Question

Select the correct answer or answers for each and explain:

1. When you take a book from your desk and put it up on a shelf above you the work done on the book depends on: (1) how fast you moved the book, (ii) depends on whether you moved it straight up or along an arched path, (iii) depends on how high the shelf is, (iv) depends on the mass of the book. d. A 80-kg baseball player while running at a speed of 5 m/s slides on the ground and comes to a stop in 3 m. True statements are: (i) Since the player had a KE at the beginning and no KE at the end energy is not conserved, (ii) From this information it is possible to determine the magnitude of the frictional force that stopped the player, (iii) ) From this information it is NOT possible to determine the magnitude of the frictional force that stopped the player, (iv) The work done by the frictional force is 1000 J.

2. A 80-kg baseball player while running at a speed of 5 m/s slides on the ground and comes to a stop in 3 m.  True statements are: (i) Since the player had a KE at the beginning and no KE at the end energy is not conserved, (ii) From this information it is possible to determine the magnitude of the frictional force that stopped the player, (iii) ) From this information it is NOT possible to determine the magnitude of the frictional force that stopped the player,  (iv)  The work done by the frictional force is 1000 J.

Answer 1

1)

Work done by external force on a system is equal to the change in total energy of the system. If we only consider total mechanical energy(Kinetic energy K+ potential energyU) then

ΔK+ΔU=W_ext where W_ext is the work done by all forces other than gravity(in your question that is you).

1/2mΔv²+mgΔh=W_ext

where m = mass , Δv = change in velocity. Δh = change in height

Work Is Independent of Path in a Conservative Field. Gravitational field is a conservative field. So the work done by or against gravitational field is path independent.

So from above points we can conclude that statements (1),(3) and (4) are correct/True.

2)

Energy is always conserved So if you have a player sliding on a ground with friction, Initial kinetic energy it had equal the final kinetic energy of the player and heat energy produced from friction. i.e. change in kinetic energy = heat energy from friction

Yes it is possible to find magnitude of friction force, By work energy principle

Work done by friction force = Kinetic energy lost due to friction = 1/2 x mass x (change in velocity)²

= (1/2 x 80 x 5² ) = 1000 J

Force of friction x distance = 1000 J

Force of friction = 1000/ 3 = 333.33 N

So statements (2) and (4) are true/correct.