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Questions and Answers
What condition must be met for a force to do work on an object?
What condition must be met for a force to do work on an object?
- The force must be greater than the object's weight.
- The force must be applied constantly.
- The force must cause a displacement of the object. (correct)
- The force must be applied at an angle to the horizontal.
What is the SI unit of work?
What is the SI unit of work?
- Newton
- Kilogram
- Joule (correct)
- Watt
A person applies a horizontal force of 50 N to push a box across a floor. If the work done by the person is 200 J, what distance did the box move?
A person applies a horizontal force of 50 N to push a box across a floor. If the work done by the person is 200 J, what distance did the box move?
- 25 m
- 2 m
- 4 m (correct)
- 10 m
A crane lifts a $1000 \text{ kg}$ car vertically upward at a constant speed of $2 \text{ m/s}$. What is the net work done on the car after it has been lifted $5 \text{ m}$?
A crane lifts a $1000 \text{ kg}$ car vertically upward at a constant speed of $2 \text{ m/s}$. What is the net work done on the car after it has been lifted $5 \text{ m}$?
What is the work-energy theorem?
What is the work-energy theorem?
An object of mass $2 \text{ kg}$ initially moving at $3 \text{ m/s}$ has $16 \text{ J}$ of work done on it. What is its final speed?
An object of mass $2 \text{ kg}$ initially moving at $3 \text{ m/s}$ has $16 \text{ J}$ of work done on it. What is its final speed?
Under what condition is work considered positive?
Under what condition is work considered positive?
A box is pushed with a force of $10 \text{ N}$ across a frictionless surface for a distance of $5 \text{ m}$. What is the work done?
A box is pushed with a force of $10 \text{ N}$ across a frictionless surface for a distance of $5 \text{ m}$. What is the work done?
Which scenario describes negative work being done?
Which scenario describes negative work being done?
What does power measure?
What does power measure?
If a machine does $500 \text{ J}$ of work in $10 \text{ seconds}$, what is its power output?
If a machine does $500 \text{ J}$ of work in $10 \text{ seconds}$, what is its power output?
How does reducing the time to complete the same amount of work affect the power required?
How does reducing the time to complete the same amount of work affect the power required?
A $60 \text{ kg}$ student runs up a $5.0 \text{ m}$ high staircase in $3.9 \text{ seconds}$. What is the student's power output?
A $60 \text{ kg}$ student runs up a $5.0 \text{ m}$ high staircase in $3.9 \text{ seconds}$. What is the student's power output?
What best describes the relationship between work and power?
What best describes the relationship between work and power?
How does using a pulley system change the amount of work required to lift an object?
How does using a pulley system change the amount of work required to lift an object?
A machine is used to lift a heavy object. Which of the following statements is always true?
A machine is used to lift a heavy object. Which of the following statements is always true?
When is it beneficial to use switchbacks to ascend a mountain?
When is it beneficial to use switchbacks to ascend a mountain?
A horizontal force of $30 \text{ N}$ is used to slide a $10 \text{ kg}$ box across a floor at a constant speed for a distance of $3 \text{ m}$. What is the work done by the frictional force?
A horizontal force of $30 \text{ N}$ is used to slide a $10 \text{ kg}$ box across a floor at a constant speed for a distance of $3 \text{ m}$. What is the work done by the frictional force?
A $2 \text{ kg}$ ball is dropped from a height of $20.4 \text{ m}$. Calculate the final speed when it hits the ground. (Assume no air resistance, $g = 9.8 \text{ m/s}^2$).
A $2 \text{ kg}$ ball is dropped from a height of $20.4 \text{ m}$. Calculate the final speed when it hits the ground. (Assume no air resistance, $g = 9.8 \text{ m/s}^2$).
Which of the following is an example of a situation where no work is done on an object?
Which of the following is an example of a situation where no work is done on an object?
A machine has a power output of $100 \text{ W}$. How long would it take for this machine to perform $500 \text{ J}$ of work?
A machine has a power output of $100 \text{ W}$. How long would it take for this machine to perform $500 \text{ J}$ of work?
A hiker climbs a hill. Which statement is most accurate?
A hiker climbs a hill. Which statement is most accurate?
A $1000 \text{ kg}$ car accelerates from rest to a speed of $20 \text{ m/s}$ over a distance of $200 \text{ m}$ on a level surface. What is the minimum average power that the engine must supply?
