Physics of Work: Definition, Examples, and Friction

1. Physics

2. Work It is done on a body when a force acts through a distance. The definition of work involves the force acting on the body (F), the distance through which this force acts (S) and the angle (θ) between the force vector and the distance vector. In mathematical equation,

3. Work Sample work problems: 1. A puck lies on the horizontal air table. The air table reduces the friction between the puck and the table to almost zero since the puck rides on a film of air. A player exerts a force of 70 lbs. on this puck through a distance of 0.5 feet, and he is careful that his force is in the same direction as the distance through the force is applied. The player has done 35 ft-lbs of work on the puck.

4. Work 2. A sled is dragged over a horizontal snowy surface by means of a rope attached to the front of the sled. The rope makes an angle of 28˚ with the horizontal. The sled is displaced a distance of 50 ft. The worker exerts a force of 35 pounds. How much work does the worker do?

5. Work Solution:

6. Friction It is a work done when as the surfaces rub against each other. This work heats the surfaces and always results in wasted work. Previously, we talk about the weight (W), which is the downward force due to gravitational pull of the earth; and the normal force (N), the upward push of the surface to itself on the body. These two forces happens when a body is at rest.

7. Friction The W and the N are equal to each other in the following condition.

8. Friction Three kinds of friction: 1. Starting friction – the friction present at an instant when a body has been at rest; just before it begins to move under the application of a force. 2. Sliding friction – the friction present as a body is sliding over another surface. It is present when the surface of the body and the surface on which it slides are moving relative to each other.

9. Friction 3. Rolling friction – the friction between a rolling body and surface on which it rolls. All the three cases, the friction equation the same.

10. Friction is the coefficient of friction is given in the following table for different kinds of material. Material μstart μslide Steel on steel 0.15 0.09 Steel on ice 0.03 0.01 Leather on wood 0.5 0.4 Oak on oak 0.5 0.3 Rubber on dry concrete 1.0 0.7 Rubber on wet concrete 0.7 0.5

11. Friction Sample Problem: A steel is weighing 100 lbs is resting on a horizontal steel surface. How many pounds force are necessary to start the body sliding? What force is necessary to keep this body sliding at constant speed?

12. Friction Solution: Force to start sliding motion: Force to keep body sliding:

13. Exercises • How much work is done by a person in raising a 45 lbs bucket of water from the bottom of a well that is 75 ft deep? Assume constant speed. • A block is dragged over a rough, horizontal surface at a constant force of 78 lbs acting at an angle of 25˚above the horizontal. The block is displaced at a distance of 45 ft. How much work is done by the worker? • An aircraft with a weight of 85,000 lbs is towed over a dry concrete surface. What force must the towing vehicle exert to keep the airplane rolling?