Forces, Mass & Changing Motion
Aligned to MS-PS2-2 — Next Generation Science Standards.
What this lesson teaches
An object's motion changes only when the forces on it do not cancel out. The same net force produces a smaller change in motion for an object with more mass.
Worked examples
Example 1
Pushing an empty shopping cart with one hand speeds it up quickly, but the same push on a full cart barely moves it — greater mass means less change in motion for the same force.
Example 2
Doubling the force doubles the acceleration; doubling the mass halves it. That is a = F/m stated in words.
Example 3
An object with no net force does not stop — it keeps doing whatever it was doing. A puck on frictionless ice coasts forever.
Practice questions
- Plan an investigation to test how adding mass changes a cart's speed under the same push.
- Observe and record how a fan cart accelerates with one fan versus two fans.
- Explain why a net force of zero means an object's motion does not change.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Fuerzas, Masa y Cambio de Movimiento
El movimiento de un objeto cambia solo cuando las fuerzas sobre él no se cancelan. La misma fuerza neta produce un cambio de movimiento menor en un objeto con más masa.
Ejemplo 1: Empujar un carrito de compras vacío con una mano lo acelera rápido, pero el mismo empujón en un carrito lleno apenas lo mueve — más masa significa menos cambio de movimiento para la misma fuerza.
Ejemplo 2: Duplicar la fuerza duplica la aceleración; duplicar la masa la reduce a la mitad. Eso es a = F/m dicho en palabras.
Ejemplo 3: Un objeto sin fuerza neta no se detiene — sigue haciendo lo que hacía. Un disco sobre hielo sin fricción se desliza para siempre.
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