Topic 2 – Motion and forces: 2.17 Describe how weight is measured

The idea

Weight is the force of gravity acting on a body, so it is a force and is measured in newtons (N). A learner should describe the practical method: suspend or support the object from a newtonmeter (a spring balance) so that the object hangs at rest, then read the scale in newtons. Because the reading is a force, the instrument must be calibrated in newtons, not grams or kilograms; a balance or scales reading in kilograms gives mass, not weight. For a very heavy body, use a suitable range newtonmeter or a force sensor connected to a data logger. The reading equals the weight only when the object is stationary and supported solely by the meter, so the force is balanced.

What earns the marks

• States that weight is a force and is measured in newtons using a newtonmeter or spring balance.
• Describes suspending the object from the newtonmeter and reading the scale once the object hangs at rest.
• Distinguishes weight measured in newtons from mass measured in kilograms with a balance.
• Explains that a force sensor or data logger can replace a newtonmeter for larger or more precise readings.

Common mistakes and how to put them right

• Confuses weight with mass and claims a kilogram balance measures weight.

  Put it right by stating that weight is a force in newtons read from a newtonmeter.

• Reads the scale while the object is still swinging.

  Put it right by waiting until the object hangs at rest so the reading is steady.

• Uses a newtonmeter beyond its maximum range.

  Put it right by choosing a meter whose range comfortably exceeds the expected weight.

Exam tips

• Name the instrument and its unit together: newtonmeter, newtons.
• State the condition for a valid reading: the object hangs at rest.
• Contrast weight in newtons with mass in kilograms in one clear sentence.