Energy is the ability or capacity to do work. Energy is a scalar* quantity (a number) that we associatewith a system of objects, e.g., planets orbiting a sun, massesattached to springs, electrons bound to nuclei, etc. It turns out that energy possesses a fundamental characteristic which makes it very useful for solving problems. It is measured in joules
Forms of energy
chemical, nuclear, thermal, electrostatic, solar, mechanical, heat, etc
Types of energy
Kinetic energy Kis energy associated with the state of motion of an object. The faster an object moves, the greater its kinetic energy.
Kinetic energy = 1/2mv 2
Where m is the mass in kg and v is velocity in m/s
Work-Kinetic Energy Theorem
Change in the kinetic of a particle = net work done on the particle
Potential energy Urepresents stored energy, e.g., in a spring. It can be released later as kinetic energy.
Potential energy= mgh
Where m is the mass in kg, g is the acceleration due to gravity and h is the height distance
Work Wis the energy transferred to or from an object by means of force acting on the object. It is the product of force and distance. Energy transferred to the object is positive work, and energy transferred from the object is negative work.
Sample Problem 1
A 105g hockey puck is sliding across the ice. A player exerts a constant 4.5N force over a distance of 0.15m. How much work does the player do on the puck?
Work done =force x distance
= 4.5 x 0.15 =0.678joules
Power is a measure of how much work is done in a period of time – the rate of doing work. The longer it takes to do work, the less power. Power is designated by the letter P. The equation for power is; P =W/t.
The unit of measuring power is watt (W). One watt is 1J of energy transferred in one second. A glass of water weighs about 2N, and if you move it 5 m to your mouth, then you have done 1J of work. If it takes 1 second to move the glass then you are working at the rate of 1W. Since the watt is such a small unit, power is more often expressed in kilowatts (1000W).
Sample Problem 2
An electric motor lifts an elevator 9 m in 15.0s by exerting an upward force of 1.20 x 104N. What power does the motor produce in watts and kilowatts?
Force=1.20 x 104N
Using the definitions of work (W = Fd) and power (P = W/t) we can solve separately or
combine the equations to solve at one time (P = Fd/t)
P = Fd/t = (1.20 x 104N)(9.00m)/15.0s = 7200W or 7.2kW
Energy and Work
If you have ever moved, you know what work it is to pick up a box and move it to another location. Sliding it across the floor is not much better due to friction therefore; energy is expended or conserved when work is done.