Physics Chapter 14: Work, Power, and Machines

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Questions and Answers

For a force to do work on an object, what must some of the force do?

Act in the same direction as the object moves

What is the equation for work?

Work = force X distance

What is the equation for power?

Power = work/time

One horsepower is equal to how many watts?

746 watts Signup and view all the answers

Machines make work easier by changing what?

The size of a force needed, the direction of a force, or the distance over which a force acts Signup and view all the answers

What is the distance the input force acts through?

Input distance Signup and view all the answers

You can increase the work input by increasing what?

The input distance, the input force, or both at once Signup and view all the answers

What is the distance the output force is exerted through?

Output distance Signup and view all the answers

What is the number of times that the machine increases an input force?

Mechanical advantage Signup and view all the answers

What is determined by measuring the actual forces acting on a machine?

Actual mechanical advantage Signup and view all the answers

What is the equation for actual mechanical advantage?

Actual mechanical advantage = output force/input force Signup and view all the answers

What describes the mechanical advantage in the absence of friction?

Ideal mechanical advantage Signup and view all the answers

What is the equation for ideal mechanical advantage?

Ideal mechanical advantage = input distance/output distance Signup and view all the answers

What is the equation for efficiency?

Efficiency = work output/work input X 100% Signup and view all the answers

Name the six simple machines.

Lever, wheel and axle, inclined plane, wedge, screw, and pulley Signup and view all the answers

What is a rigid bar that is free to move around a fixed point?

Lever Signup and view all the answers

What is the fixed point the bar rotates around?

Fulcrum Signup and view all the answers

What is the distance between the input force and the fulcrum?

Input arm Signup and view all the answers

What is the distance between the output force and the fulcrum?

Output arm Signup and view all the answers

Give an example of a first-class lever.

Seesaw, scissors, tongs, opening a paint can with a screwdriver Signup and view all the answers

A wheelbarrow is an example of what type of lever?

Second class lever Signup and view all the answers

The mechanical advantage of a first-class lever is always what?

Greater than, equal to, or less than 1 Signup and view all the answers

The mechanical advantage of a second-class lever is always what?

Greater than 1 Signup and view all the answers

The mechanical advantage of a third-class lever is always what?

Less than 1 Signup and view all the answers

Give an example of a third-class lever.

Baseball bat, hockey stick, golf club, broom Signup and view all the answers

A simple machine that consists of two disks or cylinders, each with a different radius.

Wheel and axle Signup and view all the answers

Give an example of a wheel and axle.

Steering wheel and screwdriver Signup and view all the answers

What is a slanted surface along which a force moves an object to a different elevation?

Inclined plane Signup and view all the answers

A wheelchair ramp is an example of what?

Inclined plane Signup and view all the answers

Give an example of a wedge.

Knife blade and zipper Signup and view all the answers

Does a thin wedge or a thick wedge of the same length have a greater ideal mechanical advantage?

True Signup and view all the answers

Screws with threads that are closer or farther apart have a greater ideal mechanical advantage?

False Signup and view all the answers

A flagpole is an example of what type of pulley?

Fixed pulley Signup and view all the answers

A skyscraper window washer stands on a platform suspended by what type of a pulley?

Movable pulley Signup and view all the answers

What type of pulley is a combination of fixed and movable pulleys?

Pulley system Signup and view all the answers

A large crane lifting railroad locomotives is an example of what?

Pulley system Signup and view all the answers

How much work in N X m is done when a 10 N force moves an object 2.5 m?

25 N X m Signup and view all the answers

If a machine has a mechanical advantage much larger than one, its output force is?

Much less than its input force Signup and view all the answers

How is the work output of a machine related to its work input?

Always less Signup and view all the answers

A machine with 5 N input force and a 25 N output force has a mechanical advantage of?

5 Signup and view all the answers

The mechanical advantage of a pulley system depends upon?

The number of sections of rope supporting the load being lifted Signup and view all the answers

If the output distance is 3 cm and the input distance is 15 cm, what is the ideal mechanical advantage?

5 Signup and view all the answers

If the output force is 50 N and the input force is 12.5 N, what is the actual mechanical advantage?

4 Signup and view all the answers

What determines the class of a lever?

The location of the input force, the output force, and the fulcrum Signup and view all the answers

A steel bolt, an iron nail, and a screwdriver are what type of simple machines?

Wedges Signup and view all the answers

If the actual mechanical advantage is 2.9 and the input force is 10 N, what is the output force?

