A construction worker puts 20 J of energy in to one strike of his hammer on the head of a nail. The energy transferred to driving the nail into the wood is 8.0 J. What is the efficiency of the construction worker's hammering? (2 sig figs and answer in a percentage)

It's a snowy day and you're pulling a friend along a level road on a sled. you've both been taking physics, so she asks what you think the coefficient of friction between the sled and the snow is. you've been walking at a steady 1.5m/s, and the rope pulls up on the sled at a 32 ∘ angle. you estimate that the mass of the sled, with your friend on it, is 65 kg and that you're pulling with a force of 80 n .