Investigating Forces

A spring newton meter reads force in newtons (N). Before use, check zero with no applied force in the measuring position. Choose a range that covers the expected pull; never exceed its limit. Pull along its axis, keep the reading steady, and view the scale straight on at eye level. For a horizontal trolley test, pull horizontally. If axial tension stays the same, tilting a horizontal pull upward reduces its horizontal component, not the meter reading.

With adult supervision, secure a spring to a stable stand beside a ruler. Measure its unloaded length. Add small known weights within the equipment's safe range; wait for movement to settle. Extension = loaded length minus unloaded length. Record force and extension. Plot extension horizontally and force vertically, including the zero-load reference. Stop before overloading the spring.

Choose what you change, measure and control. To compare force and acceleration, keep trolley mass and track conditions fixed. To compare surface friction, use the same block, speed and horizontal pull direction. During constant-speed motion, pull balances friction and may differ between surfaces. For spring loads, use the same undamaged spring. Controls reduce competing explanations, not measurement uncertainty.

Newton's second law is F = ma. Acceleration is how quickly velocity changes. F is resultant external force in N, m is mass in kg and a is acceleration in m/s². Use a = F ÷ m or m = F ÷ a for a missing value. A meter measures one pull, not automatically the resultant. Subtract opposing horizontal forces before using the law.

In a spring's proportional range, F = kx, where x is extension and k is stiffness. Use k = F ÷ x, x = F ÷ k or F = k × x. Keep units consistent. A constant extension-to-force ratio shows proportionality, as does a straight graph through the origin. Equal increments alone can describe a line with a nonzero intercept.

One accurate unloaded-length measurement can be reused if the spring stays unchanged. An unloaded length of 12 cm and loaded length of 18 cm give 6 cm extension. Using 11 cm instead gives 7 cm, 1 cm too large. A baseline too short makes all extensions too large; one too long makes them too small. A graph missing the origin suggests an offset but does not prove its cause.

Record unusual readings rather than declaring them mistakes. Repeat the measurement and check the apparatus, baseline and loading range. Scatter can come from measurement uncertainty; a repeatable curved pattern can be real spring behaviour beyond the proportional range. Loss of proportionality does not by itself establish permanent deformation. The elastic limit concerns whether the spring fully recovers its unloaded shape after release.

You may first predict motion while neglecting resistance. For a 2 kg trolley pulled with 10 N, that prediction is 5 m/s². If reliable measurements give 4 m/s², the resultant is 2 × 4 = 8 N. Other horizontal forces combine to 2 N against the pull. Different combinations can give that total; acceleration alone cannot identify each force. Also check measurement uncertainty and equipment before drawing conclusions.