MOMENTUM BASICS

Before you are able to do more advance Conservation of Momentum problems, you must master the following concepts.

Calculating Momentum:

p = mv

Remember that momentum can be negative or zero, if not moving.

Ex 1: A 5 kg object going 2 m/s to the left. p = 5(-2) = -10 kgm/s.
Ex 2 : A 10 kg object going 3 m/s to the right. p = 10(3) = 30 kgm/s.
Practice on this Study Help: "Calculating Momentum".

Calculating Net Momentum:

p_net = p₁ + p₂ ... (Find the momentum of each individual object and add them together.)

Remember that momentum can be zero and that net momentum can be positive, negative, or zero.

Ex 1: A 2 kg object going 3 m/s left and a 3 kg object going 1 m/s left. p_net = 2(-3) + 3(-1) = -6 -3 = -9 kgm/s.
Ex 2: A 3 kg object going 6 m/s left and a 2 kg object going 9 m/s right. p_net = 3(-6) + 2(9) = -18 + 18 = 0 kgm/s
Practice on this Study Help: "Calculating Net Momentum".

Calculating Change of Velocity:

Change of Velocity = v_final - v_initial

Remember negatives and that a negative times a negative is a positive.

Ex 1: A 2 kg mass going 2m/s left ends up going 6m/s left. Change of velocity = -6 - (-2) = -6 + 2 = -4 m/s
Ex 2: A 3 kg mass going 4 m/s ends up going 3 m/s the other way. Change of velocity = -3 - 4 = -7 m/s
Note: "The other way means negative, if startd positive. OR, if you assume it starts going left: 3 -(-4) = 7 m/s.
Practice on this Study Help: "Calculating Change of Velocity".

Calculating Change of Momentum:

Change of Momentum = p_final - p_initial = m(v_final - v_initial) (Either way)

Remember negatives and that a negative times a negative is a positive.

Ex 1: A 4 kg mass going 10 m/s stops. 0 - 4(10) = -40 kgm/s OR 4(0-10) = 4(-10) = -40 kgm/s
Ex 2: A 4 kg mass going 6 m/s right ends up going 2 m/s left. 4(-2 - 6) = 4(-8) = -32 kgm/s
Ex 3: A 2 kg mass going 4 m/s to the left ends up going 3 m/s to the right. 2(3 - (-4)) = 2(3 + 4) = 2(7) = 14 kgm/s

Practice on this Study Help: "Calculating Change of Momentum".