Car wreck
publish date: 2023/06/20 06:43:00 GMT+10
volume_mute
Consider a wreck in which a car runs into the back of a stationary truck. Before the collision, the car has momentum forward. We will suppose that the car and truck stick together after the crash.
The law of conservation of momentum tells us what the momentum of the wreckage will be.
- What will it be equal to? (1)
- What values would we need to know to calculate the velocity of the wreckage after impact? (2)
Correct Answer
Explanation
Friction and the earth
We have thus far neglected to mention the effect of friction. We spoke of the momentum of cars and trucks immediately before and after collisions, for example, but said nothing of the fact that after a wreck the wreckage would slide to a stop. What happens to the momentum of the wreckage? The answer is that the momentum is transferred to the earth. The earth spins a little faster (or slower, depending upon whether the motion was eastward or not). it may at first glance seem hard to believe that an object as small as a truck can affect the speed of the earth, but one must remember that we are not saying that the effect would be great enough to measure. Since the mass of the earth is so tremendous, its change in speed would be very, very small. Undetectable-but not non-existent.
Consider a recoiling gun when it hits the shooter's shoulder. The system is not isolated, because it includes the entire earth. Is the change in momentum of the shooter detectable? Perhaps (he may fall over backward). if the shooter is ready, however, he braces against the expected impulse, and the momentum is transferred to the earth.
Reference
Basic Physics: A Self-Teaching Guide