HW#17: pg. 100  Rev 19 - 23; Ex 22, 23, 29

Solutions to Review Questions

19.  The system is defined as the bullet and the gun.  The bullet and the gun move in different directions, and therefore their velocities (and also their momenta) will have different signs.  When the two are added together, the net result is zero.

20.  No.  If momentum were not a vector quantity, the momentum of the bullet and the gun would both be positive, and would not cancel.

21.  An elastic collision involves a collision without lasting deformation or the generation of heat.  A collision involving bouncing objects represents an elastic collision.  An inelastic collision is characterized by deformation and/or the generation of heat.  In a perfectly inelastic collision, both objects stick together. Momentum is conserved in both elastic and inelastic collisions.

22.  The speed of car B would be identical to the speed of car A before the collision.

23.  The speed of both cars would be half of car A’s original speed.

Solutions to Exercise Questions

22.  A swarm of flying insects can have a net momentum of zero if its net velocity is zero (assuming all insects have the same mass).  For every insect flying in one direction, there can be one or more insects flying in the opposite direction.  Since velocity is a vector, some insects will have a negative velocity.  If there are equal numbers of negative velocities and positive velocities, the net momentum will be zero.

23.  A fully dressed person sitting on frictionless ice can reach shore by taking an article of clothing (such as a shoe) and throwing it in the opposite direction from shore.

29.  Linear momentum is only conserved when no external force is acting upon the system.  Since the ball is acted upon by gravity, momentum is not conserved.  If the system is defined as the Earth and ball, then momentum is conserved.  The Earth experiences a change in momentum as it accelerates toward the ball.  Because of the large mass of the Earth, its velocity will change by a negligible amount to match the change in momentum of the ball.