Interaction Pairs & Force Diagrams — Practice · Grade 8 Science

Chapter: Forces

What this practice covers

By the end of Interaction Pairs & Force Diagrams you should be able to:

12 questions, written answer, each with a worked explanation.

Vocabulary used: interaction, force diagram, receiver, normal force, net force, model, evidence, system boundary, assumption.

Problem 1

Which receiver belongs in a cart diagram?
Answer and explanation

The cart. Forces exerted on the table or on Earth belong in separate diagrams, even though they are connected interactions.

The cart. Forces exerted on the table or on Earth belong in separate diagrams, even though they are connected interactions.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 2

Why do the two forces not cancel in the cart diagram?
Answer and explanation

Only one acts on the cart. Adding a force on the person to the forces on the cart mixes receivers and gives an invalid net force.

Only one acts on the cart. Adding a force on the person to the forces on the cart mixes receivers and gives an invalid net force.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 3

Does a 4 N horizontal net force require an unbalanced vertical force?
Answer and explanation

No. Horizontal and vertical comparisons are separate in this simple model. The vertical forces may balance while horizontal motion changes.

No. Horizontal and vertical comparisons are separate in this simple model. The vertical forces may balance while horizontal motion changes.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 4

If a cart is moving right, must its net force point right?
Answer and explanation

No. It can be slowing under a leftward net force or moving steadily with zero net force. Motion direction and net-force direction are different quantities.

No. It can be slowing under a leftward net force or moving steadily with zero net force. Motion direction and net-force direction are different quantities.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 5

In an illustrative interaction a hand pushes a cart rightward with 8 N. Floor resistance on the cart is 3 N leftward. State the cart’s horizontal net force and the cart-on-hand force; explain why they differ.
Answer and explanation

The cart’s net force is 8−3=5 N rightward. The cart pushes the hand with 8 N leftward as the partner of the hand-on-cart interaction. The 3 N floor force belongs to another interaction. Net force on the cart and the pair force on the hand answer different questions.

The cart’s net force is 8−3=5 N rightward. The cart pushes the hand with 8 N leftward as the partner of the hand-on-cart interaction. The 3 N floor force belongs to another interaction. Net force on the cart and the pair force on the hand answer different questions.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 6

A book’s weight and the table’s upward force are equal. Are they a third-law pair?
Answer and explanation

No. Both act on the book. Their partners act on Earth and on the table respectively. They may balance on the book without being the two sides of one interaction.

No. Both act on the book. Their partners act on Earth and on the table respectively. They may balance on the book without being the two sides of one interaction.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 7

Evaluate and correct: Equal reaction forces prevent anything from accelerating.
Answer and explanation

Interaction partners act on different objects; acceleration depends on the net force on each object separately.

Interaction partners act on different objects; acceleration depends on the net force on each object separately.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 8

Choose one prediction from this unit. Explain a comparison that could test it, specifying observations, changed and matched conditions, and a result that would challenge it.
Answer and explanation

A cart moving rightward can have a leftward net force while slowing. Draw the motion arrow and force arrows with different labels. Use before-and-after position or speed records to check the interpretation. No. It can be slowing under a leftward net force or moving steadily with zero net force. Motion direction and net-force direction are different quantities.

A cart moving rightward can have a leftward net force while slowing. Draw the motion arrow and force arrows with different labels. Use before-and-after position or speed records to check the interpretation. No. It can be slowing under a leftward net force or moving steadily with zero net force. Motion direction and net-force direction are different quantities.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 9

A moving cart has a 10 N leftward force and a 6 N rightward force. Give its net force and explain whether the equal reaction to the 10 N force belongs in its diagram.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

Net force is 4 N leftward. The partner of that interaction acts on the other object, not the cart, so it does not cancel the cart’s 10 N force. Instantaneous motion direction needs separate information.

Net force is 4 N leftward. The partner of that interaction acts on the other object, not the cart, so it does not cancel the cart’s 10 N force. Instantaneous motion direction needs separate information.

Problem 10

A book’s weight and the table’s upward force are equal. Are they a third-law pair? Explain a condition that would change this conclusion.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

No. Both act on the book. Their partners act on Earth and on the table respectively. They may balance on the book without being the two sides of one interaction.

No. Both act on the book. Their partners act on Earth and on the table respectively. They may balance on the book without being the two sides of one interaction.

Problem 11

An interaction pair belongs on two separate objectsAn interaction pair belongs on two separate objectscarthand / pusherHand’s force ON cart: rightCart’s force ON hand: leftEqual interaction forces, different receivers; not two forces on one cart.
The red arrow represents the hand’s rightward force on the cart. The blue arrow is the cart’s equal leftward force on the hand. They act on different receivers, so they cannot be canceled within the cart’s own force sum. Other forces on each receiver still matter, and equal interaction forces do not guarantee equal acceleration.
For this unit’s illustrated case, which of these arrows belongs on the cart’s free-body diagram? Where does the other belong?
Answer and explanation

Only the hand-on-cart arrow belongs on the cart’s diagram. The equal cart-on-hand partner belongs on the hand’s diagram.

Read a force name as source-on-receiver. That naming convention makes the third-law pair explicit. Balanced forces on one receiver are a different comparison from this equal-and-opposite pair between two receivers.

Only the hand-on-cart arrow belongs on the cart’s diagram. The equal cart-on-hand partner belongs on the hand’s diagram.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 12

A cart accelerates right at 1.5 m/s² with mass 4 kg. The recorded push is 10 N right. What horizontal resistance would make the introductory force model agree, and where does the cart-on-hand partner force belong?
Answer and explanation

Required net is 4×1.5 = 6 N right, leaving 4 N resistance left. The cart-on-hand partner belongs on the hand’s diagram, not the cart’s.

Use measured acceleration to find net force.

Keep the third-law partner on its separate receiver.

Required net is 4×1.5 = 6 N right, leaving 4 N resistance left. The cart-on-hand partner belongs on the hand’s diagram, not the cart’s.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Browse all courses