as you are talking to a friend on your cell phone, you might find that you can’t hear them because they are speaking very softly. if you ask them to "speak up," from a physical perspective you are asking them to of the sound waves they are producing.

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 .