A $1000 \text{ kg}$ car accelerates from rest to a speed of $20 \text{ m/s}$ over a distance of $200 \text{ m}$ on a level surface. What is the minimum average power that the engine must supply?
A block is pulled across a rough surface at a constant speed by a force of $40 \text{ N}$. If the power of the force is $200 \text{ W}$, what is the speed of the block?
A block is pulled across a rough surface at a constant speed by a force of $40 \text{ N}$. If the power of the force is $200 \text{ W}$, what is the speed of the block?
Which of the following actions requires more power?
Which of the following actions requires more power?
A $2000 \text{ kg}$ elevator ascends $25 \text{ m}$ at a constant speed. How much work is done on the elevator by the lifting force?
A $2000 \text{ kg}$ elevator ascends $25 \text{ m}$ at a constant speed. How much work is done on the elevator by the lifting force?
Why does it require more power for a helicopter to both hover and gain altitude, versus only hovering?
Why does it require more power for a helicopter to both hover and gain altitude, versus only hovering?
Flashcards
What is 'Work'?
What is 'Work'?
Transfer of energy when a force causes an object to move.
Work Equation
Work Equation
Force multiplied by the distance over which it acts (W = Fd).
What are Joules?
What are Joules?
The standard unit of work, equivalent to one Newton-meter.
Work Kinetic Energy Theorem
Work Kinetic Energy Theorem
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What is Power?
What is Power?
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What are Watts?
What are Watts?
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Ramps and work
Ramps and work
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Kinetic Energy equation
Kinetic Energy equation
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Potential Energy equation
Potential Energy equation
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Study Notes
Work
- Work is a transfer of energy
- Work occurs when a force causes an object to move
- Work requires the application of force and movement of something
Calculating Work
- Work is calculated using the formula: W = Fd
- W represents work
- F represents force
- d represents distance
- Work is measured in Joules (J), equivalent to Newton-meters (N m)
Example
- To calculate the work done when Mrs. Kreamer applies a force of 20 N to push Wesley in his stroller for 0.4 km, convert 0.4 km to 400 m
- The work done is calculated as: W = (20 N) * (400 m) = 8000 J
- Another example of calculating work is: lawnmower pushed with constance force of 100N. if the mower moves a distance of 10 m work done is: W = (100N) * (10m) = 1000J
Work Kinetic Energy Theorem
- Net work equals the change in kinetic energy (KE) of the object and is expressed as: Wnet = ΔKE
- Initial Energy + Work = Final Energy
- Work is positive when adding energy
- Work is negative when losing energy
Work Kinetic Energy Formula Expanded
- Wnet = ΔE
- Wnet = (1/2)mvf^2 - (1/2)mvi^2
- vf represents final velocity
- vi represents initial velocity
- m represents mass
Applying the Work-Energy Theorem
- To find the height a 1 kg football needs to fall from rest to reach a speed of 20 m/s, apply the work-energy theorem
- W = ΔKE
- Fd = (1/2)mv^2 - (1/2)mv^2
- mad = (1/2)mv^2 - (1/2)mv^2
- (1)(9.8)d = (1/2)(1)20^2 - (1/2)(1)0
- Solve for d: 9.8d = 200, so d = 20.4 meters
Identifying Forces Doing Work
- A 10-N frictional force slows a moving block to a stop after a displacement of 5.0 m to the right
- Consider a 2-kg object sliding at constant speed across a friction-free surface for 5 m to the right
- Consider a 2-kg object pulled upward at a constant speed by a 20-N force for a vertical displacement of 5 m
- A 10-N force is applied to push a block across a frictional surface at a constant speed for 5.0 m to the right
Power
- Power refers to the work done over time and is measured in Watts
- One Watt (W) is equivalent to one Joule per second (1 J/1s)
- The formula for power is P = W/t
- P represents power
- W represents work
- t represents time
Calculating Power
- To determine the power of a tired squirrel (1 kg) doing push-ups by elevating its center-of-mass by 5 cm (0.05 m) in 2 seconds:
- Work done = force x distance = (1 kg * 9.8 m/s^2) * 0.05 m = 0.49 J
- Power = work / time = 0.49 J / 2 s = 0.245 Watts
- When a Marine lifts a 65.0-kg body a distance of 0.25 meters in 2 seconds, the power delivered is:
- P = W/t
- P = Fd/t
- P = [(65)(9.8)(.25)] / 2
- P = 79.6 watts
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