Forces, Mass, and Motion

Forces are pushes or pulls that can change an object's motion. The overall effect depends on the combined forces acting on the object. If forces are balanced, an object’s motion does not change. If the forces are unbalanced, the object may speed up, slow down, or change direction. Mass also matters. When the same-sized push acts on two objects, the object with less mass usually has the greater change in motion. A stronger push can cause a greater change in motion than a weaker push on the same object. Forces often come in pairs. When Mateo presses his hand against a rock at a tide pool, his hand pushes on the rock, and the rock pushes back on his hand with an equal-strength force in the opposite direction. These forces act on different objects. Motion depends on the reference frame used to describe it. Ethan sitting beside Wyatt in a moving van is at rest relative to Wyatt. To someone standing beside the road, both passengers are moving along with the van.

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