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Physics - Work and Energy

Energy is the capacity to do work. Think of energy as a stored ability to do work. Work is displacement of an object by applying force. 

Rule Number 10:
With respect to any body subjected to a constant applied force, work done by an applied force equals force applied in the direction of displacement times displacement or W = FD. Work is measured in joules. One joule equals 1 Newton meter. 

Now, suppose we take an object with a mass of 6 kg and lift it straight up from the floor with a constant force to a height of 2 m, and we want to find the amount of work done on the object by the force applied. 

In order to lift an object at constant speed, one must apply an upward force equal in magnitude to the objects weight. So, W = FD. In this case, F=mass x gravity and D=height. 

In order to find W, we multiply 6 kg by 9.8 m per seconds squared by 2 m to get 117.6 joules. So, the work done on the object against gravity is 117.6 joules. 

Energy can assume a variety of forms. One form of energy is called kinetic energy. And it represents the actual fact that a thing is in motion. Kinetic energy like all forms of energy represents a stored capacity to do work. 

Rule Number 11: 
The kinetic energy of any object is equal to one-half mass times velocity squared or KE=1/2 mv2. Kinetic energy is measured in joules. 

Another form of energy is called potential energy. This form of energy is not in motion. When an object is picked up from the ground, it gains potential energy. More specifically, it gains gravitational potential energy. 

Potential energy is energy that is ready to perform work. When we pick up an object from the floor and put it on the table, we give it gravitational potential energy equal to the amount of work that would be required to lift it from the ground to the table. 

Gravity applies a force on the object equal to its weight. Work is force multiplied by displacement. So, potential energy equals the object weight multiplied by the height of the table. 

Rule Number 12: 
Gravitational potential energy equals mgh where m is the mass of the object, g is 9.8 m per second squared and h is the height of the table. 

Let us say you take a 0.5 kg baseball and throw it up into the air at an angle. Now, suppose at sometime after it is thrown, the baseball has traveled 4 m vertically upwards and it’s velocity at that moment is 6 m per second. 

If for that particular moment we want to figure out how much work was done on the baseball by the applied force, we can do it without even knowing the force with which the ball was thrown, the angle at which it was thrown, or the amount of time passed since it was thrown. 

Rule Number 13: 
For objects near the surface of the earth, work done by a net force equals change in kinetic energy plus gravitational potential energy or W=ΔKE + ΔPE.