Momentum and Energy

 

F x t = m x v

Conservation of Momentum

    • Momentum can be transfered from one object to another. The value of the momentum of one object is exchanged to other objects during an interaction such that the total momentum of all objects remains the same after the interaction.
  • Momentum is decreased for one object but increased for the other object by exactly the same amount so the total momentum of the system remains a constant.
  • Net Momentum(before collision)=Net Momentum(after collision)
  • How does firing a gun demonstrate conservation of momentum?
  • A 50 kg girl on roller skates pushes off a 100 kg boy(also on roller skates)and moves at 2 m/s? How fast is the boy moving?
  • An elastic collision is when objects rebound without lasting deformation or generation of heat M*V=M*V
  • An inelastic collision is when objects are deformed or heat is generated. In a perfectly inelastic collision, the objects stick together. M*V=(M+M)*v
  • One object moving at 10 m/s collides with a second object of identical mass standing still. If they stick together , at what velocity do they move?
  • Conservation of Angular Momentum- the total angular momentum of an isolated system is constant
  • Angular momentum =mvr
  • How does an ice skater increase the speed at which they spin?

 

Energy and Work

 

Matter and energy make up the universe. Energy is the mover of mass. Energy is abstract and is best viewed as it changes form.

  • Energy is a quantity(Joule-J) associated with the state of a system
  • Energy can easily change forms and locations. Energy is both a thing and a process. Sometimes it stored and becomes invisible.
  • Stored energy(called Potential Energy)can be later released to produce motion, like the chemical energy in your food gives your body the energy to move
  • Total Energy(in system)=potential energy+kinetic energy+thermal energy
  • Energy can change from one form to another but is never lost.
  • Energy is the property of a system that enables it to do work
  • Work accomplished by energy is measured in Joules(J).Both work and energy are measured in Joules(Newton*Meters)
  • Work(J)= Force(Newtons) applied thru a distance(Meters) or (f x d). How long in distance you apply a force affects its energy.
  • A force of 10N is applied to a 20 kg rock for 10 sec and the rock moves 2m. How much work is done?The rock does not move, how much work is done?

 

Potential Energy

  • Potential Energy(PE) is stored energy. PE is converted into other forms to do work.
  • Types of Potential Energies(PE).
    • Chemical-Energy stored in fuels such as foods, gas and batteries. The energy is stored in the bonds between atoms.
    • Electrical-Energy created by power utilities
    • Nuclear- Energy(from nucleus of atom)created by converting mass to energy
    • Elastic - Energy from a coiled spring or drawn bow
        • Molecules try to keep the same arrangement because of their electrical forces and resist being deformed.
        • Bouncing Ball Example
    • Gravitational-Energy stored in an object raised to a height
      • PEgrav(J)= mass*gravity*height

    What happens to the potential energy of an object if you double its height?

    How much potential energy does an object have if its mass is 2 kg and it is raised to a height of 3 meters?

Kinetic Energy

Kinetic Energy is energy of motion.

  • KE(J)=1/2 mv^2
  • How much KE is in a 5kg rock thrown at 2m/s?
  • If I am sitting motionless, could I still have energy? If so, what type? Do I have momentum?

Thermal Energy

 

Work-Energy Theorem

  • Work-energy theorem- Work equals change in kinetic energy.(FxD=1/2xMxV^2)
  • If you double your velocity, what happens to your KE?
  • Two football players collide head on. Player one has a mass of 200kg and runs at 1m/s. Player two has a mass of 100 kg and runs at 2 m/s. Compare their momentums and kinetic energies. Which one wins the battle?
  • When you are driving at 90 km/hr, how much more distance do you need to stop than if you were driving at 30 km/hr?

Conservation of Energy

(also called the 1st law of thermodynamics)- In a closed system the total amount of energy,including heat, is conserved. In other words, although the kind of energy in a given system can change, the total amount cannot. Energy is converted from one form to another and is not lost.

Energy exchanges drive all interactions

 

 

Power

Machines

 

Inclined Plane,Wedge and Screw

 

Efficiency

  • Efficiency- What percent of input energy is converted to useful work?
      • Work out/work in= % efficiency or work done/energy used = % efficiency
      • It is impossible to have an 100% efficient machine. Some energy is always lost as thermal
      • If a machine with 30% efficiency uses 300J of energy, how much useful work can be done?
A machine puts out 100 W of power for every 1000 W put into it. The efficiency of the machine is

Sources of Energy