Kinetic Energy and Work

What is kinetic energy?

Kinetic energy K is the energy associated with the state of motion of an object. The faster the object moves, the greater is its kinetic energy. When the object is stationary, its kinetic energy is zero but it will have potential energy.

The difference of kinetic energy and Potential Energy

kinetic energy is the energy that an object contains because of a particular motion. On the other hand, potential energy is the stored energy, because of its state of rest.

potential energy increases as weight and height increases

 Examples of Kinetic Energy:

An airplane has a large amount of kinetic energy in flight due to its large mass and fast velocity.

Kinetic energy is not a vector. So a tennis ball thrown to the right with a velocity of 5 m/s, has the exact same kinetic energy as a tennis ball thrown down with a velocity of 5 m/s.

How can we calculate kinetic energy?

 kinetic energy (KE) is equal to half of an object's mass (1/2*m) multiplied by the velocity squared.

Example problem

What is the Kinetic energy of 145 g baseball with a velocity of 42.5 m/s? 

Solution

This video is a short explanation of Kinetic energy

it also contains additional example problems about kinetic energy

Warning!!!Decrease your volume XD

*The initial velocity of Problem 2 is zero because the locomotives are parked or not moving* 

Work

Work "W" is energy transferred to or from an object  by means of force acting on the object

1.)The energy transferred to the object is positive work
2.)The energy transferred from the object is negative work

In Physics, work is the displacement of an object due to force.

Work done by  gravitational force 
Work done by  spring force
Work done by a variable force

These three topics include forces and displacement so they are considered as subtopics for work. I also included the topic of Power in terms of work

Work done by  a Gravitational force

We already know that

For the gravitational force, we consider the change of distance in terms of y-axis

So the equation should be

The gravity is always downward

So we can say that the work done by a gravitational force is negative in every opposing force

Example problem

A mechanic lifts a 40-kg engine off the ground at a constant speed of 3m/s.

after the engine is lifted 2 meters, calculate the work done by the gravitational force 

Solution

the initial position of the engine is from the ground

so our initial height is Zero

Work done by  spring force

where:

k=is the spring force

most of the spring force problems the initial value at 0 m

therefore, our equation will be

Example problem

find the work done of a spring having a spring force of 200 N/m if the spring is compressed 0.025 meter?  

Given

spring constant=200N/m

Distance=0.025

Work done by  variable force

Example problem

find the work done to lift a 20 kg from the floor to a height of 3 m when the variable force f(x) is given in newtons.

Given force

Solution

Power in terms of work

Power is the rate of doing work or using energy

Example problem

It takes 12000J of work to lift an elevator to the 3rd floor of a building ,if this is done in 8 seconds what is the rate of work?

Solution

it takes 1500 watts to lift the elevator to the 3rd floor

1730, space, start text, m, i, slash, h, r, end text, right parenthesis

referrence
https://www.khanacademy.org/science/ap-physics-1/ap-work-and-energy/kinetic-energy-ap/a/what-is-kinetic-energy