Dropping Textbooks

Today, when I dropped my textbook in the back of the car, it made a loud thumping noise. Later, I dropped my book on my bed, and it barely made any noise at all. Why was this, I wondered?

Physics, of course.


My bed is much more soft and squishy than the floor of the car, so when the textbook falls onto it the book sinks in. This makes the time of the collision much longer than when it falls on the hard floor. Since J (impulse) = change in time x force. If you say that each time the book was dropped from about the same height, the initial velocity before the book impacted was about the same. The final velocity will always be zero, after it hits the surface. This means that the impulse was the same in both collisions, the collision with my bed and with the car floor. J/t = F, so the greater the time, the less the force. The time in the collision with my bed is much longer than the collision with the car floor, so the force in the first collision is less. The greater the force, the more noise the collision makes.

Rolling balls that collide!

So yesterday sitting in robotics, as I was trying to think of something to write my physics blog about, I caught sight of the little green balls that we have to use in the VEX game. Basically, we have to build a robot to get them to the other side of the wall as the opposing robot is dumping the balls on our side of the wall.

Naturally, the first thing I thought of (since I had just been working on Physics homework -_-) was momentum. Let's pretend that there aren't any friction forces, and that the balls are moving in 2 dimensions... despite my efforts I couldn't get the balls to collide and move perfectly in opposite directions.
The moving ball, the one that comes from the left, has momentum that equals it's velocity times it's mass. We'lll assume that the second ball, the one that starts in the middle of the screen, starts from rest (yes, I do realize that it appears to be rolling slightly, but we can pretend it's not for the sake of making this easier) since it has zero velocity.
After the balls collide, there is a change in momentum - the momentum of the ball from the left decreases, and the other ball's momentum increases. In other words, both balls have an impulse value, and the energy is transferred between the balls. According to conservation of momentum, the total momentums of the balls at the beginning and end of the video will be the same. This means that all the momentum that the first moving ball lost was transferred to the other ball.
Since we are assuming that this is a frictionless environment, we can assume that this is an elastic collision - hence, the total kinetic energy stays constant.

Jumping Berens~ Conservation of Energy!

In my never-ending quest to flaunt the rules without actually breaking them and getting in trouble, I decided it'd be a good idea to post my blog at exactly 11:59 pm. Of course, this is all due to hours of planning, not because I almost forgot to do the blog at all and realized that at 11:30pm. No, of course I would never really be that forgetful.

Unfortunately, this plan was thwarted by the insistence of blogger to write the post times in some strange, non-Hawaii time zone. I'll have to be satisfied with the knowledge that I sneaked in- I mean, carefully planned to post- right before the deadline.

Today, as I watched my little brother Beren hyper-ly jump around the room while he looked as his birthday presents, naturally the first thing that came to mind was that image of the little kid on the trampoline in our physics book, where you're supposed to find the PE and KE.
For some strange reason, he isn't smiling in this picture, don't ask me why. To prove his normal levels of cuteness, I'll find another, better picture, where I'm not asking him for help on my Physics homework at 11pm at night... like this one.
Ok, never mind. In that one, he just looks evil. Like he's plotting, or something. Back to Physics.

In the first picture, he hasn't yet left the ground, and in the second one he is (theoretically) at the highest point he'll reach. This means that in the first picture, kinetic energy = .5mv^2, and PE = 0, while in the second picture, PE = mgh while KE = .5mv^2. According to the Law of Conservation of Energy, the inital PE + the initial KE = the final KE + the final PE. This means that if I knew either the inital velocity or the velocity at the highest point, I would be able to solve for the unknown velocity. Of course, this would also only work in a world where there was no friction or air resistance.

If there was no friction or air resistance, think how much higher and faster he would jump.... O_O

Now to wait 10 more minutes so I can post right at 11:59... *laughs evilly*

Pushing Pull Doors... = fail...

Last night, when my family ate at KuruKuru sushi (YUM, sushi...), I failed to open the restaurant door. Why, you may ask?

Because I have an crippling inability to read, which results in humiliation in front of large crowds of diners as I stubbornly continue to push on a door labeled "Pull". It's a regrettable talent.

Certainly, I felt like I was doing a considerable amount of work. But work is defined in our textbook as "the product of displacement and force", and that door clearly wasn't going anywhere. Since the door wasn't moving, there was ZERO displacement. And everyone knows that zero times anything will always equal zero. No matter HOW MUCH FORCE I used trying to get that stubborn door open, it just wouldn't budge.

Therefore, I was doing zero work, and still managing to humiliate myself. It almost leaves me wishing I had been doing some kind of work, because that would mean, well, that the door was opening...