At 1 because the cart is still at the top
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50N is your force and the acceleration is -9.8m/s^2 due to gravity.
So, you just plug that in.

BUT you know that mass cannot be negative, so you just disregard the negative sign and the mass of the rock is 5.102 grams.
A falling raindrop
Kinetic energy and potential energy are both applied when a body or object is falling.
Two analogies:
1. The wires are like hoses full of water. As soon as you turn on the water, water is pushed out the end of the hose.
2. Although the electrons only move at 1mm/s, the electomagnetic pulse travels through the circuit at near speed of light (varies btw .1c to .9c). It is this pulse that provides the pressure to push the electrons out the end of the wire into the light