Topic 10 Unbalanced forces — what needs James (0)

Topic 10 · Unbalanced forces · Type 1 · bank item PP-1 · tests FOR3-044

EasyMCQType 1 — a described pair, the sled moving forward, the pull forward stronger than the drag back — what does it do

Yemi's sled moves forward along the snow, and his pull forward is stronger than the snow's drag back. What does the sled do?

1. a) It keeps the same speed.
2. b) It speeds up. ← key
3. c) It slows down.
4. d) It changes direction.

Why it is this type: Easy because: one step — the stronger force is named and its direction is the direction the sled moves; the rule gives the happening.

The why line (what a student reads after answering): Yemi's pull forward is the stronger force. / It acts in the direction the sled is already moving. / So the sled speeds up.

Recommendation: Approve as the exemplar

Topic 10 · Unbalanced forces · Type 2 · bank item PP-2 · tests FOR3-046

EasyMCQType 2 — a DRAWN pair, the back arrow longer, MOVING cue, "rolling to the right" — what does the object do

The drawing shows the two forces on a stroller rolling to the right along a path. What does the stroller do?

1. a) It speeds up.
2. b) It keeps the same speed.
3. c) It changes direction.
4. d) It slows down. ← key

Figure: a dad pushes a purple stroller to the right, both oblong arms to the handle; a MOVING cue above the stroller; from the stroller body's centre a short red arrow HIS PUSH right and a long blue arrow THE DRAG BACK left. No speed word.

Why it is this type: Easy because: one reading (which arrow is longer) then the rule.

The why line (what a student reads after answering): The arrow back is longer than the arrow forward. / So the drag back is the stronger force. / It acts against the direction the stroller is moving, so the stroller slows down.

Recommendation: Approve as the exemplar

Topic 10 · Unbalanced forces · Type 3 · bank item PP-3 · tests FOR3-047

EasyMCQType 3 — a described push from the side on a rolling thing — what does it do

A marble rolls straight across the floor, and Gwen flicks it from the side. What does the marble do?

1. a) It rolls slower along the same path.
2. b) It rolls faster along the same path.
3. c) It rolls off on a new path. ← key
4. d) It keeps rolling in the same direction.

Why it is this type: Easy because: one step — a push from the side on a moving thing; the four options are the topic's three changes and the balanced case, in parallel form.

The why line (what a student reads after answering): Gwen's flick pushes the moving marble from the side. / Nothing pushes back against her flick, so the forces are unbalanced. / So the marble changes direction and rolls off on a new path.

Recommendation: Approve as the exemplar

Topic 10 · Unbalanced forces · Type 4 · bank item PP-4 · tests FOR3-044

MediumMCQType 4 — an object is speeding up — which sentence describes the two forces on it

A go-kart is speeding up along the track. Which sentence describes the two forces on it?

1. a) The push forward is stronger than the drag back. ← key
2. b) The drag back is stronger than the push forward.
3. c) The push forward and the drag back are the same strength.
4. d) No force acts on the go-kart.

Why it is this type: Medium because: the rule is run backwards — from the observed change to the state of the forces — against the slowing case, the balanced case and the catalogue's "no force on a moving thing" belief.

The why line (what a student reads after answering): The go-kart is speeding up, so the forces on it are not balanced. / An object speeds up when the stronger force acts in the direction it is already moving. / The go-kart moves forward, so the push forward is stronger than the drag back.

Recommendation: Approve as the exemplar

Topic 10 · Unbalanced forces · Type 5 · bank item PP-5 · tests FOR3-046

MediumMCQType 5 — four drawings J K L M of sleds all sliding RIGHT (stated; MOVING cue on each), two arrows each from the sled's centre — which shows the one slowing down

Four sleds slide to the right along the snow. Which drawing shows the sled that is slowing down?

1. a) drawing J
2. b) drawing K ← key
3. c) drawing L
4. d) drawing M

Figure: four panels J K L M, each a sled on snow with a grey MOVING cue pointing right above it and two arrows from the sled's centre on two parallel lines: J forward longer (a girl pulls); K back longer (a girl pulls); L equal (a girl pulls); M a boy pushes from behind with oblong arms, the red forward arrow longer. No speed word, no tick or cross.

Why it is this type: Medium because: four drawings are read and three refused — two speeding (one by a pull, one by a push) and one balanced — before the slowing one is picked. Direction check (vault: A force-and-motion item states the direction the thing is already moving): if the sleds moved LEFT, J and M would be the slowing ones — so the direction is load-bearing, stated and drawn. Exactly one option is right.

The why line (what a student reads after answering): A sled slows down when the stronger force acts against the direction it is moving. / In J and M the arrow forward is longer: those sleds speed up. In L the arrows are equal: that sled keeps its speed. / In K the arrow back is longer, so the drag back is the stronger force and sled K slows down.

Recommendation: Approve as the exemplar

Topic 10 · Unbalanced forces · Type 6 · bank item PP-6 · tests FOR3-047

MediumMCQType 6 — a rolling ball — which of these would send it onto a new path

A ball is rolling along the grass. Which of these would send it onto a new path?

