Work Power Momentum Review

Consider a tug of war in which two teams are pulling on a rope.  They are evenly matched and no motion takes place.

Is work done on the rope?
On the pullers?
On the ground?

 

ANSWER:

No work is done (no motion – same vertical distance)

 


Roads going up mountains are constructed with switchbacks with the road weaving back and forth along the face of the slope such that there is only a gentle rise on any portion of the roadway. Does this require any less work to be done by a car climbing the mountain compared to driving on a roadway that is straight up the slope? Why are switchbacks used?

 

Ignoring Fƒ:

the same amount of work is done.

Including Fƒ:

more work is done (longer period of contact with surface)

 


A sled with bricks has a total mass of 18.0 kg and is pulled at constant speed by a rope. The rope is inclined at 20.0° above the horizontal and the sled moves a distance of 20.0 m on a horizontal surface. The coefficient of kinetic friction between the sled and the surface is 0.500.

 

A. What is the tension of the rope?

79.4 N

B. How much work is done by the rope on the sled?

1.49 kJ

C. What is the mechanical energy lost due to friction?

-1.49 kJ

 


A 70 kg base runner begins his slide into second base when moving at a speed of 4.0 m/s. The coefficient of friction between his clothes and Earth is 0.70. He slides so that his speed is zero just as he reaches the base.

 

A. How much mechanical energy is lost due to friction acting on the runner?

-5.6 x 102 J

B. How far does he slide?

  m

 

Adapted from a GAVL Source