Peoples Physics Concepts

7.4. Inelastic Collisions http://www.ck12.org

7.4 Inelastic Collisions

• Solve problems involving inelastic collisions.

Key Equations

∑pinitial=∑p

final The total momentum does not change in closed systems

Example 1

Question: Two blocks collide on a frictionless surface. Afterwards, they have a combined mass of 10kg and a

speed of 2.5m/s. Before the collision, block A, which has a mass of 8.0kg, was at rest. What was the mass and

initial speed of block B?

Solution: To find mass of block B we have a simple subtraction problem. We know that the combined mass is 10kg
and the mass of block A is 8.0kg.

10kg− 8 .0kg= 2 .0kg

Now that we know the mass of both blocks we can find the speed of block B. We will use conservation of

momentum. This was a completely inelastic collision. We know this because the blocks stuck together after the

collision. This problem is one dimensional, because all motion happens along the same line. Thus we will use the

equation

(mA+mB)vf=mA×vA+mB×vB

and solve for the velocity of block B.

(mA+mB)vf=mA×vA+mBvB⇒

(mA+mB)(vf)−(mA)(vA)

mB

=vB

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