Car Velocity Lab

 

Car Velocity Lab

 Conclusions:

How do the predicted velocity and the measured velocity compare in each case?  Did your measurements agree with your initial prediction?  If not, why?  

We used the equation above to determine our predicted initial velocity. The predicted velocity and measured velocity were very similar in comparison. As always, there is uncertainty when using Video Analysis to measure data. The measurements we calculated did agree with our initial predictions within uncertainty. I believe the variation in our data came from the fact that the model we used to make our predictions was "ideal", meaning we did not account for the friction of the system (only the two masses and distance) or the effective mass of the pulley.

Does the launch velocity of the car depend on its mass?  The mass of the block?  The distance the block falls?  Is there a choice of distance and block mass for which the mass of the car does not make much difference to its launch velocity?

The launch velocity depends on: the hanging mass, the mass of the car, and the distance the hanging mass falls before reaching the ground. The mass of the hanging object affects the initial acceleration by the amount of tension it creates. The larger the mass of the hanging object, the more tension in the system, and thus a larger magnitude of initial velocity. The distance the hanging mass falls also affects the initial velocity. If the distance is smaller then the tension being applied only lasts a short amount of time; whereas if the distance (between the ground and the object) is large, then the force applied by the hanging mass will last a longer amount of time. Finally, the mass of the car is also a factor in initial velocity, because if the mass of the car is large, then it will take a considerable amount of force to move the car initially. With a large enough hanging mass at a high enough distance, the mass of the cart would be negligible.

If the same mass block falls through the same distance, but you change the mass of the cart, does the force that the string exerts on the cart change?  In other words, is the force of the string on object A always equal to the weight of object A?  Is it ever equal to the weight of object A?  Explain your reasoning. 

The tension force of the string is not always equal to the weight of the object. In a static system (when the acceleration is 0), the tension force will be equal to the weight. However, in a dynamic system the tension must also account for the objects acceleration and mass.

Was the frictional force the same whether or not the string exerted a force on it?  Does this agree with your initial prediction?  If not, why?

No, the frictional force varies based on the system. The frictional force that the car experiences while the hanging mass is still falling is less than the frictional force the car experiences when the hanging mass reaches the ground. This is because at the point where the hanging mass is still falling, the system has the force of hanging mass (creating the tension that acts on the cart) and kinetic energy of the cart to oppose friction. Once the mass is no longer acting on the system, the friction is only opposing the kinetic energy of the cart, and eventually slows it down. So, we can say that the magnitude of frictional force is changing throughout the duration of motion. This does not agree with our prediction of friction being a constant force.