Name:______Date:______

Unit 7 – Momentum and Its Conservation

7.3 – The Conservation of Momentum

* To study momentum changes in collisions, we must use a ______, ______. A system is closed if objects neither enter nor leave it. It is isolated if no net external force is exerted on it.

Conservation of Momentum –

1-  Two Body Push Apart

This problem involves the explosion of two masses. A spring between two masses will push them apart. The names of the masses are mass 1, or m1, and mass 2, or m2.

The masses move apart, each with its own velocity. The velocity of m1 is v1’; the velocity of m2 is v2'. In the diagram below, m1 is shown moving to the left after the explosion. However, this is not necessarily so. It could also be moving to the right after the explosion. Its movement depends upon the size of the impulse produced by the spring.

2-  Two Body Stick Together Collision

This problem involves the collision of two masses which stick together. The names of the masses are mass 1, or m1, and mass 2, or m2.

Before the collision:

·  Both masses are in motion, moving in positive directions. (See note below.)

·  Mass 1 is behind mass 2.

·  Their velocities before the collision are v1 and v2.

·  The velocity of mass 1 is greater than the velocity of mass 2. So, mass 1 will eventually catch up to mass 2.

Before

After the collision:

·  The masses stick together and move off to the right as one object. Therefore both masses are traveling at the same velocity after the collision. This mutual velocity is called v'.

After

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3-  Total Momentum, Two Body System, Two Dimensions

Top of Form

Bottom of Form

Imagine two objects in motion. This will be our system of two objects.

We will calculate the total momentum of this system. This situation does not consider the collision of the two objects, just the total momentum of the two objects.

Decide upon values for the masses of the two objects. Call these two masses m1 and m2.

Decide upon values for the sizes and directions of the velocity vectors for these masses. Call these velocities v1 and v2 for m1 and m2 respectively.v1 should be directed South of East as shown

v2 should be directed North of East as shown here:

______

* Law of Conservation of Momentum – the momentum of any closed, isolated system does not change!

The total momentum before an interaction equals the total momentum after the interaction

______(equation)

Ex1: Ball A of mass 0.355 kg has a velocity of 0.095 m/s. It collides with ball B of mass 0.710 kg moving in the same direction at a velocity of 0.045 m/s. After the collision, ball A continues in the same direction with a velocity of 0.035 m/s. What is the velocity of ball B after the collision?

Homework: 1. Read pgs 180 – 188

2. Do pgs 185 # 5 – 8

3. Do pgs 188 – 189 # 9-11

Directions: Answer the following questions concerning the conservation of momentum using the equations below. Show all of you work .

p = mv Ft = D (mv) impulse = FDt

pbefore = pafter

net momentum before = net momentum after

1. When these two freight cars of different mass collide and couple, what will be their resultant velocity?

2. A 2 kg blob of putty moving at 4 m/s slams into a 6 kg blob of putty at rest. What is the speed of the two stuck-together blobs immediately after colliding?

3. A large locomotive with a mass 4 times that of the smaller motionless railroad car collides and couples together. What is their combined speed after the collision?