1. a) a push from behind
2. b) a push from the front
3. c) a push from the side ← key
4. d) no push at all

Why it is this type: Medium because: the student holds the three atoms side by side and picks the one push whose direction gives a change of direction.

The why line (what a student reads after answering): A push from behind acts in the direction the ball is moving and speeds it up; a push from the front acts against it and slows it down. / With no push, the ball keeps rolling in the same direction. / Only a push from the side changes the ball's direction, so only that push sends it onto a new path.

Recommendation: Approve as the exemplar

Topic 10 · Unbalanced forces · Type 7 · bank item PP-7 · tests FOR3-047

MediumMCQType 7 — a car drives straight, round a bend, straight again at the same speed — when are the forces unbalanced

A car drives along a straight road, round a bend, and along a straight road again, all at the same speed. When are the forces on the car unbalanced?

1. a) the whole time
2. b) only on the straight parts
3. c) never
4. d) only round the bend ← key

Why it is this type: Medium because: the pack's L9 point asked as a WHEN — the student separates the turning moment from the straight moments and refuses "the whole time".

The why line (what a student reads after answering): Balanced forces leave the motion as it is; unbalanced forces change it. / On the straight parts, at the same speed, nothing about the car's motion changes. / Round the bend its direction changes, so the forces are unbalanced only while it goes round the bend.

Recommendation: Approve as the exemplar

Topic 10 · Unbalanced forces · Type 8 · bank item PP-8 · tests FOR3-044

MediumtypedType 8 — two opposite forces on a moving toy car, one stronger than the other — type the word we use for these forces

TEXT ENTRY

Two opposite forces act on a moving toy car, and one force is stronger than the other. Type the word we use for these forces.

Accepted answers (typed): unbalanced · unbalanced forces · un balanced · un-balanced · unbalance · unballanced · unbalenced · unbalansed · not balanced · not-balanced (the list PR-9 passed the 29 Sep adversarial read with; the grader lower-cases, drops a final period and a leading article, accepts a one-word key's plural).

Why it is this type: Closed because: the answer is the topic's one coined term, recalled — not on the page, not in view (James's three tests: closed, useful, not an MCQ in disguise). Mirrors Topic 9's typed "balanced", which James called "a good use of a free response". Medium, as Topic 9 tags its twin.

The why line (what a student reads after answering): One force is stronger than the force against it, so the two forces are not balanced. / Forces that are not balanced are called unbalanced forces.

Recommendation: Approve as the exemplar

Topic 10 · Unbalanced forces · Type 9 · bank item PP-9 · tests FOR3-047

ProposedMediumProposed: Type 9 — a hoop rolling in the direction of the drawn dashed arrow, a shoe taps its side once and comes away — what does the hoop do AFTER the tap

A hoop rolls along the playground in the direction of the dashed arrow, and Hettie's shoe taps its side once and comes away. What does the hoop do after the tap?

1. a) It rolls straight in its new direction. ← key
2. b) It curves back to its old direction.
3. c) It keeps turning more and more.
4. d) It rolls straight in its old direction.

Figure: seen from above, a green hoop rolling up a grey playground along a dashed arrow (its head shows the direction); a shoe touching its left side; one red arrow from the hoop's centre pointing right, chip THE TAP. No path ahead.

Why it is this type: The tap pushed the hoop from the side, so its direction changed. / A moving thing changes direction only while something pushes it from the side. / After the tap nothing pushes the hoop from the side, so it rolls straight in its new direction.

Recommendation: Approve the proposed type

Topic 10 · Unbalanced forces · Type 10 · bank item PP-10 · tests FOR3-044

ProposedHardProposed: Type 10 — a skater glides right on smooth ice; a friend keeps pushing her with the same steady push — what happens to her speed while the push acts

An ice skater glides to the right across smooth ice, which hardly drags her back. A friend behind her keeps pushing her with the same steady push. What happens to her speed while the push keeps acting?

1. a) It gets faster, then stays at one speed.
2. b) It stays the same the whole time.
3. c) It keeps getting faster the whole time. ← key
4. d) It gets faster, then slower.

Why it is this type: The push forward is stronger than the ice's tiny drag back, so the forces are unbalanced and the skater speeds up. / The push keeps acting, so the forces stay unbalanced the whole time. / So she keeps getting faster the whole time the push acts.

Recommendation: Approve the proposed type

Topic 10 · Unbalanced forces · Type 11 · bank item PP-11 · tests FOR3-046

ProposedHardProposed: Type 11 — a beanbag tossed straight up, still going up — which forces act on it now

A girl tosses a beanbag straight up. It has left her hand and is still going up. Which forces act on the beanbag now?

1. a) Earth's pull down and her push up.
2. b) Her push up only.
3. c) No force at all.
4. d) Earth's pull down only. ← key

Why it is this type: Her push acts on the beanbag only while her hand touches it. Once it has left her hand, nothing pushes it up. / Earth pulls the beanbag down the whole time. / So the only force on it is Earth's pull down, and that pull is what slows the beanbag as it goes up.

Recommendation: Approve the proposed type