Why are fuel cells considered a "clean" energy source?

Acricket ball of 70g moving with a velocity of 0.5 m/s is stopped by a player in 0.5s what is the force applied to stop the ball

Ethanol has a heat of vaporization of 38.56 kj/mol and a normal boiling point of 78.4 c. what is the vapor pressure of ethanol at 14 c?

What kind of waves require a medium to travel? question 6 options: light waves mechanical waves all waves need a medium to travel electromagnetic waves