Two ice skaters push off each other and glide apart. No table of data is needed to know their total momentum afterward. A law fixes it in advance.
The last lessons built the pieces: a system's total momentum, the word conserved, and the internal-external sort. The law bolts them together.
The law of conservation of momentum: the total momentum of a system stays constant when the net external force on the system is zero.
The condition is the whole job. Internal forces never change a system's total momentum, however violent they are.
The skaters show the law at work. Take the two skaters as the system. Their forces on each other are internal. The Earth's gravity on each skater is balanced by the upward force from the ice, and friction from the smooth ice is tiny. The net external force is roughly zero.
So the law applies: the system's total momentum stays constant. Whatever it was before the push-off, it is the same during and after.
When the condition fails, the law makes no promise. A large external friction acts on two carts crashing on a rough track, and their total momentum can change while it acts.
The law is the reason the matching tables in the last lesson matched. The runs that failed were runs where an external force acted.
The net external force on a system is zero while its two objects collide. What does the law of conservation of momentum say about the system's total momentum?
Two gliders interact on an air track where friction is negligible, so the net external force on the two-glider system is roughly zero. Before they touch, the system's total momentum is 9.0 kg·m/s rightward. What does the law say about the total momentum while they collide and after they separate?
The net external force on the two-glider system is roughly zero.
The law of conservation of momentum: the total momentum of a system stays constant when the net external force on the system is zero.
So the total momentum stays 9.0 kg·m/s rightward, during the collision and after it.
The total momentum stays 9.0 kg·m/s rightward throughout.
Two carts collide while a large external friction acts on them. Does the law of conservation of momentum promise that the system's total momentum stays constant here?
Jorge says: while the net external force on a system is zero, the system's total momentum stays constant, even while the objects inside collide violently with each other. Is he correct?
Which statement is the law of conservation of momentum?
Take rightward as positive. Friction is negligible on an air track, so the net external force on a two-glider system is roughly zero. Before the gliders collide, the system's total momentum is +8.5 kg·m/s. Enter the system's total momentum after the collision, in kg·m/s.
In which of these situations does the law of conservation of momentum promise a constant total momentum?