Students who demonstrate understanding can: Apply Newton's Third Law to design a solution to a problem involving the motion of two colliding objects.*
Clarification statement: Examples of practical problems could include the impact of collisions between two cars, between a car and stationary objects, and between a meteor and a space vehicle.
Assessment boundary: Assessment is limited to vertical or horizontal interactions in one dimension.
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
Forces always come in pairs. When a car bumps into a wall, the car pushes on the wall and the wall pushes back on the car, with a force of the same size in the opposite direction. That is Newton's third law, and it is true for every interaction, whether a feather lands on a table or two trucks collide.
The surprising part for many students is that the two forces are equal even when the objects are very different. When a small car hits a large truck, the truck pushes on the car exactly as hard as the car pushes on the truck. The car is damaged more because the same force acting on less mass produces a bigger change in motion, and because the car's structure is weaker.
Students apply this idea as engineers. Since you cannot remove the force pair, safety designs spread the force over a longer time or a larger area: bumpers, crumple zones, egg-drop cushions and padded helmets. Work is limited to collisions along a straight line, horizontally or vertically.
In a car and truck collision, students expect the truck to exert more force. The forces are equal; the car changes motion more because it has less mass.
Action and reaction act on different objects, so they cannot cancel each other. Only forces on the same object can balance.
Students say a wall just sits there. A wall deforms very slightly and pushes back, which is why your hand stops when you press on it.
Padding does not break the third law. It lengthens the collision time so the force at each moment is smaller, which reduces damage.
A toy cart carrying an egg rolls into a wall and the egg cracks. Use Newton's third law to explain why, and propose a design change.
Answer: The wall exerts an equal and opposite force on the cart; a crushable bumper and cushioning lengthen the collision time, lowering the force on the egg so it survives.
Pair two students on rolling chairs or skateboards and have one push the other: both roll apart, which makes the force pair visible. Then run a bumper design challenge with carts, ramps and eggs or clay figures so students can test and revise.
Assessment may show a collision between unequal objects and ask how the forces compare, or ask students to choose the best bumper material from test data. The answer about force size is always equal.
Original questions written for this standard. Choose an option or type your answer, then press Check. Every question has a worked explanation.
Answer: A) The forces are equal in size and opposite in direction
Newton's third law: the car and truck push on each other with forces of equal size in opposite directions.
Answer: D) The equal force changes the motion of the lower-mass car more
Equal forces produce a bigger change in motion for the object with less mass, and the car's structure is weaker.
Answer: C) They act on different objects
Only forces on the same object can balance. The two forces in a pair act on two different objects.
Answer: forward (also accepted: forwards)
The water pushes the swimmer forward with an equal and opposite force, which moves the swimmer through the pool.
Answer: A) Add a crushable foam bumper
A crushable bumper makes the stop take longer, which lowers the force at each instant.
Answer: 3 (also accepted: third, 3rd, third law)
Newton's third law states that for every force there is an equal and opposite force on the other object.
Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object.
A full lesson with slides, activities and an exit ticket on newton's third law in collisions, pitched to grades 6-8 and editable in PowerPoint or Google Slides.
Make a lesson βA printable, differentiated worksheet on MS-PS2-1 with an answer key, ready in about a minute.
Make a worksheet βTurn newton's third law in collisions into a quiz students answer online that marks itself, with a class summary for you.
Build a test βNo. Any collision in one dimension works: carts, balls, a meteor hitting a spacecraft, or a dropped object hitting the floor. Cars are a common, relatable example.
Not necessarily. The focus is on applying the third law to explain a collision and design a solution, using qualitative evidence or simple measurements.