Maine Grade 3 Science

Explain a change in motion using the forces in a model

Compare balanced and unbalanced pushes on a model cart, distinguish motion from force, and use Maine Grade 3 science references.

The Maine science standards directory identifies the standards adopted in 2019 as current. It states that the proposed later revision was not authorized. The linked PDF used here presents that 2019 Learning Results framework; its 2026 file date does not establish a new adoption. Use the Maine Science, Technology, and Engineering Standards (2019 Learning Results) for the selected performance expectation and its complete supporting information. This activity selects a Grade 3 expectation. It is one part of a science course, not a complete curriculum.

A resting cart and two opposing pushes

Imagine a cart at rest on a level track. In a simplified model, vertical forces balance and the only horizontal forces are two pushes in opposite directions. In Trial A the pushes have equal strength. In Trial B the rightward push is stronger. Students draw arrows for both pushes and predict what happens just after the pushes begin. The table gives the model conditions rather than data from a real cart. Explain why the prediction differs between A and B, and identify what would need observation in an actual investigation.

Explain the evidence or model

In Trial A the equal opposing horizontal pushes balance. Their combined effect does not change the cart’s motion, so a cart that began at rest stays at rest in this model. In Trial B the pushes do not balance: there is an overall rightward effect. The cart begins moving to the right. Both trials involve pushes, so it is incorrect to describe Trial A as a cart with no forces. Use arrow direction and relative strength together. The stronger arrow in B helps explain the change; seeing an arrow that points right by itself would not tell us whether another force cancels it.

What to check in the explanation

The model starts from rest. Balanced forces on an already moving object would not by themselves make it stop. Also, real tracks can introduce friction, slopes, and unequal wheel behavior. The model explicitly leaves those horizontal effects out; an actual classroom investigation must consider them. The table gives no measured push strengths, time intervals, or distances, so it cannot support a claim that the cart travels a particular number of centimeters. Do not infer that a cart traveling right necessarily has an overall rightward force: its motion and a change in its motion are different observations. The selected task focuses on explaining a change from the stated starting condition.

Plan the next activity

1. Mark the cart’s initial motion before drawing force arrows.
2. Compare the arrows’ directions and strengths in each trial.
3. Explain the prediction and separate it from an observation that still needs testing.

With teacher supervision, plan a low-speed classroom trial on a suitable surface. Decide how to keep the cart, starting position, and track consistent, and describe what evidence would count as beginning to move. Repeat trials rather than treating a single unexpected result as a new rule. If the cart drifts under apparently equal pushes, check whether the pushes really match and whether other effects were present. Record the observation first, then revise the model or setup. Use the forces lesson to practice describing pushes before choosing related independent questions.