MomentumMomentum

Chapter 6Chapter 6

Momentum (Symbol = p) [Unit = kg m/s] Inertia in motion The Quantity of MotionMomentum = Mass • Velocity

p = m • vNewton’s 2nd Law

Impulse = change in momentum

( )

f i

f i

f i

v vvF ma a t tv v

t F m ttFt mv mv

Impulse (Symbol = I)[Unit = Nsec] A force in a certain time

Concept: In order to change momentum, we need a force in a certain amount of time!

f ipI Ft mv mv

f ipI Ft mv mv

Changing MomentumDifference between hitting concrete bridge vs. yellow barrels full of pea gravelSeat belt vs. Air bag

Δp = Ft Δp = Ft

0 - m v = FT 0 - m v = Fthttp://www.regentsprep.org/Regents/physics/phys01/impulse/default.htm

Momentum is a vector, so direction is important!

A 4.0 x 104 N rocket is at rest when it fires its thrusters. If the thrusters put out an average force of 5.0 x 103 N over 20 sec, then (a) What is the impulse the rocket experiences?(b) What is the rocket’s final velocity?

(a) Impulse I = F tI = 5000 (20)I = 100,000 Nsec

(b) v I = p = mvf – mvi

100,000 = 4080 vf – 4080(0) vf = 24.5 m/s