Forces — Chapter Practice, Grade 8

40 N

Net force = 60 − 20 = 40 N (forward).

is either at rest or moving at constant velocity

A zero net force means no change in motion — the object could be still, or cruising at a steady velocity.

30 N

Net force = 500 − 470 = 30 N, in the direction of the stronger team.

balanced, since velocity isn't changing

Constant velocity means no acceleration, which means the net force is zero — the forces are balanced.

15 N

Net force = 30 − 10 − 5 = 15 N to the right.

With no friction or other horizontal forces acting on it, the net force is zero, so by Newton's first law the puck's velocity never changes — it keeps gliding at the same speed and direction indefinitely.

5 N right; velocity changes toward the right.

Choose right as positive; add 14+(−9)=5 N.

The acceleration is toward the right, regardless of the cart's initial motion.

Response criteria

• Use the stated conditions.
• Show the evidence or calculation.
• Explain the conclusion and any stated limitation.

Opposite directions require subtraction, not addition.

Net force is 0 N; the cart continues at constant velocity in an inertial-frame model.

State what the supplied observations or conditions establish, and separate that from any assumption.

No. Its leftward speed initially decreases. Its velocity can become rightward later if the force continues.

10−12 = −2 N with right positive: 2 N left. That predicts leftward acceleration, not necessarily leftward current velocity.

Choose the receiver before adding arrows. A sign records direction along the chosen line. Net force is the combined result of the real forces, not an extra push that exists in addition to them.

10−12 = −2 N with right positive: 2 N left. That predicts leftward acceleration, not necessarily leftward current velocity.

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.

Net 9 N right; acceleration 9/3 = 3 m/s² right. Its initial leftward motion slows because acceleration opposes velocity.

Subtract opposing forces on the same receiver.

Compare acceleration direction with the supplied velocity.

Net 9 N right; acceleration 9/3 = 3 m/s² right. Its initial leftward motion slows because acceleration opposes velocity.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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

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.

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.

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.

490 N

W = mg = 50 × 9.8 = 490 N.

117.6 N

W = mg = 12 × 9.8 = 117.6 N.

its mass

Mass is the amount of matter in an object and doesn't change with location, unlike weight.

rubber on dry pavement

Rough, grippy surfaces like rubber on dry pavement create much more friction than smooth or lubricated ones.

Air resistance pushes up on the feather much more relative to its weight, because of its large surface area and very low mass, which slows its fall a lot. In a vacuum with no air resistance, a feather and a rock would actually fall at the same rate.

10 kg

m = W ÷ g = 98 ÷ 9.8 = 10 kg.

30 N on Earth; the same 3 kg mass has a smaller weight on the Moon.

Weight near Earth is 3×10=30 N.

A different gravitational field changes force, but not the object's amount of matter or inertia.

Response criteria

• Use the stated conditions.
• Show the evidence or calculation.
• Explain the conclusion and any stated limitation.

The book's weight is still present even though it is not falling.

Left, opposing the box's sliding relative to the floor.

State what the supplied observations or conditions establish, and separate that from any assumption.

Mass remains 3 kg; weight changes with the local gravitational field.

Use the illustrated study titled Gravity: masses, distance and free fall. Keep the observations, model and conditions separate.

The force rises from 8 to 16 relative units when one mass doubles at fixed separation, and falls to 2 when separation doubles at the reference masses. These model comparisons support attractive, mass- and distance-dependent interaction under the stated conditions. Feather/stone differences in air also involve drag and inertia; an ideal negligible-drag comparison does not depend on mass in the same way. Separate receivers for the interaction pair and name the model limit.

The force rises from 8 to 16 relative units when one mass doubles at fixed separation, and falls to 2 when separation doubles at the reference masses. These model comparisons support attractive, mass- and distance-dependent interaction under the stated conditions. Feather/stone differences in air also involve drag and inertia; an ideal negligible-drag comparison does not depend on mass in the same way. Separate receivers for the interaction pair and name the model limit.

Response criteria

• Compare one changed factor at a time.
• Explain attraction and appropriate force receivers.
• Distinguish gravity-only motion from significant drag.

Horizontal velocity 6 m/s; vertical velocity 0; vertical acceleration −10 m/s². Gravity continues acting downward.

Separate the two velocity components.

Distinguish zero vertical velocity from zero acceleration.

Response criteria

• State all three values with units.
• Keep gravity downward.
• Avoid claiming zero total velocity.

2 s; 5 m; 4 m respectively, in the same no-drag model.

Vertical motion is unchanged.

Range = 2 m/s × 2 s = 4 m.

