Solve: (a) As the package slides, work is done by the frictional force which acts at to the displacement. The normal force is Thus for (b) Work is done by the component of the gravitational force parallel to the displacement. and the work of gravity is: (c) since the normal force is perpendicular to the displacement. (d) The net work done on the package is: 7.9. Set Up: For part (a), use to calculate the work W done by the person in lifting himself against gravity. Divide by 0.20 to account for the fraction of energy used that appears as work. Solve: (a) The person must exert an upward force equal to his weight, so and The energy used is and the number of food calories is thus, (b) The other 80% of the food energy is converted to forms of energy other than mechanical energy, such as the ther-mal energy released by the action of the person’s muscles. 7.10. Set Up: Use to calculate the work done by the gravity force on each object. Solve: (a) and (b)
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