Material:- a catapult, rubber bands, tape, a box with four different surfaces , measuring meter, notebook, pen
Method:-
- The catapult was placed on the cement floor.
- Two rubber bands were attached to either legs of the catapult. Afterward the launcher of the catapult was attached to the rubber bands.
- Then the launcher was pulled to a distance of 15 cm from the place where the rubber bands were attached to the legs of the catapult. The spot was marked with a tape.
- The box was placed on the floor against the launcher of the catapult whilst its glass side exposed downward to the cement.
- The launcher was steadily released so that the box could be pushed to slide on the cement floor.
- Then the distance traveled by the box was marked by a tape. The closest back tip of the box was used as the mark of the distance travelled by the box.
- A measuring meter was used to record the distance traveled.
- Step 5 up to step 7 were repeated five times.
- Step 4 up to step 7 were repeated for the remaining surfaces of the box, namely wooden, fabric and sandy.
Method for controlling variables:-
- Two equivalent rubber bands were chosen and were attached to the legs and launcher of the catapult. The rubber bands were not changed during the whole test so that the elasticity of the rubber bands remained constant.
- The launcher of the catapult was always pulled 15 cm. The distance was marked using the tape.
- The mass of the pushed object should be constant throughout the test. Therefore the same box was used.
- The same catapult was used for the whole duration of the test.
- The test was performed on a cement floor only. It was also performed on the same area of the cement floor so as to have the same type of friction that is present due to dirt, humidity and other particles on the floor. Therefore a tape was used to mark the area where the catapult was placed on the cement.
- The whole test should be done in one room only so as to keep the humidity factor constant.
Results:-
The table below shows five measurements of the range of the catapult using four different surfaces of a box. The uncertainty in the measuring tape is taken to be ± 1 to compensate for human and systematic error. A mean distance is also presented for each surface.
Table 1. The range of a catapult in centimeter when exposing different surfaces of a 236g box on a cement floor.
Conclusion
As it can be seen from the data presented in Table 1 as well as Graph 1 the box with its glass surface exposed to the cement floor was able to travel farthest and had the longest range. Secondly, using its wooden surface the box was able to travel the second longest distance. Using sandy and fabric surfaces resulted in the box traveling approximately the same distance. Therefore the two surfaces had relatively the same kind of effect on the catapult in terms of its range. Thus it is possible to arrive at the conclusion that the range of the catapult was clearly affected by the change in the surface type of the box.
The effect of different surfaces on the range of a catapult can be explained in terms of friction. When a certain body travels in a certain direction a frictional force acts against it in the opposing direction (a kinetic frictional force) and at rest there is also static frictional force acting on it. By overcoming these forces a body can move. In this case as the box moved due to the force gained from the catapult, the different type of surfaces had their own coefficient of friction with the cement floor. Coefficient of friction expresses the friction between two surfaces and is dependent on the material used. In this case it is also dependent on the pressure between the two surfaces. Since the glass surface was able to rub relatively easily with the cement floor it had a lower coefficient of friction with the cement floor compared to the other surfaces and thus is able to travel the farthest distance. On the other hand using the same amount of force the box was able to travel the shortest distance using its sandy and fabric surfaces. This was because the surfaces were not able to rub relatively well with the cement floor. Thus they had a relatively higher coefficient of friction.
Evaluation and Limitation
The test was suitable to perform in a school environment. It can also be done without the need of assistance from others. However there are certain problems that can be improved. Sometime as the launcher of the catapult is released, the catapult is slightly thrown backwards which might take away some force from the push on the box. The cement floor had certain dirt particles on it that might have affect the range depending on the surface used. Pulling and releasing the wooden launcher of the catapult using a hand can cause friction between the hand and the catapult resulting in a reduced speed or inaccurate range.
Improvements
The catapult can be fixed to the floor using a screw to prevent it from moving. The cement floor can be wiped and dried to remove dirt and other particles. An automatic release system with hooks on the launcher of the catapult can be used for efficiency.
