Check a force-model boundary
Check a force-model boundary using supplied evidence and a solved model.
About 25 minutes, plus any extended practice
Supplied materials
- All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
- Paper or a browser document for a labeled drawing, table or explanation.
- Worked problem inputs: A floor pushes up on a cart, and the cart pushes down on the floor. Explain why these do not cancel on the cart’s free-body diagram.
- Assessment case 1: A cart-only free-body diagram includes the floor’s force on the cart. Should the cart’s force on the floor also cancel it there? Which inference or prediction is best supported?
- Assessment case 2: Which scientific label fits the specifically described step or feature? Supplied case: Floor pushes on cart.
- Assessment case 3: Which scientific label fits the specifically described step or feature? Supplied case: Cart pushes on floor.
- Assessment case 4: Which scientific label fits the specifically described step or feature? Supplied case: All cart forces combined with signs.
- Assessment case 5: Which scientific label fits the specifically described step or feature? Supplied case: Earth pulls on cart.
- Assessment case 6: The cart attracts Earth gravitationally while Earth attracts the cart. What is warranted for the cart diagram? Which inference or prediction is best supported?
Learn the idea
A free-body diagram lists forces acting on one chosen body. An action-reaction partner acts on another body and should not be added as if both act on the same object.
Chosen body means object whose motion is analyzed. External force means force acting on chosen body from surroundings. Interaction partner means equal opposite force on another body. Net force means vector sum of forces on chosen body.
For the supplied mechanism problem, A floor pushes up on a cart, and the cart pushes down on the floor. Explain why these do not cancel on the cart’s free-body diagram. The two forces act on different bodies. The cart diagram includes the floor’s upward force and other forces on the cart, not the cart’s force on the floor. Net force must sum one body’s forces.
- Read the supplied problem and mark the structures, processes or quantities named: A floor pushes up on a cart, and the cart pushes down on the floor. Explain why these do not cancel on the cart’s free-body diagram.
- Draw the relevant boundary, pathway or comparison and label it using the scientific ideas above.
- Write a causal explanation or calculation, showing how each observation supports the result.
- Compare your reasoning with the worked solution; name a condition that would change or limit the inference.
See the method in action
Worked supplied problem: A floor pushes up on a cart, and the cart pushes down on the floor. Explain why these do not cancel on the cart’s free-body diagram. Solution: The two forces act on different bodies. The cart diagram includes the floor’s upward force and other forces on the cart, not the cart’s force on the floor. Net force must sum one body’s forces. This conclusion uses the stated observations and the mechanism explained above; it does not establish every unmeasured feature of a real organism, device or place.
Check your reasoning
Your answer is temporary and is not saved to an account. Download your current answer before leaving or reloading. Written work on paper or in another app stays in your own file.
Helpful tips
- Use the feature “object whose motion is analyzed” to justify chosen body; do not sort by the name alone.
- Distinguish external force (force acting on chosen body from surroundings) from a different process occurring in the same situation.
See the answer and explanation
No; the reaction acts on the floor, a different body.
Net force sums forces on the chosen body; paired interaction forces act on different bodies.
Take it further
New free-body task: a book rests on a table. Expected: draw Earth’s downward gravitational force and the table’s upward contact force on the book. Place the book’s force on the table in a separate table diagram, not as a book force.
Nothing here is saved to an account or sent to a teacher. The answer download includes your current choice and any checked feedback. It does not include choices from the separate More practice section. Download that section separately.
More practice: try new cases
Choose an answer for each problem. Explain your method on paper, then use the feedback to decide what to revisit.
Your choices are temporary. Feedback appears after you check all the answers.
Nothing here is saved to an account or sent to a teacher. Keep your own notes or download them before leaving. The score describes these questions; it does not certify subject mastery.
Check the subject sources
These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.