Question

A) How many joules of energy does a 100-watt light bulb use per hour? Express your answer using two significant figures.
B) How fast would a 65 kg person have to run to have that amount of energy? Express your answer using two significant figures.

Answer 1

A) $3.6\cdot 10^5 J$

The power used by an object is defined as

$P=\frac{E}{t}$

where

E is the energy used

t is the time elapsed

In this problem, we have

P = 100 W is the power of the light bulb

t = 1 h = 3600 s is the time elapsed

Solving for E, we find the amount of energy used by the light bulb:

$E=Pt = (100 W)(3600 s)=3.6\cdot 10^5 J$

B) $1.1 \cdot 10^2 m/s$

The kinetic energy of an object is given by

$K=\frac{1}{2}mv^2$

where

m is the mass

v is the speed of the object

In this problem, we have a man of mass

m = 65 kg

we want its kinetic energy to be

$E=3.6\cdot 10^5 J$

Therefore, we can calculate its speed from the previous formula:

$v=\sqrt{\frac{2K}{m}}=\sqrt{\frac{2(3.6\cdot 10^5 J)}{65 kg}}=105.2 m/s = 1.1 \cdot 10^2 m/s$