Ohm's Law Resource

This video was very helpful because it really focuses on the basics of Ohm's Law as far as equations go, and simple ways we use it in problem solving.  The diagrams were also very clear

VOLTAGE Resource

Click on this link...
VOLTAGE RESOURCE WEBSITE!!
This website explains voltage through an animation of charges flowing through a circuit.
There is also another tab you can click to more deeply explore different ways we see voltage used in everyday life and how it relates to us

MASTERS of the Mousetrap Car

Maiya and I built a mousetrap car to put our total Physics knowledge to the test, applying most of the concepts we've learned throughout the year so far.

Our car came in 1st place overall

with a record-breaking speed of 5 meters in 2.2 seconds

Therefore, we reached a speed of 2.27 m/s

-Maiya Eldridge and Catherine Eckerd-

Masters of the Mousetrap Car

Yes, 2.2 seconds.  Don't believe it? Watch this video:

We took many factors into account, most of which we connected to physics concepts (in red) we've learned so far, all labeled on the picture below:

1. Light frame - We used wood (from Lowe's) for the base to keep our car light.  We did not want a heavy frame because acceleration = force / mass, therefore if we decrease our total mass, knowing that the force would remain constant, we would have a greater acceleration
2. Small wheels - we used small wheels (from a toy car I found) because of rotational inertia.  Small wheels distribute their mass closer to the axis of rotation, therefore will spin much faster than larger wheels.  Also, the small wheels kept the car closer to the ground, therefore keeping it's center of gravity lower, keeping our car more stable, therefore faster.
3. Axel - We used a metal rod (also from toy car) as an axel because the metal is slippery, so it would decrease friction between the axel and the wooden frame.
4. Rubber bands - We used rubber bands around the wheels (from Carson) because the rubber would increase friction between the wheels and the ground, so no power would be lost by the wheels slipping
5. Mouse Trap - The mouse trap (from Mrs. Lawrence) increased the energy we put into the car by winding it up because of it's tightly coiled spring, so when we let it go, it snapped together quickly, pulling the string with a great speed
6. Wooden Arm - We used a wooden plank (also from Lowe's) as a lever arm.  We made our lever arm long because lever arms increase the distance the string connected to it travels therefore increasing the velocity at which the string is pulled
7. Fishing Line - The fishing line (from Carson) connects the lever arm to the wheels' axel (by hot glue), pulling the wheels with the speed from the lever arm.

PHYSICS BEHIND MOUSETRAP CAR

This project really showed how Physics applies to mechanics and real-life machines, such as cars.  You can see the importance of each of Newton's 3 Laws in the performance of a car.

• Newton's 1st Law: An object in motion will stay in motion unless acted upon by another force.  This is important to a car because, to make a car as efficient as possible, you want to eliminate as much as possible all these outside forces that will slow it down.  We did this by trying to eliminate the force of friction between the frame and the axel to keep it from slowing down
• Newton's 2nd Law: Acceleration is proportional to force, and inversely proportional to mass.  This was very important to our car because we really focused on creating a light frame so our car could accelerate quickly.
• Newton's 3rd Law: For every action there is an equal and opposite reaction.  This is important because we were able to create an action reaction pair with the car and the ground.  As the car moves forward, it's pushing on the ground, and the ground is pushing back.  This is what makes the car moves.  Knowing this, we realized how important it was for the wheels to have a great force of friction with the ground.

The two frictions present are the friction between the ground and the wheels and the friction between the axel and car frame.  Between the wheels and the ground, we used friction to our advantage by adding rubber bands to the wheels to increase their relation with the ground.

We decided less wheels was better because there will be less friction between axels and the car itself, therefore less energy lost, therefore more efficient.  Using large wheels seemed to increase distance, but not speed.  Small wheels increase rotational inertia, therefore increase speed.

Conservation of energy was important in designing our mousetrap car.  Because a mousetrap car only emits so much energy, it was important to realize that if our car wasn't moving very far, we were losing energy in some way making the car less efficient.

We created a lever arm about 5 inches long.  Because our lever arm was so long in comparison with the rest of the car, we were able to increase the velocity our axel was rotating at.  The lever arm increased the radial distance from the axis of rotation of the mousetrap's arm.  Increasing this radial distance increases the tangential velocity at which the string is moving, therefore moving the wheels with more speed.

