5.8 Summary & Keep the Curiosity Alive

Key Points

  • A force is a push or pull on an object, arising from its interaction with another object. At least two objects are always involved. The SI unit is the newton (N).
  • A force can start motion, change speed, change direction of motion, or change shape — or several of these at once.
  • Contact forces act only on touching: muscular force and friction. Friction always acts opposite to motion and comes from surface irregularities.
  • Non-contact forces act from a distance: magnetic, electrostatic and gravitational. Like poles/charges repel and unlike attract; gravity is always attractive.
  • Weight is the Earth’s gravitational pull on an object, measured in newtons with a spring balance. Mass is the amount of matter (kg/g).
  • Mass stays the same everywhere; weight can vary (≈ one-sixth on the Moon).
  • An object in a liquid feels an upward buoyant force (upthrust). It sinks if its weight exceeds the buoyant force, and floats if they are equal.

A Closing Challenge: Keep the Curiosity Alive

Think It Through

  1. Match the force to its example: muscular · magnetic · friction · gravitational · electrostatic — with: a child lifting a bag · a compass needle pointing north · a ball stopping on its own · a fruit falling · a rubbed balloon attracting hair.
  2. True or false? (i) A force is always needed to change an object’s speed. (ii) Friction makes a rolling ball speed up. (iii) There is no force between two charged objects placed a little apart.
  3. Two rubbed balloons are brought near each other. What happens, and why?
  4. The slipping puzzle. Why do we slip on ice or a polished floor — and how does this connect to friction being “a necessity, not just a problem”?
  5. Moon weight. An object’s weight on the Moon is one-sixth of its weight on Earth. What causes this? Does its mass also become one-sixth?

You’ve now met the invisible pushes and pulls behind all motion. Next, we follow one special force — pressure, the force spread over an area — out into the open air, where it stirs up winds, storms and cyclones.