Momentum Impulse Energy Work Power

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? • F = N = kg m / s2 • Ft = kg m / s2 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 2

KE = ½ mv

Kinetic Energy • KE = ½ mv2 or = ½ kg(m/s)2 • kg m2/s2 = 1 Joule • Joule is the SI unit used to measure energy

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 mv2

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/

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