direction (from north to west) by pulling ‘manually’ on it with a rope, you should see that, on average, you would have to pull in a south west direction.
To calculate the average acceleration you use:
a = (v – u)/t
In this equation a, v (final velocity) and u (initial velocity) are vectors. The direction of ‘a’ is the same as the direction of v – u. So you have to do a vector subtraction.
In this question u = 10m/s north and v = 10m/s west
v – u is the same as v +(-u). And -u is the the same magnitude as u but in the opposite direction.
So -u = 10m/s south (that’s where south comes in)
To work out v – u we need to add together:
v = 10m/s west and
-u = 10m/s south
The magnitude of v- u, is |v-u| = √(10² +10²) = √200 = 10√2
The magnitude of acceleration is |a| = |v-u|/t = 10√2 / 5 = 2√2 m/s².
From a simple diagram you will see (because v and -u have equal magnitudes) that the direction of v+(-u) is south west.
Watch the video-lesson below, from 16minutes 16seconds, for further explanation of the direction.
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