Presentation on theme: "Physics 3.3. Work WWWWork is defined as Force in the direction of motion x the distance moved. WWWWork is also defined as the change in total."— Presentation transcript:
Work WWWWork is defined as Force in the direction of motion x the distance moved. WWWWork is also defined as the change in total energy. WWWWork is given in N·m, or Joules. WWWWork is a scalar quantity. Why? LLLLift a weight, put it back where it came from, displacement is zero, but work was done.
??? A constant force of 1900 newtons is required to keep and automobile having a mass of 1.0 E3 Kg moving at a constant speed of 20. meters per second. The work done in moving the automobile a distance of 2.0 E3 meters is: W = Fd = ∆E T
Power Power is the rate at which work is done Power is a scalar quantity. Power equal work divided by time. P = W / t Units of power are the watt (j / s).
??? A 3.0 kilogram block is initially at rest on a frictionless, horizontal surface. The block is moved 8.0 meters in 2.0 seconds by the application of a 12 – newton horizontal force. What is the average power developed while moving the block? P = W / t = Fd / t = Fv
??? A motor having a power rating of 500. watts is used to lift an object weighing 100. newtons. How much time does the motor take to lift the object a vertical distance of 10.0 meters? P = Fd / t
Potential and Gravitational Potential Energy: The energy an object has because of its height. PE = mass x acceleration due to gravity x height ∆PE = mg∆h. Units of gravitational potential energy is joules or kg∙m² / s².
??? An object weighing 15 newtons is lifted from the ground to a height of 0.22 meter. The increase in the object’s gravitational potential energy is approximately: ∆PE = mg∆h.
Hooke’s Law Force The force (F s ) placed on a spring is equal to the spring constant k times the change in distance from the rest position ∆x. F s = k ∆x
??? A 10-newton force is required to hold a stretched spring 0.20 meters from its rest position. What is the spring constant? F s = k ∆x
Potential Energy of a Spring A compressed or stretched out spring stores potential energy and can be calculated 2 ways: 1.The area of a F s vs ∆x graph. 2.PE s = ½kx²
??? A 10.-newton force is required to hold a stretched spring 0.20 meter from its rest position. What is the potential energy stored in the stretched spring? PE s = ½kx²
Kinetic Energy The energy associated with motion. The unit of kinetic energy is joules or kg∙m² / s². kg∙m² / s². KE = ½mv².
??? The kinetic energy of a 980-kilogram race car traveling at 90. meters per second is approximately: KE = ½mv².
??? An Object moving at a constant speed of 25 meters per second possesses 450 joules of kinetic energy. What is the object’s mass? KE = ½mv².
??? If the speed of a car is doubled, the kinetic energy of the car is: KE = ½mv².
Work and Energy Work changes into potential of kinetic energy. When there is friction, work also changes into internal energy.
??? An average force of 20. newtons is used to pull back the string of a bow and arrow 0.60- meters. As the arrow leaves the bow, its kinetic energy is: W = Fd = ∆E T
Conservation of Energy: Energy cannot be created or destroyed. Ideal mechanical system –Total energy equals potential and kinetic energies. There is no friction or air resistance. Non-ideal mechanical system –Total energy equals potential, kinetic, and internal energies. There is friction and/or air resistance.
A Pendulum and a Mass on a Spring Both can be used to show the relationship between KE and PE. During an oscillation, the rest position represents 0 PE and max KE. The two points where there is a change in direction, and furthest from the rest position is where KE is zero, and PE is maximum.