As stated above, tangential velocity was important in the movement of our lever arm.  For our wheels, rotational inertia was especially important because we kept the mass of the wheels distributed closer to the axel by using small wheels, therefore keeping a smaller rotational inertia.  Less rotational inertia means a greater rotational velocity.

The spring does no work on the car.  We do work on the spring by winding up the lever arm, but the spring snapping the trap shut is simply due to the energy we gave it in winding it.  We cannot calculate the potential or kinetic energy of the car because it's energy does not come from motion.  The spring did not exert a force on the car because it was part of the car, creating a system.

REFLECTION
Our final design was actually very close to our original design:

One change we made was rather than suspending the mousetrap inside the frame, we connected it to the frame.  This was because our frame didn't allow the space to suspend the mousetrap because of the small width of our wheel axels.  Another aspect we were unable to do was the cork on the back axel of the car.  We were unable to do this because there was too little friction between the cork and the axel, and it was too slippery.  It was much more effective to connect the string directly to the axel.One aspect we didn't incllude in our plan but was extremely vital to the sucess of our car was adding a lever-arm, which we did last in construction.  One major difficulty we encountered was in our choice of string.  We began with thick pink string that was slippery and difficult to work with.  The string got tangled in our wheels, and sometimes kept the car from moving at all.  After we switched to the yellow fishing wire, we kept in mind how important it was to CAREFULLY wrap the string around the axel so it wouldn't get tangled and cause the car to stop.

If I were to do this project again, I would have been more careful in aligning the holes on both sides of the planks for where the axels go through because one was higher than the other, and this may have caused the car to be unsteady.  Also, I would have found a better way to add friction to the wheels because the rubber bands slipped off almost every test run.  Perhaps we could have secured them better.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          

UNIT 5 REVIEW

In this Unit, we covered major topics:

1. Work and Power
2. Work and Kinetic Energy Relationship
3. Conservation of Energy
4. Inclined Planes and Machines

Work and Power
The equation we use for work is:

         Work = Force * Distance
One important thing to remember is that in order to calculate work, the force and distance MUST BE PARALLEL 
  For instance, if someone picks up a box, you can calculate the force because the force (weight of the box) is parallel with the distance they lift it.
However, if the same person then carries the box while walking, the work cannot be calculated because the distance is now forward, and not parallel with the force of the box.

One example we used in class was walking up the stairs. To see how to calculate the work for this example, watch this helpful video:

Power:

Power is how quickly work is done

The equation for power is 

                  Power = Work / Time

The units for power are (Joules/ Second), which is the same as one WATT

Another unit commonly used when referring to power is horsepower.

              1 horsepower = 746 Watts

Work and Kinetic Energy
A term new to us is "kinetic energy".  Kinetic energy is the energy of movement, or motion.
How is kinetic energy connected to work? The change in Kinetic energy equals work, or:

                   Work = ∆ Kinetic Energy

To be able to use Kinetic Energy in problem solving, we must remember this equation:

             KE = 1/2 m v²
Because work is a change in kinetic energy, use this set up to find work:

        ∆KE = KE final - KE initial

Potential Energy

Potential Energy is the energy an object has because of its position, how far it is from the ground, and how much gravity is acting upon it.  

Potential Energy can apply to an object in motion or an abject at rest.

Kinetic Energy and Potential Energy are inversely related.  You can see this in a swinging ball on a string:

Notice where the Potential energy is the greatest, the kinetic energy is the least

and where Kinetic Energy is greatest, Potential Energy is the least

Inclined Planes and Machines

Why do movers use ramps to move boxes into an truck?

It is because using an inclined plane decreases the force needed to move the box.

How is this so?

Think back to work.  We know work = force * distance.  Since force cannot change (because of conservation of energy), the only way to decrease the force exerted is to increase the distance:

Work = Force * Distance

Work = Force * Distance

The ramp above does not reduce the amount of work you must do to get the box to the top, but it does reduce the FORCE you must exert to get it there.  It does this by INCREASING THE DISTANCE you push the box.  So overall, you're really still doing the same amount of work, but your force is more spread out in more distance.

This is how machines work.  

