Physics 3.3

Work WWWWork is defined as Force in the direction of motion x the distance moved. WWWWork is also defined as the change in total energy. WWWWork is given in N·m, or Joules. WWWWork is a scalar quantity. Why? LLLLift 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 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.