Response criteria

• Keep flight time and height unchanged.
• Calculate range with units.
• State no drag and same return height.

They land together (after 1 s using g = 10 m/s²). An upward component changes initial vertical velocity, so the original comparison no longer applies.

Horizontal motion does not change the shared vertical acceleration.

Distinguish horizontal from upward launch.

Response criteria

• Use equal-height and vertical-velocity conditions.
• State the no-drag limit.
• Explain why an upward throw is different.

Its mass remains the same if no matter is added or lost. Its gravitational weight decreases because the local field is weaker.

A 2 kg object has weight about 19.6 N near Earth using 9.8 N/kg. On a location with weaker gravitational field, the same object retains 2 kg mass but has lower weight. State which quantity a scale reports.

Its mass remains the same if no matter is added or lost. Its gravitational weight decreases because the local field is weaker.

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.

At the top after 0.8 s: horizontal velocity 3 m/s, vertical velocity 0, vertical acceleration −10 m/s². Total flight is 1.6 s, so range is 4.8 m under the same-height ideal model.

Vertical velocity loses 10 m/s each second, reaching zero after 0.8 s.

Symmetric ideal same-height return takes twice that time; multiply by horizontal speed.

At the top after 0.8 s: horizontal velocity 3 m/s, vertical velocity 0, vertical acceleration −10 m/s². Total flight is 1.6 s, so range is 4.8 m under the same-height ideal model.

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.

repel

Like poles repel.

Only the separation

Changing only separation isolates its effect.

No; a magnet exerts a force on an iron clip across a gap.

Charge separation, or polarization, can produce a net attraction even while the paper remains neutral.

The comb's field can slightly rearrange charges in the paper.

Opposite charge closer to the comb can experience a stronger attraction than the farther like charge's repulsion.

Response criteria

• Use the stated conditions.
• Show the evidence or calculation.
• Explain the conclusion and any stated limitation.

Distinguish net charge from uneven distribution of charge.

They repel: like charges repel in the simple electrostatic model.

State what the supplied observations or conditions establish, and separate that from any assumption.

The magnet produces a field at the compass; the compass's magnetic needle experiences a turning effect.

That would change another cause of force, confounding the intended distance comparison.

That would change another cause of force, confounding the intended distance comparison.

Response criteria

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

The objects affect one another without direct contact and remain coupled across the supplied paper barrier. The measured force decreases with separation in this range. These illustrative records do not establish a universal inverse-square rule for all magnet geometries or show that every barrier behaves like paper. A field is a model of the spatial influence, not a material stream of air.

The objects affect one another without direct contact and remain coupled across the supplied paper barrier. The measured force decreases with separation in this range. These illustrative records do not establish a universal inverse-square rule for all magnet geometries or show that every barrier behaves like paper. A field is a model of the spatial influence, not a material stream of air.

Response criteria

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

Use the illustrated study titled Field strength and stored energy. Keep the observations, model and conditions separate.

At fixed 0.2 A, changing turns from 10 to 20 raises the illustrative lift count 3→5; at fixed 20 turns, current 0.2→0.3 A changes it 5→7. These proxy outcomes support questions and a bounded trend, not a direct calibrated force law. Moving repelling magnets closer requires input work; separating attracting magnets can require input work instead. Energy is associated with the pair’s arrangement and can transfer into later motion.

At fixed 0.2 A, changing turns from 10 to 20 raises the illustrative lift count 3→5; at fixed 20 turns, current 0.2→0.3 A changes it 5→7. These proxy outcomes support questions and a bounded trend, not a direct calibrated force law. Moving repelling magnets closer requires input work; separating attracting magnets can require input work instead. Energy is associated with the pair’s arrangement and can transfer into later motion.

Response criteria

• Isolate turns/current using the proper records.
• Distinguish proxy measurement from force.
• Model potential energy, orientation and work.

Positive: upward. Negative: downward. Field direction uses the positive-test-charge convention.

Identify the definition of electric-field direction.

Reverse the force direction for a negative test charge.

Response criteria

• Name both directions.
• Explain the sign convention.
• Distinguish field from a path.

Bound electron clouds shift slightly, leaving a relatively positive near side and negative far side. Equal total amounts remain; closer attraction can exceed farther repulsion. The paper need not have net positive charge.

Use the polarization figure rather than inventing charge creation.

Separate local charge redistribution from total net charge.

Response criteria

• Describe the near and far sides.
• Keep total paper charge zero.
• Explain the unequal-distance effect.

Outside N to S; inside S to N. Each cut piece has both N and S poles.

Trace a complete loop.

