PhysicsLAB: Momentum and Energy

The relationship between conservation of energy and conservation of momentum is an extremely important one. During every collision, momentum is conserved. Remember that conservation of momentum is actually a restatement of Newton's Third Law.

Occasionally kinetic energy is also conserved during an elastic collision. When this type of collision is being references problems will explicitly state that a steel ball or a "superball" bounces elastically off a surface. The implication is that the ball will return to the same height from which it is released. That is, its original gravitational potential energy is transformed into kinetic energy which remains constant throughout the collision allowing the ball to return to its original height. Those collisions are rare and far between.

But even when collisions lose KE, and are considered to be inelastic, we often use conservation of energy methods to analyze the behaviors of the objects involved. We do this by considering the energy content of the system before the objects collide and then again after they collide - just NOT during the collision. Given below are some classic examples.

Refer to the following information for the next two questions.

[ballistic pendulum] A 10-gram bullet is fired from a rifle at a speed of 700 m/sec into a 1.50-kg wooden block suspended by a string that is two meters long.

	After the collision, through what vertical distance (h) does the block rise?

	How much KE is lost during the collision?

Refer to the following information for the next question.

[table projectile] A block having a mass of 800 grams is at rest near the edge of a frictionless table. A second block, of mass 600 grams, is sliding towards it at a speed of 4 m/sec.

	If the table is 1 meter above the floor, and the blocks stick together after the collision, how far from the end of the table will they strike the floor?

Refer to the following information for the next two questions.

[impact collision] A block having a mass of 600 grams is sliding towards the edge of a frictionless table at a speed of 4 m/sec. On the floor, at the base of the table is a 200 gram cart, initially at rest. The table is 1 meter above the floor.

	How far from the base of the table should the cart be placed?

	If the block sticks to the cart after the collision, how fast will the block-cart combination begin to move across the floor?

Refer to the following information for the next question.

[skid marks] A 2000-kg car moving with an unknown speed strikes a 1000-kg car that is initially at rest. After the collision, the two cars stick together and skid to a stop on asphalt, where the friction coefficient is 0.65. The police measure the skid marks to be 40 meters long.

	Determine the speed of the 2000-kg car just before the collision.