Question

why work requires force

you know from experience that if work is to be done, a force must be exerted. this is because no movement will occur unless resistance is overcome. resistance is a force that opposes movement. you may want to lift a 10 - pound box. the box weighs 10 pounds because gravity is pulling it toward the earth with a force of 10 pounds. in this case gravity is a resistance. even an effort of 9 pounds is not great enough to lift the box because the resistance of gravity is greater than 9 pounds. it will require a force of at least 10 pounds to lift a 10 - pound object. gravity is often one of the resistances that must be overcome before work can be done. the force of gravity opposes an upward effort to lift an object.

probably the greatest resistance involved in work is friction. as two surfaces rub against each other, there is opposition to the movement. friction can be defined as the resistance of rubbing. if you attempt to push a heavy desk across the floor, you know that a slight push will not move it. a force sufficient to overcome friction must be applied before the desk will move. the smoothness of the two surfaces that rub together helps to determine the amount of friction. in general, the rougher the surfaces, the greater the resistance of friction.

inertia, another resistance, must be overcome to start moving something. inertia of rest is the property of matter to resist moving if it is at rest. before a parked truck will move, a great force must be applied to overcome the resistance of its inertia.

a fourth resistance that must be overcome in some types of work is molecular attraction. the molecules of a material resist being pulled apart. when you tear a piece of paper, you must use enough force to pull the molecules apart. sawing, cutting, and breaking are all work activities in which the resistance of molecular attraction must be overcome.

fill in the blank.

1. ____________________ is a force that opposes movement.
2. the force of ____________________ opposes an upward effort to lift an object.
3. probably the greatest resistance involved in work is ____________________.
4. in general, the ______________ the surfaces, the greater the resistance of friction.
5. the ____________________ of a material resist being pulled apart.
6. ____________________ can be defined as the resistance of rubbing.

Explanation:

1.

From the text: "Resistance is a force that opposes movement." So the answer is Resistance.

The text says: "Gravity is often one of the resistances that must be overcome before work can be done. The force of gravity opposes an upward effort to lift an object." So the answer is gravity.

The relevant sentence is: "Probably the greatest resistance involved in work is friction." So the answer is friction.

Answer:

Resistance

2.