Identify both poles on each smaller magnet.

Response criteria

• Give both directions.
• Keep both poles on each piece.
• Distinguish magnetic poles from electric charge.

Changing coil current changes the magnetic interaction and force on the coil. The attached cone vibrates and disturbs air, producing sound waves.

Follow the supplied cutaway connections.

Explain a mechanism between each pair of parts.

Response criteria

• Connect current to magnetic force.
• Connect coil to cone motion.
• Connect repeated cone motion to air vibration.

Means approximately 3.33, 5.33 and 6.33 clips. Lifting performance increases in this tested range with comparable current and setup. The response varies and is a proxy, not force in newtons or proof of proportionality.

Add each three-trial set and divide by three.

Compare means and retain the individual variation.

Response criteria

• Calculate all three means.
• Hold current and setup comparable.
• State proxy and range limits.

It represents where and in what direction a source can influence suitable test objects. It does not show physical strings or prove identical force at every location.

A magnet can attract a suitable object before touching it. Compare the same objects at different distances. Do not assume every shiny metal is strongly attracted, because magnetic response depends on the material.

It represents where and in what direction a source can influence suitable test objects. It does not show physical strings or prove identical force at every location.

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.

The negative charge feels force right, opposite the positive-test-charge convention. A neutral paper piece can polarize while keeping balanced total positive and negative charge.

Use the field convention before choosing the test-charge force.

Distinguish charge redistribution from transfer that changes net charge.

The negative charge feels force right, opposite the positive-test-charge convention. A neutral paper piece can polarize while keeping balanced total positive and negative charge.

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.

3 N

The upward and downward forces balance.

smaller

Pressure = force ÷ area.

Gravity acts downward and buoyancy acts upward; their balance gives zero net force.

400 Pa initially; reducing the area by half doubles pressure to 800 Pa.

Pressure=force/area=200/0.5=400 Pa.

Halving area gives 200/0.25=800 Pa.

Response criteria

• Use the stated conditions.
• Show the evidence or calculation.
• Explain the conclusion and any stated limitation.

Use displaced fluid volume, not the container's total water volume.

18 N upward, balancing its weight.

State what the supplied observations or conditions establish, and separate that from any assumption.

Yes. Shape can change enclosed volume and average density; a hollow object can displace enough liquid to balance its weight while a compact one cannot.

The larger-area output piston moves a shorter distance when displaced fluid volumes match.

The larger-area output piston moves a shorter distance when displaced fluid volumes match.

Response criteria

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

Using force = pressure × area, the piston forces are 20 N and 80 N. In an ideal hydraulic arrangement, the larger force acts through a smaller distance while displaced volumes balance, so energy is not created. The displaced-fluid model gives an 8 N buoyant force; floating equilibrium also requires the object’s weight to be balanced.

Using force = pressure × area, the piston forces are 20 N and 80 N. In an ideal hydraulic arrangement, the larger force acts through a smaller distance while displaced volumes balance, so energy is not created. The displaced-fluid model gives an 8 N buoyant force; floating equilibrium also requires the object’s weight to be balanced.

Response criteria

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

No. Fluid pushes on all surfaces, including sides and top/bottom. Pressure varies with depth; comparing the complete surface forces is needed to explain net buoyancy.

Depth means vertical distance below the fluid surface. Pressure is force per area, and direction belongs to the force on each surface. A single selected arrow cannot represent every force acting on an immersed object.

No. Fluid pushes on all surfaces, including sides and top/bottom. Pressure varies with depth; comparing the complete surface forces is needed to explain net buoyancy.

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.

The string supplies 3 N upward. Buoyancy alone is smaller than weight; the third force is needed to make the held object’s net force zero.

List all forces on the object.

Use 6+T−9 = 0 without calling the held state unsupported floating.

The string supplies 3 N upward. Buoyancy alone is smaller than weight; the third force is needed to make the held object’s net force zero.

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.

Displaced volume is 300 mL, giving displaced water mass 0.30 kg and buoyancy 3 N up. The block weighs 0.50×10 = 5 N down. With other support absent and initial drag neglected, net force is 2 N down. It begins accelerating downward while water still provides a 3 N upward buoyant force.

700−400 = 300 mL.

300 mL of water has mass 300 g = 0.30 kg, so Fᵦ = 0.30×10 = 3 N.

Object weight = 0.50×10 = 5 N.

5−3 = 2 N down, with buoyancy still acting.

Response criteria

• Subtract the initial reading and use the fluid density to get buoyancy.
• Use object mass separately to find weight.
• Compare forces and distinguish buoyancy from their net result.