Grade 11 · Physics · Mechanics
It is moving, so where is the force pushing it?
A worksheet on Newton's three laws, with a teacher page
أ. يوسف الجبوري
مدرسة ج
Eight situations where the student draws the forces with a pen before seeing a single number, so the mistake shows up in the drawing rather than the arithmetic.
This page is exactly what a teacher's published work will look like. Only the teacher is not real yet.
What the student does
- 1
Read the situation, do not calculate
Eight short situations, one question each: which forces act, and which way does the resultant point? No numbers yet.
- 2
Draw the forces by hand
An empty box beside each situation. They draw the body as a dot and the forces as arrows of their own — nothing to pick from a list.
- 3
Name the pair
For each force they write who is pushing what. The action-reaction pair then plainly sits on two bodies rather than one.
- 4
Calculate last
Once the drawing is finished, they put numbers into the second law (F = m × a) — for four of the eight situations only.
What's inside
- 8 situations, each with an empty box to draw in
- A «who acts on what» table for the action-reaction pair
- 4 numerical problems that come after the drawing, not before
- A teacher page naming the expected mistake in each situation
Outcomes
Identifies the forces acting on one body and leaves out the ones acting on anything else.
Concludes that the resultant force on a body moving at constant speed is zero.
Writes an action-reaction pair on two different bodies rather than on the same one.
Works out acceleration from resultant force and mass in four situations.
What the teacher focused on
They do the arithmetic right and get the problem wrong, because they drew a force that is not there. So I hold the numbers back until the drawing is finished.
Teaching strategies
- Draw before you calculate
- One body per box
- The expected mistake written on the teacher page
He has the student draw the arrows by hand in an empty box rather than drag them from a ready list. A list hands over the correct set of forces before they have thought about it, and the whole question is which forces are there at all. He also refused to let any number appear on the first page, because a student who sees a number starts substituting before they have understood the situation.
Questions from inside it
- 1A ball rolls along smooth level ground at constant speed. Draw the forces on it. Is one of them in the direction of motion?
- 2A book of weight 12 newtons rests on a table. How large is the force the table exerts on the book, and on which body does its reaction act?
- 3A horse pulls a cart with 400 newtons, and the cart pulls the horse with the same 400 newtons. Explain how the two of them move at all.
Curriculum
- Grade
- Grade 11
- Subject
- Physics
- Unit
- Mechanics
- Taught in
- Palestine
Where this lesson sits in the curriculum
The same lesson appears in these countries' curricula. One page, five paths to it.
When to use it
The explanation
The first law says a body stays as it is — at rest, or moving in a straight line at constant speed — unless a resultant force acts on it. Motion itself needs no force; what needs a force is a change in motion. The second law measures that change: resultant force equals mass times acceleration (F = m × a), so a resultant of zero means an acceleration of zero and a speed that does not change. The third law says forces come in pairs: if A pushes B, then B pushes A with an equal force in the opposite direction. The condition everyone forgets is that the two forces of a pair act on two different bodies, which is exactly why they never cancel each other out.
Common mistakes it catches
Draw the forces on a ball rolling across ice: weight down, the surface pushing up, and nothing forward at all. Constant speed is evidence that the resultant is zero, not that something is pushing. A force changes speed; it does not maintain it.
Both of those act on the book, so they cannot be a pair. The partner of the book's weight is the book pulling on the Earth; the partner of the table pushing the book is the book pushing the table. At every arrow ask: which body does this force act on?
The two act on different bodies, so they never appear in the same sum. Look at the cart alone: the horse pulls it forward, friction pulls it back, and the difference is what moves it.