The outer surface of a skier’s clothes of emissivity 0.7000.700 is at a temperature of 5.505.50 °C. Find the rate of radiation if the skier has a surface area of 1.601.60 m2 and the surroundings are at −20.0−20.0 °C.

121.0 W

Explanation:

We use the equation for rate of heat transfer during radiation.

Q/t = σεA(T₂⁴ - T₁⁴)

Since temperature of surroundings = T₁ = -20.0°C = 273 +(-20) = 253 K, and temperature of skier's clothes = T₂ = 5.50°C = 273 + 5.50 = 278.5 K.

Surface area of skier , A = 1.60 m², emissivity of skier's clothes,  ε = 0.70 and σ =  5.67 × 10⁻⁸ W/m²K⁴ .

Therefore, the rate of heat transfer by radiation Q/t is

Q/t = σεA(T₂⁴ - T₁⁴) = (5.67 × 10⁻⁸ W/m²K⁴ ) × 0.70 × 1.60 m² × (278.5⁴ - 253⁴) = 6.3054 × (1918750544.0625) × 10⁻⁸ W = 1.2098 × 10² W = 120.98 W ≅ 121.0 W

122.1 W.

Explanation:

Thermal radiation is defined as the electromagnetic radiation generated by the thermal motion of particles in matter.

Mathematically,

q = σ * ε * A * (Ts^4 - Ta^4)

where,

q = heat transfer per unit time (W)

σ = The Stefan-Boltzmann Constant = 5.6703 x 10^-8 (W/m^2.K^4)

Ts = absolute temperature of the surroundings in kelvins (K)

ε = emissivity of skier's clothes = 0.70

Ta = absolute temperature of the object in kelvins (K)

A = area of the emitting body (m2)

= 5.6703 x 10^-8 * 0.7 * 1.6 * (278.65^4 - 253.15^4)

= 122.1 W.

d

explanation:

v=uf/(u-f)

= 5.10/(5-10) convex lens has positive focal length

=50/-5

=-10 cm (formed at other side)

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.