Linear momentum is a new vector quantity, defined simply as the product of an object's mass and velocity:
Explore this directly below.
Adjust mass, speed, and direction. The momentum vector always points exactly where the object is heading — only its length depends on mass and speed.
Momentum is the natural tool for analyzing collisions — a model for any interaction where the forces exerted between the objects involved are much larger than any net external force acting on them during the interaction itself.
Two carts colliding on a track — exactly like this unit's header art — is the classic example: whatever normal forces or friction the track exerts externally are negligible compared to the contact force between the carts themselves during the collision.
An explosion is the mirror image of a collision: a model for an interaction in which forces internal to a system push objects within that system apart, rather than bringing them together.
Explore all three scenarios from this unit's header art directly below — elastic rebound, perfectly inelastic collision, and explosion — and see how each cart's momentum changes from before to after.
The three scenarios from this unit's header art. Pick one and watch how momentum for each cart changes from before to after.
Cart 1 approaches, bounces backward; cart 2 moves off in the direction cart 1 came from. Internal forces acted only during the brief contact.
Two carts sit motionless, connected by a compressed spring. When released, the spring pushes them apart, one moving left and one moving right. Is this a collision or an explosion, and why?