Mouse-Trap Vehicle Practical

To build a mousetrap vehicle, from a kit of parts, then perform in a race against other team vehicles.

Objective:

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1. To design and build a high-performance (high speed) vehicle based upon fundamental principles of mechanics.

2. To Determine the most successful design from the results of a race against other vehicle designs

3. Calculate the performance of a theoretical vehicle and describe differences that exist between theory and practise.

Background Knowledge:

Building a mouse-trap for speed…

A vehicle built for speed releases its energy quickly or at a high power output. This means the acceleration is proportional to its energy release, which is high/large. Below are some of the ways in which greater acceleration can be obtained.

* Using a short lever arm

* Having a large wheel to small axle ratio

* Light weight body

* Low rotational inertia (wheels)

* Good traction

Using a short or long lever arm does not affect the amount of energy released from the trap, but does affect the rate at which the energy is released. Therefore having a small lever arm will produce greater acceleration; however having to short an arm will produce wheel spin. Long arms will therefore decrease the pulling power (acceleration), but will increase the endurance.

Inertia = The resistance an object has to a change, in its state of motion.

As the mousetrap will start from a stationary position it is vital that the wheels have a low inertia to enable good acceleration.

Any types of friction on the car will decrease acceleration and overall speed. To reduce friction the axles must be lubricated where they rub against the frame, also the wheels must be firmly attached to the axle to prevent slip. The only friction that is needed is the friction between the wheels and the ground, which is needed to accelerate quickly.

Theoretical Calculations:

When the angle is zero the torque is not zero as there is a constant of 0.20542, which is worked out by plotting a trend line about the theoretical points. However at this point k can be any number, as any number multiplied by zero is still always zero.

Angle of spring lever (Radians)

Measured torque (Nm)

Spring constant k (Nm/Rad)

0

0.20542

Infinity

0.349

0.24

0.687

0.698

0.38

0.544

1.047

0.47

0.449

1.396

0.52

0.372

1.745

0.6

0.344

2.094

0.69

0.33

2.443

0.73

0.299

2.793

0.82

0.294

The constant k should be calculated using the trend line (change (y)/change (x)), which is shown below in the form y=mx+c

The trend line shown on the graph above using the points from the theoretical values gives the equation:

Y=0.22337?+0.20542

Based on the equation T=k?+c the k and m (y=’m’x+c) values must be the same, Torque is on the y axis and theta on the x axis, therefore…

k=0.22337

To work out the potential energy the ? value where torque is zero is needed. This can be worked out easily using T=k?+c where T=0, k=0.22337 and c=0.20542.

0=0.22337?+0.20542 0.22337?=-0.20542 ?=-0.20542/0.22337

?=-0.91964radians

Now using the PE equation below you can work out potential energy with the theta values ranging from 0.91964 radians and 2.094+0.91964 radians (2.094 is the value at 120 degrees, at which the mousetrap has fully sprung) and with k=0.22337.

PE = 0.22337/2(3.014042-0.919642) = 0.920139141Nm

Assuming only 85% on the potential energy is converted to kinetic energy and the vehicle mass is 150g, you can work out the velocity using…

KE =

To work out the kinetic energy you need to multiply the potential energy by 0.85 giving…

0.920139141×0.85 = 0.78211827joules

This value can now be said to equal , therefore:

0.78211827 = 1/2 x 0.15 x v2 0.78211827/0.075 = v2 v = 3.229m/s

Equipment:

* Mouse-trap

* Three pieces of lightweight wood (30cm/1cm/1cm)

* Strong fishing line

* Balloons

* Four cd’s

* Strong glue

* Two axles

* Lubricant

* Four plastic wheel mounts

* Two plastic axle mounts

* Lever arm

Method:

1. Create the sub frame using the three pieces of lightweight wood and the strong glue. The frame is large enough to contain all the mousetrap and plenty extra, which can be cut once tests have taken place.

2. Modify the mousetrap and attach the lever arm.

3. After attach the mousetrap to the sub frame (near the centre)

4. Next construct the front wheel assembly holder using a thin metal axle and the given axle mount. Fix this temporally to the front of the car.

5. Now mount and assemble and temporally mount the rear axle to the back of the car. A notch should be cut into the rear alxe so the line can be attached later.

6. Cut two of the cd’s down to half there diameter, to use as the front wheels and attach them to the wheel mounts. Also attach the full size cd’s to the other two wheel mounts. Balloons should be cut and pulled around the cd’s to greatly improve the traction.

7. Put a number of washers on the axle at either end. There should be enough of the rod left at either side to accommodate the wheels but not too much rod or the wheels will rub against the mousetrap. Remember the further the wheels from the car the greater inertia they create.

8. Attach the wheel mounts to the axles creating a car structure, adding plenty of lubricant to the joining between the axle and the sub frame.

9. Now by trial and improvement move the rear axle up or down car either improving the cars acceleration or endurance.

10. The fishing line should be attached to the lever arm and to the rear axle. Tie a small loop in the line so that the tip of the loop ends at the rear axle. Cut the loose ends off the line.

11. The lever arm must meet the rear axle perfectly and should be cut to fit once the rear axle is in its required position.

12. The front axle can now be fixed when in the desired position, making sure the car steers straight.

Results:

Below are all the race times for our class, Group 5 was my group and the fastest time was 2.93 seconds over a 5m track.

The most successful design is obviously group1 with a fastest speed of 2.86 seconds. I believe the reason they were able to beat our car was the fact that their wheel to axle ratio was smaller than ours. This provided them with optimum speed, but still had the ability to complete the track. They also developed their own wheels providing them with extreme amounts of traction. However I feel our car was of a good standard and could of easily been improved.

Improvements:

If I were to build a car again I would spend more time working out the mechanics involved to insure the car is working to its full potential. Physically the wheels where the worst aspect of my car, they lacked traction and the balloons where constantly breaking and slipping. Originally to gain more power I twisted the spring round one more revolution, but did not calculate that we would need a considerable amount more traction on the wheels. This was soon observed after our first race where the majority of our power was in the form of wheel spin.