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Work is the way a force transfers energy to or from an object when the object moves through a distance. When a person pushes a crate across the floor, chemical energy in the muscles can become kinetic energy of the crate, thermal energy from friction, and sound energy. This idea matters because it connects forces, motion, and energy in one measurable quantity. Work helps explain everything from lifting a backpack to braking a bicycle.
Key Facts
- Work by a constant force: W = Fd cos theta
- Positive work increases an object's energy, while negative work decreases it.
- The SI unit of work and energy is the joule: 1 J = 1 N m.
- Kinetic energy: KE = 1/2 mv^2
- Work-energy theorem: W_net = Delta KE
- Power is the rate of doing work: P = W/t
Vocabulary
- Work
- Work is energy transferred when a force acts on an object as it moves through a distance.
- Energy
- Energy is the ability to cause change, such as motion, heating, deformation, or sound.
- Force
- A force is a push or pull that can change an object's motion or shape.
- Displacement
- Displacement is the change in position of an object, including both distance and direction.
- Power
- Power is the rate at which work is done or energy is transferred.
Common Mistakes to Avoid
- Using distance instead of displacement in W = Fd cos theta is wrong when direction matters, because work depends on the component of motion along the force.
- Forgetting the angle factor is wrong because only the component of force parallel to the displacement transfers energy as work.
- Assuming all applied work becomes kinetic energy is wrong because friction and other forces can transfer some energy into heat, sound, or deformation.
- Treating work and power as the same quantity is wrong because work measures total energy transfer, while power measures how quickly that transfer happens.
Practice Questions
- 1 A student pushes a crate with a horizontal force of 80 N for 5.0 m. How much work does the student do on the crate?
- 2 A 12 kg box speeds up from 1.0 m/s to 4.0 m/s. What is the net work done on the box?
- 3 A person carries a heavy bag at constant height while walking across a level room. Explain whether the upward force from the person does work on the bag during the horizontal walk.