Question

Bob lifts a 0.9kg cup of coffee, initially at rest on the table, up to his mouth. The cup comes to rest after a vertical displacement of 0.7m. (NO IMAGE GIVEN)

a. In the process of being lifted, how much work was done on the cup by Bob?

b. Bob then hold the cup stationary for 1.8s. How much work was done on the cup by Bob during this time?

c. Realizing that the cup is empty, Bob lowers the cup 1.1m downward into the sink, where it comes to rest. How much work was done on the cup by Bob in the process of being lowered?

Answer 1

The work is done by a force (F) in displacing an object by S is given by

                                                 

Where is the angle between the displacement vector and the direction of the applied force.

The only force acting on the cup is the weight of the cup.

The weight of the cup is

                                                 

So, the total workdone by the gravity on the cup is

                                                 

a.

So, the workdone by Bob is 6.3 J.

b. In this case, the cup was in stationary position. So, the displacement of the cup is zero.

hence, the workdone on the cup is 0 J.

c. In this case the workdone by the gravity on the cup is

                                        

So, the workdone by Bob is -9.9 J.