MS-PS2-2: How Force and Mass Change Motion
Also called NGSS MS-PS2-2 · Topic: Forces and Interactions · Physical Science
The Official Wording
Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object.
Clarification statement: Emphasis is on balanced (Newton’s First Law) and unbalanced forces in a system, qualitative comparisons of forces, mass and changes in motion (Newton’s Second Law), frame of reference, and specification of units.
Assessment boundary: Assessment is limited to forces and changes in motion in one-dimension in an inertial reference frame and to change in one variable at a time. Assessment does not include the use of trigonometry.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What MS-PS2-2 Means in Plain English
Middle schoolers plan an investigation showing that how much an object's motion changes depends on two things: the total of all forces on it and its mass. A bigger unbalanced force speeds it up faster, and more mass makes the same force less effective.
Motion stays along one line, students change one variable at a time, and no trigonometry is needed. Balanced forces keep motion steady, an idea from 3-PS2-1. This leads to HS-PS2-1, where students describe the math linking force, mass and acceleration.
MS-PS2-2 I Can Statements
- I can plan a fair test that changes one variable at a time.
- I can show that a bigger force speeds an object up faster.
- I can show that more mass makes the same force change motion less.
MS-PS2-2 Worked Example
A tabletop test with a lab cart pulled by hanging masses over a pulley.
How do the pulling force and the cart's load change how fast the cart speeds up?
Set It Up: A 500 g lab cart, a clamp-on pulley at the table edge, string, 50 g hooked masses, 500 g of extra load and a stopwatch, with a 1 m track marked on the table.
- Hang 50 g from the string, release the cart from the start line and time it over 1 m.Empty cart, 50 g pull: 1.6 s
- Double the pull to 100 g with no load on the cart.Empty cart, 100 g pull: 1.1 s
- Go back to a 50 g pull and add the 500 g load to the cart.Loaded cart, 50 g pull: 2.2 s
- Run each setup three times and average the times.Averages follow the same pattern
- Take off the hanging mass and set the cart down at rest.Balanced forces: the cart stays put
A bigger pull made the cart cover the meter faster, and a heavier load made it slower to speed up under the same pull.
Why it works: An unbalanced force changes motion, and the change is larger when the force is bigger and smaller when there is more mass to move.
Common Mistakes with MS-PS2-2
Changing force and mass together
Adding load and more hanging mass in the same run
Try this: Change only the hanging mass in one set of runs, then only the load in another, so each result has one cause.
Thinking motion needs a constant force
“Once you stop pushing, the cart has no force, so it stops instantly.”
Try this: A moving object keeps moving unless a force like friction acts on it. Forces change motion, they are not needed to keep it.
Ignoring friction
Adding up only the pull of the string
Try this: Sum every force on the cart, including friction against the motion, to find the unbalanced force that actually changes its speed.
Where MS-PS2-2 Fits
Practice MS-PS2-2
Quick Check
- Two students push identical carts, one harder. Which cart speeds up more?
- The same push moves an empty cart and a loaded cart. Which speeds up more?
- A book rests on a table. Are there forces on it?