29 Signup and view all the answers

How much work does a 50 N force do when lifting a box 2 meters?

100 joules Signup and view all the answers

How much work does a 1 kW motor do in one minute?

60,000 joules Signup and view all the answers

What is the best way to increase power?

Increase the amount of work done in a given amount of time or do the work in less time Signup and view all the answers

When a machine does work, a trade off occurs. What best exemplifies the trade off?

A pulley changes the direction of your force, but requires a greater input force Signup and view all the answers

A variation of an inclined plane is a?

Wedge Signup and view all the answers

Work input is greater than the work output when shooting a gun. Where does some of this go?

Friction, heat, and sound Signup and view all the answers

A mechanical device requires 420 joules of work to do 230 joules of work lifting a crate. What is the efficiency?

55% Signup and view all the answers

Inclined planes reduce effort force by?

Increasing the distance through which the force is applied Signup and view all the answers

How will a lubricant affect the efficiency of a simple machine such as a pulley?

It will decrease friction which increases the work output Signup and view all the answers

If two swimmers compete in a race, does the faster one automatically develop more power?

The faster swimmer might, but if the slower swimmer exerts a greater force, she might develop more power Signup and view all the answers

A force of 11 N is applied to the handle of a screwdriver being used to pry off the lid of a paint can. As the input force moves through a distance of 0.3 m, the screwdriver does 3 joules of work on the lid. What is the efficiency?

91% Signup and view all the answers

Study Notes

Work, Power, and Machines

Work occurs when a force acts in the same direction as the object moves.
Formula for work: Work = Force × Distance.
Power is defined as the rate at which work is done: Power = Work / Time.
One horsepower is equivalent to 746 watts.

Mechanical Advantage

Machines simplify work by altering the size, direction, or distance of a force.
Input distance refers to the distance over which the input force acts.
Increasing either the input distance or input force enhances work input.
Output distance is the distance over which the output force is exerted.
Mechanical advantage quantifies how much a machine amplifies an input force.
Actual mechanical advantage is determined by the forces acting on the machine:
- Actual Mechanical Advantage = Output Force / Input Force.
Ideal mechanical advantage is calculated without accounting for friction:
- Ideal Mechanical Advantage = Input Distance / Output Distance.
Efficiency can be calculated using the formula: Efficiency = (Work Output / Work Input) × 100%.

Simple Machines

The six types of simple machines include levers, wheels and axles, inclined planes, wedges, screws, and pulleys.
A lever consists of a rigid bar that pivots around a fulcrum (fixed point).
The input arm extends from the fulcrum to the input force, while the output arm extends from the fulcrum to the output force.

Lever Classes

First-class levers have the fulcrum located between the input and output forces (e.g., seesaws, scissors).
Second-class levers have the output force between the fulcrum and input force (e.g., wheelbarrows) and always have a mechanical advantage greater than 1.
Third-class levers have the input force between the fulcrum and output force (e.g., baseball bats) and always have a mechanical advantage less than 1.

Other Simple Machines

Wheel and axle systems consist of two disks or cylinders of different sizes (e.g., steering wheels, screwdrivers).
Inclined planes provide a slanted surface to facilitate lifting (e.g., wheelchair ramps).
Wedges are variations of inclined planes and include objects like knife blades.
Screws feature closely spaced threads, providing a greater ideal mechanical advantage.

Pulleys

Fixed pulleys change the direction of a force (e.g., flagpoles).
Movable pulleys can lift heavier loads more easily (e.g., window washers).
A pulley system combines fixed and movable pulleys for greater mechanical advantage.

Work and Efficiency Calculations

Work done can be measured in Newton-meters (N × m).
Relationships between input and output forces can be expressed as ratios indicating a machine's mechanical advantage.
Input distance and output distance play key roles in calculating ideal mechanical advantage.
Efficiency reflects the ratio of useful work done compared to total work input, often affected by factors such as friction and heat loss.

Practical Applications

Lubricants can reduce friction, thereby enhancing a machine's efficiency.
When two swimmers compete, more power can be generated by greater force, irrespective of their speed.
The efficiency of mechanical devices can be assessed using specific numerical examples, revealing insights into work-loss during operation.

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Test your knowledge on the concepts of work, power, and machines with these flashcards from Chapter 14 of your physics textbook. Each card presents key definitions and equations essential for understanding this fundamental topic in physics.