In which situation is a person doing work on an object? a. A school crossing guard raises a stop sign that weighs 10 N.
b. A student walks while wearing a backpack that weighs 15 N.
C. A man exerts 350 N force on a rope attached to a house.
d. A worker holds a box 1 m off the floor.

Trumpeter a holds a b-flat note on the trumpet for a long time. person c is running towards the trumpeter at a constant velocity. person b is running away from the trumpeter at the same rate. person d is standing still the whole time. which person hears a frequency that is lower than the b-flat?

Acave rescue team lifts an injured spelunker directly upward and out of a sinkhole by means of a motor-driven cable. the lift is performed in three stages, each requiring a vertical distance of 11.0 m: (a) the initially stationary spelunker is accelerated to a speed of 4.80 m/s; (b) he is then lifted at the constant speed of 4.80 m/s; (c) finally he is decelerated to zero speed. how much work is done on the 78.0 kg rescue by the force lifting him during each stage

If a balloon is taken outside on a very cold day, what will occur?