Investigate how forces and mass affect changes in motion

Learning goal: Use evidence to explain changes in an object's motion, distinguishing the effects of force strength, force direction and mass. A force is a push or pull. A change in motion can be speeding up, slowing down or changing direction; it is not simply having a position somewhere else.

Consider two horizontal forces on a cart. A pull of three force units to the right and a pull of three units to the left balance. Their opposing effects cancel. If the cart is initially at rest, it remains at rest in this simplified model. If it is already moving, balanced forces let it continue at constant speed in the same direction. Balanced forces do not necessarily stop a moving object.

If a five-unit pull acts rightward and a two-unit pull acts leftward, the forces are unbalanced, with a combined effect equivalent to three units rightward. A cart initially moving rightward speeds up. A cart initially moving leftward slows down while the rightward effect acts; if that effect continues long enough, the cart can stop and begin moving rightward. The force direction describes how motion changes, not always the direction the object is currently traveling.

Forces can also act vertically. A book at rest on a table has Earth's downward gravitational pull balanced by the table's upward support. The upward support is a force even though the table does not visibly lift the book. These two forces act on the same book. Do not replace the upward support with the book's downward push on the table, which acts on a different object.

Real rolling carts experience resistance. After a brief push ends, a cart usually slows because friction and other resistance remain. It does not slow because moving objects must always run out of an invisible supply of motion. To identify forces, state which object receives each push or pull and whether contact continues. A hand no longer touching the cart cannot keep exerting its earlier contact push.

Compare speed changes under stated conditions: each cart starts from rest and is pulled along a straight track, in one direction, for the same time. Under those conditions, for the same cart, a greater unbalanced force produces a greater change in speed. If two carts of different mass, each starting from rest, experience the same unbalanced force for the same time, the less massive cart has the greater change in speed. These conditions matter. A force that acts against a cart that is already moving first slows it, as in the leftward example above, and can stop it and start it moving the other way, so the speed change alone would not show how strong that force was. Starting every trial from rest avoids this. Compare changes from the same starting speed. A heavier cart can already be moving faster; mass alone does not reveal its current speed.

A fair classroom test separates these two relationships. First, use one low-friction cart and change the measured pulling force while keeping mass, track, starting condition and pull duration consistent. Second, keep the measured pull the same and securely add mass to that cart. Do not change force and mass together and then claim that only mass caused the difference.

Equal-looking hand pushes are difficult to reproduce. With adult support, use a force meter to guide a steady horizontal pull over a brief timed interval, record actual force variation and compare repeated trials. Keep wheels, surface and pull direction the same. Added mass can also change rolling resistance, so acknowledge this limitation when interpreting the measured result. A simple classroom trial is evidence under its tested conditions, not a perfect friction-free model.

Predictions should describe observable changes. Record starting and ending speeds using equal-time video positions or distance and timing measurements. Speed is calculated by dividing the distance traveled by the elapsed time: a cart that moves 60 centimeters in 2 seconds averages 30 centimeters per second. The companion lesson, Measure speed and distinguish constant from changing motion, develops this measurement; together the two lessons cover 5.PS.1. Distance traveled after release can be affected by resistance and the duration of the original push; it is not a direct measurement of force. Revise a claim if uncontrolled differences could explain the results.

Worked model: In a simplified horizontal-force diagram, arrows of equal length point left and right. The cart is already moving right at a steady speed. The prediction is continued rightward motion at that speed, not an immediate stop. The diagram represents only the horizontal comparison; vertical forces are balanced separately.

Worked critique: A student pushes an empty cart gently for one second and a loaded cart hard for three seconds. The loaded cart travels farther. This does not isolate the effect of mass because both force and duration changed. Repeat with the same measured pull and duration, then discuss resistance and trial variation.

Guided investigation: On a clear, level floor or low track with a safe stopping area, an adult helps operate a small cart and force meter. Secure added masses, keep fingers away from wheels and avoid launching objects. Record at least three trials per condition, with mass, pull, duration and measured motion. Stop and revise the setup if a steady pull cannot be maintained.

Independent performance: Draw two opposite forces on one object, compare their strengths and predict the motion change from a stated starting motion. Report one actual investigation with a variable changed, quantities held consistent, repeated observations and a limitation. Distinguish your collected evidence from this lesson's idealized examples. Study the connected speed lesson to measure motion rather than guessing from how far a cart eventually rolls.