If the collision is perfectly elastic, what are the final velocities of the two pucks? Figure 9. The top diagram shows the pucks the instant before the collision, and the bottom diagram show the pucks the instant after the collision.
The net external force is zero. Conservation of momentum seems like a good strategy; define the system to be the two pucks. There is no friction, so we have a closed system. We have two unknowns the two final velocities , but only one equation.
The comment about the collision being perfectly elastic is the clue; it suggests that kinetic energy is also conserved in this collision. That gives us our second equation. The initial momentum and initial kinetic energy of the system resides entirely and only in the second puck the blue one ; the collision transfers some of this momentum and energy to the first puck. There are our two equations in two unknowns. The algebra is tedious but not terribly difficult; you definitely should work it through.
The solution is. Notice that after the collision, the blue puck is moving to the right; its direction of motion was reversed. The red puck is now moving to the left. Show Answer. At the beginning of the fight, Thor throws his hammer at Iron Man, hitting him and throwing him slightly up into the air and against a small tree, which breaks. From the video, Iron Man is standing still when the hammer hits him. The distance between Thor and Iron Man is approximately 10 m, and the hammer takes about 1 s to reach Iron Man after Thor releases it.
The tree is about 2 m behind Iron Man, which he hits in about 0. Thus, with the correct choice of a closed system, we expect momentum is conserved, but not kinetic energy. We use the given numbers to estimate the initial momentum, the initial kinetic energy, and the final kinetic energy. Because this is a one-dimensional problem, we can go directly to the scalar form of the equations. If so, this would represent an external force on our system, so it would not be closed.
At a stoplight, a large truck kg collides with a motionless small car kg. The truck comes to an instantaneous stop; the car slides straight ahead, coming to a stop after sliding 10 meters. How fast was the truck moving at the moment of impact?
Similarly, we know the final speed of the truck, but not the speed of the car immediately after impact. A useful strategy is to impose a restriction on the analysis. Suppose we define a system consisting of just the truck and the car. But if we could find the speed of the car the instant after impact—before friction had any measurable effect on the car—then we could consider the momentum of the system to be conserved, with that restriction.
Can we find the final speed of the car? Yes; we invoke the work-kinetic energy theorem. Friction is the force on the car that does the work to stop the sliding.
With a level road, the friction force is. This is an example of the type of analysis done by investigators of major car accidents. A great deal of legal and financial consequences depend on an accurate analysis and calculation of momentum and energy.
What is the mistake in this conclusion? Conservation of momentum is crucial to our understanding of atomic and subatomic particles because much of what we know about these particles comes from collision experiments. At the beginning of the twentieth century, there was considerable interest in, and debate about, the structure of the atom. It was known that atoms contain two types of electrically charged particles: negatively charged electrons and positively charged protons.
The existence of an electrically neutral particle was suspected, but would not be confirmed until The question was, how were these particles arranged in the atom? Were they distributed uniformly throughout the volume of the atom as J. They bombarded a thin sheet of gold foil with a beam of high-energy that is, high-speed alpha-particles the nucleus of a helium atom.
The alpha-particles collided with the gold atoms, and their subsequent velocities were detected and analyzed, using conservation of momentum and conservation of energy. If the charges of the gold atoms were distributed uniformly per Thomson , then the alpha-particles should collide with them and nearly all would be deflected through many angles, all small; the Nagaoka model would produce a similar result.
If the atoms were arranged as regular polygons Lewis , the alpha-particles would deflect at a relatively small number of angles. What actually happened is that nearly none of the alpha-particles were deflected. None of the existing atomic models could explain this. Eventually, Rutherford developed a model of the atom that was much closer to what we now have—again, using conservation of momentum and energy as his starting point. The Thomson model predicted that nearly all of the incident alpha-particles would be scattered and at small angles.
Rutherford and Geiger found that nearly none of the alpha particles were scattered, but those few that were deflected did so through very large angles. Community Bot 1. I'm not sure about the meaning of the second question. Hope this helps. Towne Springer C. Towne Springer 1, 1 1 gold badge 8 8 silver badges 11 11 bronze badges.
During recoil the potential energy is converted back into the kinetic energies of the particle. After collisions the particles do not return to their initial configurations - Permanent deformations being produced or due to loss of energy in other forms. Avyakta Purush Avyakta Purush 6 6 bronze badges.
Roger Vadim Roger Vadim Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. Featured on Meta. Now live: A fully responsive profile. Linked 1. Related 0. Hot Network Questions. Question feed. Physics Stack Exchange works best with JavaScript enabled. Certain collisions are referred to as elastic collisions. Elastic collisions are collisions in which both momentum and kinetic energy are conserved.
The total system kinetic energy before the collision equals the total system kinetic energy after the collision. If total kinetic energy is not conserved, then the collision is referred to as an inelastic collision. The animation below portrays the elastic collision between a kg truck and a kg car. The before- and after-collision velocities and momentum are shown in the data tables. In the collision between the truck and the car, total system momentum is conserved.
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