What a learner can do afterwards
- Calculates work done when a force drags, lifts or pushes something a stated distance
- Converts between joules, watts and seconds to find any missing quantity
- Explains why an upward supporting force does no work on a bag moving horizontally at constant height
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Work is energy transferred by a force. For a constant force along the motion, work W = F × d. Use force F in newtons and distance d in metres to get work in joules. Identify the force and the object it acts on; work is not an energy store.
A bag carried horizontally at constant height moves perpendicular to its upward supporting force. That force does zero work on the bag. Lifting the bag does positive work against gravity and increases the bag-Earth gravitational store. The person can use energy even when the upward force does no work on the bag.
Average mechanical power is P = W ÷ t. Weight F = m × g. For a steady vertical lift, useful work = weight × height = mgh. Choose g = 10 N/kg. A 30 kg crate weighs 30 × 10 = 300 N. Lifting it 2 m in 4 s, ignoring other losses, does 300 × 2 = 600 J of useful work and gives 600 ÷ 4 = 150 W average useful output. Electrical input power may be higher.
For a job with several stages, add the work done in all stages and add their times. Average power for the whole job is total work divided by total time. Do not average stage powers unless the stages take equal times.
A push on a wall does zero work on that wall if the wall has no displacement. In W = F × d, its distance is zero. This does not mean the person's muscles use no energy. The claim concerns the push acting on the wall.
Rearrange P = W ÷ t to get W = P × t or t = W ÷ P. Watts are joules per second, so multiplying work by time does not give power. Check both the operation and the units. Use division by time when calculating average power.
Force along the distance makes joules, joules per second make watts.
Adapted from OpenStax College Physics, sections 7.1 and 7.7, openstax.org.
Rewritten readings were written for this dataset. Linked videos are from Khan Academy.
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