Mousetrap car, you don't go very far...

We were assigned to build a mousetrap car which could travel a distance of five meters. This project was very challenging, but at the end our car ended up going 5.21 seconds in five meters. 

We hot glued to mouse trap to a wooden platform, and attached axels on the back and front by drilling holes and securing them with eye bolts. We fastened a colored pencil to the trap as our lever arm, and attached the string from the pencil to the back axel using zip ties. When the string is would around the back axel, the lever arm pulls the string forwards, and the back wheels move with it. 

THE PHYSICS

àNewton’s First Law

Newton’s First Law states that objects in motion will stay in motion and objects at rest stay at rest until an unbalanced force acts upon it. According to this law, the car would supposedly stay in motion forever, however friction acts on the wheels causing it to stop eventually.

àNewton’s Second Law

Newton’s Second Law states that acceleration is directly proportional to force, and is indirectly proportional to mass. In terms of our car, if we had a larger force causing it to move, it will have a larger acceleration. This also means that the more mass the car has, the harder it will be to accelerate.

àNewton’s Third Law

Newton’s Third Law states that every action has an equal and opposite reaction. In this case, the action reaction pair is the car pushing the ground backwards, and the ground pushing the car forwards

àFriction

Friction is caused by weight and quality of the surface. The first time we rolled our car, we noticed that the wheels weren’t gripping to the ground like they should have been. In order to remedy this, we applied a few layers of electrical tape so that the wheels would have more traction and would move better across the floor. Although we wanted friction on our wheels, we didn’t want it on our axel. We applied vaseline to the axel to get the string to pull off more easily with less resistance. Both of these small changes related to friction helped the car move more efficiently.

àWheel size

Originally, we had two large wheels made of CD’s in the back, and two very small wheels in the front. Even though all of the wheels moved well and were stable, the difference in size of the two sets of wheels were an issue. Both sets of wheels had the same rotational velocity, or number of revolutions per second. However, the back wheels had a smaller tangential velocity than the front wheels, because they are larger. The smaller front wheels had a larger tangential velocity in order to cover a much larger distance to keep up with the back wheels. Since the front wheels were spinning so much, they weren’t really taking the car anywhere. We decided to replace the small wheels with larger ones, which solved the problem and helped the car travel another 2.5 meters.

àLever arm

The lever arm was attached to the mousetrap, and essentially is what made the car go. A torque is factor that causes an object to rotate.

Torque=lever arm x force

Essentially, we want a smaller lever arm, because this will cause the force to be greater, which will propel the car forward faster. Furthermore, the lever arm increases the time and distance that the force is acting on the axel. However, I you have very large wheels, you must have a longer lever arm since you will have a large rotational inertia

àConservation of energy

It is important to remember that energy can be neither created nor destroyed. It can however, be converted into different types of energy such as kinetic (movement) and potential (position) energy. When the car was at rest, it had a certain amount of potential energy. Then when the car started speeding up, some of that potential energy was gradually turned into kinetic energy. As the car slowed down, some of its kinetic energy changed back to potential energy. However, the amount of energy remained the same the whole time.

àWork calculation

Interestingly enough, the car was actually not doing work before, during, or after if moved. We know that work equals force times distance. But it is very important to remember that force and distance must be in the same direction in order for work to be done. The spring acting downwards on the car is not work, because the car moves forwards.

REFLECTION

As stated earlier, the only drastic design change was switching small wheels out for larger ones. Smaller things like adjusting lever arm attachments, and dealing with friction were other things we did to improve the car.

The biggest problem with our car was when in the beginning, it would not go in a line, but rather in a complete circle. This was because one of the back wheels wasn’t touching the ground completely when it rolled. To fix this, we applied a few layers of tape around the wheel to make it taller. We also added a little weight to the side. These fixes corrected the turn, and we finally got the car to move down the hallway.

If I were to do this project again, I would research more and develop a solid plan before beginning. We didn’t really know what we were doing at first, and more organized planning would have benefited us greatly in the end. 

