Calculate the kinetic energy of this object in joules.

Question

An object with a mass of 26.9 pounds is moving with a uniform velocity of 46.8 miles per hour. Calculate the kinetic energy of this object in joules.

(Enter your answer to three significant figures.)

Hints:

EK = ½(mass)(velocity)2

1 lb = 453.59 g

1 km = 0.62137 mi

1 J = 1 kg m2/s2

Answer 1

This is a matter of unit conversions. Since KE = 1/2 mv²and m is mass in kg and v is velocity in meters per sec we need to convert the given values into these units.

mass = 26.9 lbs x 453.59 g/lb x 1 kg/1000 g = 12.20 kg

velocity = 46.8 mi/hr x 1 km/0.62137 mi x 1000 m/km x 1 hr/3600 sec = 20.9 m/sec

KE = 1/2 (12.20 kg)(20.9 m/sec)² = 2670 kg m²/sec² = joules

Answer 2

KE = ½(mass)(velocity²) the equation is given for you. We have to convert lb to kg and miles per hour to m/s.

KE = 1/2 (26.9*453.59*10^-3)((46.8/60*60*0.62137)²) = 2.65*10^-3 J