Honors Physics 2/25/11--BREAK!!!

Hello everybody!!

So today in physics class, we did the accelerating truck problem.We know that the the mass of the weight is 10kg, the angle between the roof and the rope is 55°, and that the truck is accelerating. So for this problem, we needed to draw a quantitative force diagram for the weight to determine all the forces acting upon the weight.

First, we draw a system boundary around the weight and find out what contact forces are acting on the weight...We know that the rope is pulling up on the weight causing a Tension force and that Gravity is pulling down on the weight. The Tension force is unknown while the Gravitational force is 100N because 10kg X 10 N/kg=100N.
Our next step is to find the Tension force by making a triangle with what we know. So we draw a perpendicular dashed line from the roof to weight which is the Gravitational force (100N). With this, we made a right triangle and we can now use trig!! So, we take the sin55=100N/Force of T to find the Tension force (122N).

What!?!? The forces aren't balanced!!! But...but...the picture looks like it is not accelerating. What the deal?!? Well, what's actually happening is that since the weight is in the accelerating truck, the weight is also accelerating at a constant acceleration. So, we need to find the acceleration. But how??

We need to use the equation Acceleration (A)=Force unbalanced (Fun)/Mass (m). There is just one little problem...we don't know the force unbalanced!! Here's how we find it.
To find the force unbalanced, we need to use vector addition (which means we line up our arrows tip to tail).
The gray dashed line represents the force unbalanced. Now all we need to do is find what the actual value is. Just plug in your numbers (122N for T and 100N for G). Use the Pythagorean Theorem to find the force unbalanced.
Now that we have the force unbalanced, we can find the acceleration!!

A=Fun/m

***IMPORTANT***
Remember that Fun is the force unbalanced (NOT THE TENSION FORCE OR THE GRAVITATIONAL FORCE!!!)

So the weight is accelerating at 7 m/s^2 with the truck. We DO NOT put an arrow for the acceleration on the force diagram because acceleration is NOT a force!!


We also did a lab where we only changed the mass of the weight. We were to find the angle between the roof and the rope. We used 1kg, 5kg, 15kg, 20kg, 50kg, and 100kg. No matter what the mass was, the angle was 55 degrees just like the orginal problem. This is true because the acceleration stayed the same. We did not change the acceleration, therefore, the angle did not change!

Mr. Dickie assigned a Extra Credit assignment as well. Over break, we are to make an accelerateometer (of some sort) to find the acceleration of our cars. We were to make an accelerateometer, drive with it in the car (however it may work), take a picture of the angle of the accelerateometer, and then measure the angle. Once you have the angle, find the acceleration...GOOD LUCK!!! Also...please be safe and have somebody do this with you. Thank you!!

If you have any questions, comments, corrections, etc. please leave a comment!! = )

Have a FUN break!!!!!!!