Unit 5 Summary

WORK AND POWER Work=force X distance →Work is measured in joules, and the force and distance must be parallel in order for work to be done So walking up the stairs is doing work, but holding an object above your head (force upwards) while walking across the room is not doing work, since the force and distance are in different directions. Power=joules/seconds Measured in watts →time is a factor in power, unlike time If you push a 20N box for 10m in 5 seconds, how much work and power are you generating?

Check out our video! WORK AND KINETIC ENERGY Energy is the ability to do work, which requires both mass and speed Work = change in kinetic energy

Kinetic energy is energy that requires movement. Remember that... KE final- KE initial= change in KE Potential energy is energy that does NOT require movement, but rather height (position of object). Here are the formulas for the two…

A 100kg car travels 10 meters in 5 seconds. Find the kinetic energy

Unfortunately, the car falls off a cliff and falls 200 meters to the ground. Find the potential energy of the car.

CONSERVATION OF ENERGY Change in kinetic energy = change in potential energy Work = f x d Work in = work out (Force in)(Distance in)=(Force out)(Distance out) energy in= energy out Say it takes 500 joules of energy for a roller coaster to run. From start to finish, the roller coaster will have 500 joules, because energy cannot be created or destroyed. However, energy shifts from potential (height) to kinetic (movement) depending on the position of the cars.

As you can see in the diagram, when the roller coaster is higher up on the tracks, it has a higher potential energy because it has height. Similarly, it has more kinetic energy at the bottom because it is in motion, and has less height. Why do airbags in cars keep us safe? When in a car crash, you go from moving to not moving despite how you are stopped. So whether you hit a tree, a brick wall, or a person, your change in kinetic energy is going to be the same. Remember… Ke= 1/2mv^2 Change in KE= Ke final- Ke initial If the change in KE is the same (which it is) so is the work. The airbag will increase the distance that the force acts on you, so the force will be smaller. Less force= less injury

MACHINES The purpose of a machine is to increase the distance at which an object moves, while decreasing the force used to move it. It is important to remember that machines do not change the amount of work done, they just make it easier since you don’t have to use such a great force to accomplish your task. (insert diagram) Work= fd (insert ramp diagram) work out (small distance) work in (big distance) work in= work out no change in work Say you have to lift a 20N refrigerator 5 meters onto a moving truck.

The work out is always the smaller distance, and the work in is the larger distance.

Remember that machines do not change the amount of work done. They just redistribute the amount of force you exert the object over a certain distance. —>So even though energy is conserved, you can never get more work out of a machine than you put in. In fact, it is impossible for a machine to be 100% efficient. This is because some energy escapes through the form of sound and heat. Efficiency= workout/work in x 100

Unit 4 Summary

I. ROTATIONAL AND TANGENTIAL VELOCITY

Tangential velocity is the speed at which an object travels along a circular path

àdepends on the object’s distance from the axis

Rotational velocity is the number of revolutions that an object makes per unit of time (RPM’s)

Tangential velocity is directly proportional to rotational speed and radial distance

So say hypothetically, you put a raisin on the outside of a record, and one closer to the middle of the record...which has the greater a. rotational velocity, and b. tangential velocity

A. The raisin on the outside has to travel faster in order to cover a larger distance than the one on the inside. They will have the same number of revolutions per minute, therefore the same rotational velocity

B. We already established that the raisin on the outside must go faster because it is traveling a longer distance in order to keep up with the raisin on the inside. Therefore, the outside raisin has the greater tangential velocity.

How do train wheels work?

Train wheels are narrow on the outside, and wide on the inside. They will have the same rotational speed (RPM’s), but different tangential speeds. This is because the wider inside has to move faster too keep up with the narrow inside, so that they will have to same RPM’s. When the wide part is resting on one of the tracks, it has a faster speed that causes the train to curve towards the middle of the track, and self correct.

