Folks, the live webinar associated with the 5 Steps to a 5 AP Physics 1 book took place at 3:30 on Tuesday March 7. Here is the recording from the show!
I'm not on any social media! But if you'd like to ask questions or share thoughts, please put them in the comments here. They won't show up instantly... but I'll be happy to respond to people after the show!
And this thread will last a while. Even if you watched the show on demand rather than live, still put your thoughts here! I'll respond soon.
P.S. In the event, physics teachers Kristin Sumner and Nicole Murawski answered a LOT of the questions in the chat! Thank you both!
Greg
Here's the comment from the anonymous contributor... I will respond! I just removed the screenshots. :-)
ReplyDeleteHi Mr. Jacobs,
I was working on an AP Workbook scenario (Unit 5 : Momentum) and I am having trouble understanding the answer to the second graph on Part C (Initial energy + Work = Final Energy), where KE after the collision is not equal to both the car and the truck. The truck has mass 3m and the car has mass m, and they travel at different velocities with respect to their mass and law of conservation of momentum, however their KE's are not equal to each other after the collision?
Got it! In the collision you describe, the problem states clearly that the carts stick together. Below are the three facts about energy in collisions:
ReplyDelete1. In an elastic collision, mechanical energy of the system is conserved.
2. Collisions for which objects stick together can not be elastic.
3. Collisions for which object bounce off each other may or may not be elastic.
Whenever two carts stick together, mechanical energy (i.e. kinetic plus potential energy) is converted to internal energy (i.e. thermal energy). Not only do the car and truck not have to have the same KE as each other after the collision, the *total* KE should not be the same before and after collision.
So how do you solve this problem if you can't use energy? Use momentum. Any time you see a collision, start with momentum concepts. The total momentum of two objects is conserved when the objects collide.
In this case, the objects have different momentums from each other before and after collision! But the total momentum is the same before and after. After the collision, the car and truck must have the same *speed* as each other because they stick together; but the more massive truck will thus have more momentum than the car.