Pages 298-299: Questions # 1,3,6,7,9,10 [answers in bold print]

1.    What is the difference between inertia and linear momentum?

Inertia is the resistance of a body to a change in its state of linear motion and is measured by its mass.  Momentum is the "quantity of motion" a body has, taking into account both its mass and its velocity (M=mv); the greater the velocity, the greater the momentum and the greater the mass of a moving body, the greater its momentum.  The momentum of a body at rest is zero, whereas its inertia is not zero; its inertia is equal to its mass.

3.    Define linear impulse.

Linear impulse is the product of the force applied times the time that force is applied to a body, and the magnitude of the impulse is equal to the change in momentum of the body receiving the force:
F(t )= mv2 - mv1

6.    Using the concepts of impulse and momentum, cite several exammples of how body movements are used to absorb shock when a body must stop a moving object or when the moving body meets a stationary object.

Sudden deceleration demands a fairly large force acting for a short time, whereas, if the time during which the deceleration occurred was longer, less force need be applied.  In catching a ball, less force need be applied to the ball by the hands if the ball is gradually decelerated to zero velocity by bending the arms to absorb the shock.  Similarly, landing from a jump requires bending of the legs to gradually bring the momentum of the body to zero so that less force is applied during the longer time of deceleration.  Any example in in which a moving body is contacted by a source of force to slowly decelerate the body, rather than abruptly, is appropriate:  entering water feet first rather than in a "belly flop," contacting the ground with the feet in jogging, changing body shape upon receiving a blow so that the momentum of the body's center of gravity is slowly accelerated rather than abruptly accelerated.

7.    Use the conservation of linear momentum principle to tell the direction of motion of these two players (the system) after they collide: Player A's mass is 80 kg and runs north at 5m/sec into player B.  Player B's mass is 90 kg and runs south at 2 m/sec into player A.

Player A has 80kg x 5m/s = 400N-sec of momentum north.  Player B has 90kg x 2m/sec = 180 N-sec momentum south.  Prior to impact the total momentum of the two-player system is +400 N-sec + (-180 N-sec) = 220 N-sec north.  After collision, the two players' mass (80kg +90 kg = 170kg) will move north, reflecting the conservation of linear momentum of a system free of external forces (the impact forces between the two players were internal to the two player system).  Therefore:
        Momentum before = Momentum after
        +220 N-sec          = +220 N-sec
                                     = 170kg x  X m/sec
+220 N-sec/170kg       =  ? m/sec
                                     = 1.29 m/sec

Both players move north at 1.29 m/sec

9.    Explain how gravitational or elastic potential energy is given to a body or object by something possessing kinetic energy.

The kinetic energy of a mass serves to deform a reformable (elastic) body.  The deformed body then has elastic potential energy which can be transformed into kinetic energy upon recoil.  Gravitational potential energy is given to a ball struck upward into the air by a volleyball player.

10.    If you double your running speed, how is the magnitude of your momentum changed?  How is the magnitude of your kinetic energy changed?

Doubling your running speed will double your momentum, whereas it will quadruple your kinetic energy because of the squared velocity factor in kinetic energy.