Students who demonstrate understanding can: 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.
Official wording from the Next Generation Science Standards (NGSS Lead States, 2013). NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the NGSS were involved in the production of this page, and they do not endorse it. View on nextgenscience.org
An object's motion only changes when the forces on it are unbalanced. If the pushes and pulls add up to zero, a resting object stays put and a moving object keeps the same speed in the same direction. That is the heart of Newton's first law. Add an unbalanced force and the object speeds up, slows down or changes direction.
How much the motion changes depends on two things. A bigger net force produces a bigger change, and a bigger mass produces a smaller change for the same force. Push an empty shopping cart and a full one equally hard: the empty one picks up speed faster. This is the qualitative version of Newton's second law.
Students plan investigations to gather evidence for these relationships, changing one variable at a time, choosing a reference point for motion, and stating units such as newtons and kilograms. Everything stays in one dimension, and no trigonometry is required.
Students think a rolling ball stops because its push runs out. It slows because friction is an unbalanced force; with no friction, it would keep rolling.
A book on a table has gravity and the table's push acting on it. They are balanced, so the book stays still.
With the same applied force, more mass means a smaller change in motion, not a larger one.
Net force is the combined effect: forces in opposite directions subtract. A 10 N push against 4 N of friction gives a 6 N net force.
A sled is pulled to the right with a force of 50 N while friction acts to the left with 30 N. What is the net force and what happens to the sled's motion?
Answer: The net force is 20 N to the right, so the sled accelerates to the right; with more mass, the same net force would change its motion less.
Use carts on a track with spring scales or rubber bands for a consistent pull, and add washers or masses to change the load. Students can time the cart over a fixed distance and compare. Force diagrams with labeled arrows help students reason about net force before they test.
Questions often show force arrows and ask for the net force, or describe two investigations and ask which one tests the effect of mass fairly. Train students to spot the variable that changes and the ones held constant.
Original questions written for this standard. Choose an option or type your answer, then press Check. Every question has a worked explanation.
Answer: 0 (also accepted: 0 n, zero)
Equal forces in opposite directions balance: 12 - 12 = 0 N, so the box's motion does not change.
Answer: C) Friction from the grass is an unbalanced force opposite to the motion
Friction acts against the motion. That unbalanced force slows the ball down until it stops.
Answer: D) The empty cart speeds up faster
With the same force, the object with less mass has a bigger change in motion.
Answer: A) The force pulling the cart
To test mass, the applied force must be kept constant so mass is the only variable that changes.
Answer: 20 (also accepted: 20 n, 20 n to the right)
Subtract opposite forces: 30 - 10 = 20 N, acting to the right.
Answer: C) Gravity and the desk's upward push are balanced
Gravity pulls the book down and the desk pushes up with an equal force, so the forces are balanced and the book stays still.
Answer: newton (also accepted: newtons, n)
Force is measured in newtons (N).
Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object.
Analyze data to support the claim that Newton's second law of motion describes the mathematical relationship among the net force on a macroscopic object, its mass, and its acceleration.
A full lesson with slides, activities and an exit ticket on net force, mass and motion, pitched to grades 6-8 and editable in PowerPoint or Google Slides.
Make a lesson βA printable, differentiated worksheet on MS-PS2-2 with an answer key, ready in about a minute.
Make a worksheet βTurn net force, mass and motion into a quiz students answer online that marks itself, with a class summary for you.
Build a test βThe emphasis is on qualitative comparisons of force, mass and change in motion. Simple net force arithmetic along a line is common, but formal second law calculations are not required.
It is the point of view used to describe motion. A passenger in a moving car is still relative to the car but moving relative to the road.