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Momentum, Energy, Work and Power Section 1 Momentum Momentum • Momentum is related to how much force is need to change the movement of an object • Momentum equals mass times velocity • Momentum is a vector p=mv Momentum • In collisions or other encounters, momentum is transferred between objects • In any encounter, the total momentum of the objects does not change – it is conserved Law of Conservation of Momentum Conserving Momentum How is momentum conserved when a cannon is fired? The mass and velocity of the cannon ball must equal the mass and velocity of the cannon Units for Momentum • mv = kg m / s • The SI unit for momentum is a kilogram meter per second • Is that the only way to get momentum? 2 • F = N = kg m / s 2 • Ft = kg m / s X s = kg m / s Ft = Δmv Impulse • Ft is known as inpulse measured in Newton seconds • In a collision or other interaction Impulse = change in Momentum • Seatbelts, airbags and crumple zones increase the time of acceleration in a crash decreasing the force on you making the crash more survivable J = Ft Momentum, Energy, Work and Power Section 2 Energy Energy • Energy is the ability to make change • The total energy of a system is called mechanical energy • Energy is divided into two types: Kinetic and Potential Mechanical Energy = KE + PE Kinetic Energy • Energy in motion • For objects in motion, the amount of kinetic energy is dependent on its mass and its velocity • The kinetic energy of something in motion =1/2 mass times velocity squared KE = ½ 2 mv Kinetic Energy • KE = ½ or = ½ • kg m2/s2 = 1 Joule • Joule is the SI unit used to measure energy 2 mv 2 kg(m/s) James Joule 1818 - 1889 Question? • Does 10 kg moving a 5 m/s have the same energy as 5 kg moving at 10 m/s? • Remember KE = ½ m/v2 NO Potential Energy • Stored energy in a motionless object – has the potential or ability to cause change –Elastic - objects that are stretched or compressed –Chemical – chemical bonds –Gravitational – Height Potential Energy • PE - energy stored in things above the earth • PE depends on mass, height and acceleration due to gravity PE = mgh PE • PE = m X 9.8 m/s2 X height • That’s kg X m/s2 X m or kg m2/s2 • kg m2/s2 = 1 Joule, the same as kinetic energy • What is the PE of a 10 kg mass 5 m above ground? • PE = 10 X 5 X 9.8 = 490 J Law of Conservation of Energy • Energy may change from one form to another, but the total amount never changes • As an example, what happens in a swing? • If the energy of a swing decreases, some other object must increase its energy by an equal amount Momentum, Energy, Work and Power Section 3 Work Work - Defined • Transfer of energy that occurs when a force makes an object move • The object must move for work to be done • The motion of the object must be in the same direction as the applied force Work is Related to Energy • Energy is always transferred from the object doing the work to the object on which the work is done • Work is done on an object only when a force is being applied to the object and the object moves Calculating Work • Work equals force (in Newtons) times distance W=FXD Work Units • W = FXD • W = N X m or kg m2/s2 • Kg m2/s2 are the same units as energy • Therefore work is measured in Joules Question Time If you hold 100 kg over your head, are you doing any work? NO! Momentum, Energy, Work and Power Section 4 Power Work and Time • The relationship between work and time is called power • Power equals work / time P=W/T Power Units • P = W/T • P = Joules/sec • Joule/sec = Nm/sec = watt (W) • Since work and energy are related, power also can be calculated by dividing energy by time James Watt Scottish scientist and inventor 1736 - 1819