Answer:
Explanation:
The rate at which heat will be radiated is given by the expression
E = e Aσ ( T⁴ - T₀⁴ )
E is heat radiated , e is emissivity , A is area of surface , σ is stephan's constant T is temperature of the object and T₀ is temperature of the surrounding .
For all the objects given , e , σ T and T₀ are same so E will solely dependent on area of the surface
surface area of cube= 6 r² ,
surface area of sphere = 4 π r²
= 12.56 r²
hemisphere = 2 π r²
= 6.28 r²
12.56 r² >6.28 r² > 6 r²
heat radiated by sphere > heat radiated by hemisphere > heat radiated by cube .
Answer:
A
Explanation:
From a Solenoid we know that a magnetic fiel is always inversely proportional to lenght L or BL = constant

As I is constant



Answer:


Explanation:
As the disc is unrolling from the thread then at any moment of the time
We have force equation as

also by torque equation we can say



Now we have



Also from above equation the tension force in the string is


As the question is about changing in frequency of a wave for an observer who is moving relative to the wave source, the concept that should come to our minds is "
Doppler's effect."
Now the general formula of the Doppler's effect is:

-- (A)
Note: We do not need to worry about the signs, as everything is moving towards each other. If something/somebody were moving away, we would have the negative sign. However, in this problem it is not the issue.
Where,
g = Speed of sound = 340m/s.

= Velocity of the receiver/observer relative to the medium = ?.

= Velocity of the source with respect to medium = 0 m/s.

= Frequency emitted from source = 400 Hz.

= Observed frequency = 408Hz.
Plug-in the above values in the equation (A), you would get:


Solving above would give you,

= 6.8 m/s
The correct answer = 6.8m/s
The box is at equilibrium, so the net force on the box is zero (the force of gravity on the box is equal to the force exerted up on the box by the surface on which it rests.)
To pick up the box, our upward force must be greater than the force of gravity on the box (the weight). So, we must lift up the box with a force greater than 98 newtons. :)