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Chapter 2.1.1 Work in Mechanical Systems

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Chapter 2.1.1Work in Mechanical Systems

Principles of Technology I

Edinburg North High School

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 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 Wwith

- 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

- In this chapter we will only consider work in one dimension.

- 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.

- 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.

- In general, the work done on an object equals to the objects change in energy state.
- In Example 2.1
- In Example 2.2

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

- 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?

- Pp. 93-94, #s 1-3, 6, and 9