Study Guide – Force and Work
Name:_______________________________________
WORK:
Define work:
Work is done when a force is applied to an object and the object moves in the direction of the force.
Work can also be defined as a transfer of energy.
In order for work to be done, force and displacement must be in the same direction.
Give one example in which work is done:
A person lifting a box up from the ground to a table.
Give one example in which work is not done:
Carrying an object at a constant height without moving it horizontally (e.g., walking with a briefcase).
For the following 3 scenarios, explain why work is or is not done:
-
A woman preparing for a trip lifts her suitcase from the floor to the bed so that she may pack more easily.
Work is done because she applies a force to lift the suitcase, and the suitcase moves upward. -
A man spends 5 minutes thinking about the most efficient way to decorate his roof with Christmas lights.
No work is done because there is no physical movement or displacement occurring during this time. -
A student carries his book bag down the hallway.
Work is done if the bag is being carried forward; however, if he holds it in place while standing still, no work is done.
Identify which of the pairs in each scenario illustrates more work being done:
- _____ A boy helps a teacher by lifting a 200-N box of books 1.5 meters from the floor to the desktop.
- _____ The same boy lifts a 500-N box of books the same height.
Explain your choice:
The second scenario (lifting the 500-N box) illustrates more work being done because the work done is calculated as Work = Force × Displacement. Therefore, 500 N × 1.5 m = 750 J, compared to 200 N × 1.5 m = 300 J.
- _____ A girl throws a 1-kg softball with a force of 50 Newtons a distance of 25 meters.
- _____ The same girl throws the softball with the same force a distance of 17 meters.
Explain your choice:
The first scenario illustrates more work being done because the work done is greater with a longer distance (Work = Force × Distance). In the first case, Work = 50 N × 25 m = 1250 J, compared to Work = 50 N × 17 m = 850 J.
The formula for calculating work is Work = Force × Displacement, and the unit for work is the Joule (J).
POWER:
Define power:
Power is the rate at which work is done or energy is transferred.
How are power and work related?
Power is calculated as Power = Work / Time, which means more power is needed to do the same amount of work in less time.
Identify which of the pairs in each scenario illustrates less power being generated:
- _____ A woman pushes a cart with 95 N of force 3 m in 10 seconds.
- _____ A man pushes a cart with 95 N of force 3 m in 13 seconds.
Explain your choice:
The second scenario illustrates less power because power is inversely related to the time taken: more time (13 seconds) results in less power output (Power = Work / Time).
- _____ A 340-N student climbs the stairs in 14 seconds.
- _____ A 420-N student climbs the stairs in 14 seconds.
Explain your choice:
The second scenario illustrates more power being generated because power is based on the force (weight) exerted. The 420-N student does more work against gravity compared to the 340-N student in the same time period.
The relationship between power, force and velocity is Power = Force × Velocity.
MACHINES:
A machine is a device that makes work easier by changing the size, direction, or distance of the force applied.
Three simple machines include:
- Lever
- Pulley
- Inclined Plane
The work input for a machine is the work done on the machine (input force × input distance), while the output is the work done by the machine (output force × output distance).
If work input is not equal to work output for a machine, how does the Law of Conservation of Energy explain this “lost” energy?
The "lost" energy is usually converted to other forms, such as friction, heat, or sound, which do not contribute to useful work output.
Why is no machine 100 % efficient, or greater than 100 % efficient?
No machine can be 100% efficient due to energy losses primarily caused by friction and thermal energy loss.
The efficiency of a machine can be calculated by Efficiency = (Output Work / Input Work) × 100%.
Calculate the efficiency of the following machines:
-
1200 N of force are used to push a lever down 1.3 m. The lever raises a 1450-N boulder 0.4 m.
- Input Work = 1200 N × 1.3 m = 1560 J
- Output Work = 1450 N × 0.4 m = 580 J
- Efficiency = (580 J / 1560 J) × 100% = 37.18%
-
A machine that places the caps on plastic soda bottles requires 25 kJ of input work. The output work of the machine is 23 kJ.
- Efficiency = (23 kJ / 25 kJ) × 100% = 92%
Define mechanical advantage:
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device, or machine system.
Essay Questions:
1. In 1-2 sentences, explain how to calculate the efficiency of a toaster.
To calculate the efficiency of a toaster, measure the output energy (useful heat produced) and the input energy (electricity consumed), then use the formula: Efficiency = (Output Energy / Input Energy) × 100%.
2. A student uses a pulley to lift a log. In 1-2 sentences, explain how she can calculate the pulley’s efficiency.
The student can calculate the efficiency of the pulley by measuring the work input (force applied to lift the log times the distance pulled) and the work output (weight of the log times the height it is lifted), and then applying the formula: Efficiency = (Output Work / Input Work) × 100%.
3. In 1-2 sentences, describe an efficient simple machine.
An efficient simple machine, like an inclined plane, allows a person to lift heavy objects with less effort by spreading the required work over a longer distance, which decreases the force needed at any point while maintaining energy conservation principles.