Gravitational Mechanics

Energy that is stored in the gravitational field is called gravitational potential energy, or potential energy due to gravity.

Formula For Work

Let's see how energy gets stored in the gravitational field. First of all, remember that work equals force times distance as stated in this formula: W = F . d

Force Doing The Lifting

Now, let's consider what happens when an object is being lifted. We will consider lifting an object at constant velocity. That is, during the lift the object will not be accelerating. For the moment we will not discuss the very start of the lift, when the object must begin to move upward from a stationary position. Nor will we, for the moment at least, discuss the very end of the lift, when the object must slow to a stop after the upward motion is finished. Here, we are talking about the motion during the lift, and we will consider this motion to be directly upward and at constant speed.

If the object is being lifted at constant velocity, then it is not accelerating, and the net force on it is zero. We know that from Newton's first law of motion.

We must pull up on an object when we lift it. So, the agent doing the lifting must provide an upward force on the object. This would be the yellow applied force in the animation at left. Gravity, of course, will be pulling down. This pull down of gravity is called the weight of the object. It is the blue downward force in the animation at left.

If during the lift the net force on the object is zero, then the upward pull must be canceled by the downward pull. That is, the upward lifting force must be equal in size to the downward pull of gravity.

In summary, when you are lifting something at constant velocity, the upward pull that you provide is equal to the weight of the object.