Work in = Work out

F d = F d

They increase the distance in which you exert your force, therefore decreasing your force needed.  But overall, your work in will always equal your work out because of CONSERVATION OF ENERGY

Personal Review

Overall, I think this was the most challenging unit we have covered all year.  I think the hardest concept to grasp was the relationship between potential energy and kinetic energy.  I think the reason I didn't fully understand this was because I didn't have solid, straightforward notes to look back on.  However, after going over some problems in the book, I was able to overcome this confusion and it's a lot more clear now.

Simple Machine Resource

This video explains how a pulley works as a simple machine h=by increasing distance, therefore decreasing the amount of force needed to lift the box.  But remember, WORK stays the same

Work / Energy Resource

This video is very clear. It is mostly just a review of all we learned, but it is more concise and really tests your understanding.  It connects energy to work to power in a way that makes a lot of sense.

UNIT 4 OVERVIEW

In Unit 4, we covered 6 major topics:

1. Rotational and Tangential Velocity
2. Rotational Inertia
3. Conservation of Angular Momentum
4. Torque
5. Center of Mass/ Gravity
6. Centripetal/Centrifugal Forces

Rotational and Tangential Velocity

We measure linear motion in speed. For circular motion, however, we have to ways to express velocity:

• Tangential Velocity
• Rotational Velocity

Rotational Speed measures the number of rotations that occur in a certain unit of time.  For instance, A car measures a it's speed in "RPM", or Rotations Per Minute.

Tangential Velocity measures the distance covered per unit of time such as miles per hour (mph) kilometers per hour (km/h).  Therefore, it is still linear speed, but in a circular path.

One thing to remember about tangential velocity is that it depends on RADIAL DISTANCE.  Radial distance is distance from the AXIS OF ROTATION.  They are directly related.  Also, Tangential Speed is also directly related to rotational velocity.  We express both these relationships like so:

v ~ r w

(tangential velocity ~ radial distance * rotational velocity)

-We saw this relationship when we did the activity outside.  We linked arms and created a line, rotating around Maiya as our axis of rotation.  As people moved from closer inside of the circle to the very outside of the circle, they noticed they must walk much faster, even run, to keep the same number of rotations as the people in the inner circle.  

-For instance, the image below shows 4 students on a ferris wheel, Maiya, Catherine, Princess, and Jasmin climbing the bars

Jasmin and Princess will experience the same number of rotations, however, Princess will have to go faster (have a greater tangential speed) to reach them.  This is because her radial distance is greater.

Rotational Inertia
Rotational inertia is a property of an object to RESIST change in spin.
Rotational inertia is similar to inertia, except rather than depending on mass, it depends on DISTRIBUTION OF MASS.

The two dumb-bells shown above have equal weights, however the let's weight is distributed father from the axis of rotation than the right's.  Therefore, it has a larger rotational inertia, and is harder to spin.  The right dumb bell has a smaller rotational inertia, therefore is easier to spin.

Here's a video we created on rotational inertia:

Conservation of Angular Momentum

when calculating angular momentum, we use:

• mv before = mv after

Angular momentum is similar, but now we will use rotational inertia (RI) and rotational velocity (RV).  (these are not the real letters we use, but we will use them now for sake of space)

• RIbefore * RVbefore = RIafter * RVafter

This image shows a spinning ice skater.  We know their momentum before must equal their momentum after (conservation).  As they take their arms in, they are decreasing their rotational inertia, which will increase their rotational velocity.

Torque

Torque is the force that causes rotation.  

Torque = force * lever arm

Lever arm is the distance from the axis of rotation.

This is why we use wrenches of different lengths.  The longer wrenches have a greater lever arm, therefore require less force to unscrew a bolt.

Torque also affects balance which will be explained in the section below...

Center of Mass/ Gravity

The center of gravity for any object is it's average position of all mass

The base of support is the plane on which the object is supported by the ground.

An object will fall if it's center of gravity does not align over it's base of support:

There are two ways you can make something more stable:

1. Create a bigger base of support
2. Lower the center of gravity

This is why wrestlers bend spread their legs (#1) and bend their knees (#2), so they can't be pushed over as easy.

Centripetal and Centrifugal Forces

A centripetal force is a center- seeking force that keeps something moving in a curve:

Why don't you continue forward when your car turns?  Because of Newton's 1st law, your body wants you to.  However, the car door runs into you (centripetal force) and you run into the door (centrifugal force).  When turning, it may feel like you are the one pushing on the door, but this is not a real force.  It is named centrifugal force, however it doesn't really do anything.