Example: if you throw a ball, the work done consists of the distanceyou accelerated the ball until you let it go. Once you havethrown the ball, it will continue at a constant velocity and nofurther work is done.
If you pushed a box across a slippery floor,it might continue to slide for a shortdistance after you stopped pushing.
-is a derived unit of energy, work, oramount of heat in the InternationalSystem of Units.- This SI unit is named after JamesPrescott Joule. As with everyInternational System of Units (SI)unit whose name is derived from theproper name of a person, the firstletter of its symbol is upper case (J). James Joule – Physicist
As an object fall, its PE decreases while its KEincreases, or if its rising the PE increaseswhile KE decreases.
•Imagine that you are on a swing. think about the changes of energywhen you are swinging. at what point do you have the maxPE and Max KE energy? and what happens to the Mechanical energy as you swing?
•Energycannot be created ordestroyed•Theenergy of the universeremains constant
•Remember the swing?...what if you stop swinging? = if you stop swinging you need to rememberfriction, as you slow down on the swing the chainrubs against each other.•Plusthe rubbing of the metal chains in a swingmake the metals temperature heated.Which means that energy is still there its justthat its in a different form.
The change in the kinetic energyof an object is equal to the network done on the object.
Example: Work-energy theoremQuestionA 1 kg brick is dropped from a height of 10 m. Calculate thework done on the brick when it hits the ground assuming thatthere is no air resistance.AnswerDetermine what is given and what is required•Mass of the brick: m=1 kg.•Initial height of the brick: hi=10 m.•Final height of the brick: hf=0 m.•We are required to determine the work done on the brick asit hits the ground.
Determine the bricks potential energy at hiPE=m·g·h=(1 kg)(9.8 m/s²)(10 m)=98 JDetermine the work done on the brickThe brick had 98 J of potential energy when it was releasedand 0 J of kinetic energy. When the brick hit the ground, ithad 0 J of potential energy and 98 J of kinetic energy.Therefore KEi=0 J and KEf=98 J.From the work-energy theorem:W=ΔKE=KEf−KEi=98 J−0 J=98 JHence, 98 J of work was done on the brick.
The gravitational force has an interestingproperty that when an object is moved from oneplace to another, the work done by thegravitational force does not depend on the choiceof path.Forces like these are called conservative forces.
A force is conservative when the work itdoes on a moving object is independentof the path between the objects initialand final positions.
A force is non-conservative when the workit does on a moving object is dependent ofthe path between the objects initial andfinal positions.
Conservative Forces Non-conservative Forces Gravitational force Static and kinetic frictional forces Elastic spring force Air resistance Electric force Tension Normal force Propulsion force of a rocket
The total mechanical energy (E = KE + PE) of anobject remains constant as the object moves,provided that the net work done by externalnon-conservative forces is zero.
In the roller coaster example, we ignored non-conservativeforces, such as friction. In reality, however, such forcesare present when the roller coaster descends. The actualspeed of the riders at the bottom is 41.0 m/s. Assumingagain that the coaster has a speed of 3.0 m/s at the top,find the work done by non-conservative forces on a 55.0-kg rider during the descent.
Work Energy Powerrefers to an activity is the capacity for doing is the rate of doinginvolving a force and work. You must have work or the rate ofmovement in the energy to accomplish using energy, which aredirecton of the force. A work - it is like the numerically the same. Ifforce of 20 newtons "currency" for you do 100 joules ofpushing an object 5 performing work. To do work in one secondmeters in the direction 100 joules of work, you (using 100 joules ofof the force does 100 must expend 100 joules energy), the power isjoules of work. of energy. 100 watts.
UnitsQuantity Symbol Unit S.I. Units Directionvelocity v→ — m·s−1 or m·s−1 ✓momentum p→ — kg·m·s−1 or kg·m·s−1 ✓energy E J kg·m2·s−2 or kg·m2·s−2 —Work W J N·m or kg·m2·s−2 —Kinetic EK J N·m or kg·m2·s−2 —EnergyPotential EP J N·m or kg·m2·s−2 —EnergyMechanical U J N·m or kg·m2·s−2 —EnergyPower P W N·m·s−1 or kg·m2·s−3 — Table 1
You dont always get what you wishfor, you get what you work for.