7.4 Work

1. 7.4   Work Work, W is energy transferred to or from an object by means of a force acting on the object. Energy transferred to the object is positive work, and energy transferred from the object is negative work. Work is a scalar. Unit for work: joule (J)

2. 7.5   Work and Kinetic Energy Finding an Expression for Work Work done by a constant force: Units for Work:

3. Work–Kinetic Energy Theorem Sample Problem 7-2

4. 7.6   Work Done by the Gravitational Force Consider a tomato of mass m, thrown upward: The minus sign tells us that during the object’s rise, the gravitational force acting on the object transfers energy in the amount mgd from the kinetic energy of the object. This is consistent with the slowing of the object as it rises. During the fall of the tomato: The plus sign tells us that the gravitational force now transfers energy in the amount mgd to the kinetic energy of the object. This is consistent with the speeding up of the object as it falls.

5. Work Done in Lifting and Lowering an Object Now suppose we lift a particle-like object by applying a vertical force to it. During the upward displacement, our applied force does positive work on the object while the gravitational force does negative work on it. If the object is stationary before and after the lift,

6. Sample Problem 7-5 • An initially stationary 15.0 kg crate of cheese wheels is pulled, via a cable, a distance, d = 5.70 m, up a frictionless ramp to a height h of 2.50 m, where it stops. • How much work is done on the crate by the gravitational force during the lift? • How much work is done on the crate by the force from the cable during the lift?