Screws and jacks are simple machines that play a major role on construction sites. An auger uses a rotating screw-shaped blade to pull soil upward while the bit moves downward into the ground. A screw jack uses a threaded shaft to lift heavy loads a small distance with controlled force.
Both machines matter because they turn everyday rotation into strong, useful linear motion.
Key Facts
- Mechanical advantage = output force / input force
- Torque = force x radius, or tau = F r
- Work input = work output in an ideal machine, so F_in d_in = F_out d_out
- Thread pitch is the linear distance a screw advances in one full turn.
- For an ideal screw, mechanical advantage ≈ 2 pi r / pitch
- Smaller thread pitch gives greater lifting force but requires more turns.
Vocabulary
- Screw thread
- A screw thread is a spiral ridge that converts rotational motion into linear motion.
- Pitch
- Pitch is the distance a screw moves forward or upward during one complete rotation.
- Torque
- Torque is the turning effect of a force applied at a distance from an axis.
- Mechanical advantage
- Mechanical advantage is the factor by which a machine multiplies input force.
- Screw jack
- A screw jack is a lifting device that uses a rotating threaded screw to raise or support a heavy load.
Common Mistakes to Avoid
- Confusing pitch with thread height, which is wrong because pitch is the distance advanced per full turn, not how tall the thread ridge is.
- Ignoring the handle length when calculating torque, which is wrong because the same push force creates more torque when applied farther from the screw axis.
- Assuming a screw jack creates energy, which is wrong because it trades a long input distance for a shorter output distance with greater force.
- Forgetting friction in real machines, which is wrong because friction reduces the actual lifting force and makes the required input torque larger.
Practice Questions
- 1 A screw jack has a handle radius of 0.40 m. If a worker pushes with 120 N, what torque is applied to the screw?
- 2 A screw jack has a pitch of 5 mm. How far does the load rise after 18 full turns?
- 3 An auger and a screw jack both use screw threads, but one drills into soil while the other lifts a load. Explain how the direction of force and the purpose of the thread are different in each machine.