Chapter Work in Mechanical Systems Principles of Technology I Edinburg North High School

Notes Work in Mechanical Systems Objectives: Define work done by force or torque in a mechanical system. Explain the relationship between work, force applied, and the distance an object moves. Solve problems, given force and distance information in English and SI units.

Work in Mechanical Systems Work is done when a force moves an object through a distance (Fig. 2.1). Chain hoist Gears We will see that with rotating systems it will be easier to calculate W with 

Work Done by a Force Work – a quantity equal to the product of the applied force and the distance though which the object moves. What are the units for work? The distance and direction through which an object moves is called displacement.

Work is the product of two vector quantities: In this chapter we will only consider work in one dimension. If F and d are in the same direction – positive work If F and d are in opposite directions – negative work

Example 2.1 Work Done by A Weight Lifter (p.85) Calculate the amount of work done by a weight lifter in lifting a 200 lb barbell a distance of 5 ft.

A closer look at lifting a barbell (Fig. 2.2) Initial force to get barbell moving, a bit stronger As barbell moves at a constant speed At highest point, and held in place, no work is being done even though You feel tired because work is still being done by your muscles.

Example 2.2. Work Done to Stop a Trailer A car and trailer are moving to the right. The driver applies the brakes and stops the car in 5 m. A constant braking force of 900 N is applied over this distance. What is the direction of the braking force? How much work is done on the car by this force?

Work is done only while the force is being applied.

Work Can Change an Object’s Potential Energy or Kinetic Energy In general, the work done on an object equals to the objects change in energy state. In Example 2.1 In Example 2.2

Sample Problem A A 2 kg object is lifted onto a table 1.5 m high. Calculate the work necessary to move this object.

Sample Problem B A moving object has a KE equal to 300 J. A force is applied on the object in the same direction an increases it KE to 400 J. How much work was done on the object?

Problem Set Pp , #s 1-3, 6, and 9