II. ROTATIONAL INERTIA AND ANGULAR MOMENTUM

Watch this video to better understand rotational inertia! I would definitely click that link if I were you…

To recap, a golf ball (solid) will have less rotational inertia than a ping-pong ball (hollow). This is because the mass of the golf ball is closer to its axis of rotation

Angular momentum= rotational inertia x rotational velocity

It is also important to remember that momentum can neither be created nor destroyed

total (angular) momentum before= total (angular) momentum after

III. TORQUES AND CENTER OF MASS/GRAVITY

A torque, in essence, is a factor that causes an object to rotate. In order for a torque to be present, you must have a force and a lever arm.

àLever arm- distance from the axis of rotation to force

    Torque= force x lever arm

The center of mass is the average position of all combined masses in an object. 

àa lever arm can occur if the center of gravity is NOT above the base of support

If you are trying to make something rotate, the best way to do so is to make the lever arm larger. Similarly, if you don’t want rotation, shorten the lever arm.

When something is balanced…

The torques are equal, but forces/lever arms are different

Clockwise torque = counter clockwise torque

F x lever arm= f x lever arm

IV. CENTRIPETAL AND CENTRIFUGAL FORCES

Centripetal force is a center seeking force that causes an object to follow a curved path.

àWhen you are riding in a car and you take a sharp curve, the centripetal force causes the car to curve inwards. Due to inertia, your body will stay in the same place while the car itself turns. Essentially, the side of the car moves into you while you sit still causing you to hit the car door. This phenomenon, that you are hitting the side of your car, is called centrifugal force. However, it is just a term used to describe this experience, it is not an actual force. Be weary…

If you have ever seen a racecar track, you will notice that the track itself has an elevated slant. This can be explained with centripetal force, check out the diagram below.

Here is another example using the flying pig from class

Meter Stick Challenge

Using just a meter stick and a 100g weight, we were given the challenge to determine the mass of said meter stick.

The first thing we did was locate the balancing point on the meter stick. We did this by resting the stick on the edge of a table and determining where it balanced on its own, which was about a third of the way up the stick.

To solve, we set up an equation using the formula

 Force x lever arm= Force x lever arm

(Lever arm= distance from axis of rotation)

Note: Convert to Kg.

Use 9.8 for weight

We multiplied the weight (9.8) and lever arm (30) of one side, and set it equal to the weight and lever arm (20) of the other side

Once we got 1.47, we divided by the weight (9.8) to get the weight of the meter stick.

After we found our hypothetical answer, we weighed the meter stick, which ended up being about 146 grams. Since the two numbers are within 10 percent of each other, we know that our process was accurate.

Torques and Basketball

Have you ever wondered why basketball players stand with their legs shoulder width apart with their knees bent? Understanding torques will help us answer this question… A torque is a factor that allows an object to rotate. Torques requires both force and a lever arm. A lever arms are the distance from the axis of rotation Torque= force x lever arm When basketball players are playing ball, it is important that they stand stationary so that they won’t fall over. If they stand with their legs shoulder width apart, they are creating a larger base, and in squatting down, a lower center of gravity. These two factors reduce the chances of a torque occurring because there is no lever arm due to the low center of gravity.

Rotational Inertia and Figure Skating

As you can see in the video, the figure skater spins slower while her arms and leg are extended out, and gains speed as she pulls her limbs in closer to her torso. What causes this change?

Rotational inertia allows an object to resist changes in spinning or circular motion. The factor that affects rotational inertia is not the AMOUNT of mass of the object, but rather the DISTRIBUTION of mass and its location on the object.

That being said, when mass is closer to the axis of rotation the object will have a smaller rotational inertia (easier to move). And when mass is further from the axis of rotation, the object will have a larger inertia (harder to move).

In terms of the ice skater…

While her arms are extended out, her mass is further from the axis of rotation, thus yielding a larger inertia. With the larger rotational inertia, she will move slower. As she brings her arms and leg towards her body, she is bringing her mass closer to the axis of rotation, resulting in a smaller rotational inertia. Hence, she spins faster.