# AP Physics 1: 3.1 Changes and Conservation Laws

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If you bang your head against a moving wall while studying physics, and your head sticks to the wall and keeps moving, what is the combined velocity of the wall and your sad, over-taxed head?

AP Physics 1 | Changes and Conservation Laws |

Language | English Language |

Science Practice 2 | Using math appropriately |

### Transcript

meters per second the two particles stick together and continue to move what

is their combined velocity here are choices [Muttering of potential answers]

Alright, well two particles collide stick together

that's right we're dealing with an inelastic collision and we're stoked [Two particles colliding and sticking together]

like a dream come true granted we have strange dreams there was

this one where we were riding a dinosaur into a tunnel and never mind actually [Two people riding on a dinosaur into a tunnel]

yeah, let's just get the question all right when we're dealing with an

inelastic collision like this we have to remember that momentum is always

conserved it's like there's a law about it or something so the total momentum of

two particles before the collision has to equal the total momentum of the

particles after the collision and we know that momentum equals mass times [Two particles stuck together above a definition of momentum]

velocity we'll call these particles one and two - that's creative right well here's

the equation this says that mass times velocity of particle 1 plus the mass

times velocity of particle 2 equals the sum of the mass of both particles times [equation for the velocity of two particles]

the final velocity we can flip going to decide which particle in this problem is

1 and which is 2 we'll say the particle with the negative velocity is numero uno

and now we can plug in the numbers 1.5 grams times negative 2 meters a second

Plus 3 grams times 4 B to the second equals 4.5 grams times final velocity

well solve them for final velocity gives us answer of 2 meters of seconds and the [equation filled in with the mass and velocity readings]

answer B is correct if you ever feel like banging your head against a wall

just stop take a few deep breaths and maybe take a study break because [woman stops a man from beating his head against a wall]

studying physics can get frustrating sometimes but studying physics with a

concussion, well that won't work yeah [Boy studying physics falls off chair]