Types of Forces, Grade 9: Contact, Field, and Measuring Weight
Push a heavy box with 50 N and it does not move. How big is the friction?
Exactly 50 N. Not more. Several Grade 9 textbooks say the friction is larger than your push, which cannot be true, because a friction force bigger than the push would drag the box towards you.
That is one of four things this chapter routinely gets wrong. This page covers the types of forces, the difference between mass and weight measured rather than defined, and the four corrections worth making before the test.
Contact forces and field forces
| Type | Needs touching? | Examples |
|---|---|---|
| Contact forces | Yes | Friction, tension in a rope, the normal force from a table, applied pushes and pulls, air resistance |
| Field forces | No | Gravitational, magnetic, electrostatic |
A field force acts across a gap. A magnet moves a paperclip it never touches, the Earth pulls on a falling stone from 6 000 km away, and a rubbed balloon lifts paper from a centimetre off the bench.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Term | 3 |
| Strand | Energy and change |
| Topic | Topic 10, Forces |
| Status | No formally assessed practical task |
| Marks | None prescribed. Our worksheet is 40 marks |
Mass and weight, measured instead of defined
Every textbook defines the difference. This measures it, and it takes fifteen minutes.
- Zero a force meter while it hangs empty
- Hang a 100 g mass and record the reading in newtons
- Repeat for 200 g, 500 g and 1 kg
- Plot weight in newtons against mass in kilograms
- Draw the best straight line
| Mass | Reading you should get |
|---|---|
| 100 g | about 1 N |
| 200 g | about 2 N |
| 500 g | about 4,9 N |
| 1 kg | about 9,8 N |
The line is straight, it goes through the origin, and its gradient is 9,8. Mass is what you hung on the hook. Weight is what the meter read. The gradient is the exchange rate between them, and on the Moon it is 1,63.
| A 2 kg brick | On Earth | On the Moon |
|---|---|---|
| Mass | 2 kg | 2 kg, unchanged |
| Weight | 19,6 N | about 3,3 N |
Nothing about the brick changed. Only the field it is sitting in. That is the whole difference, and a bathroom scale marked in kilograms is why almost nobody believes it.
Four things worth correcting
1. Static friction does not exceed your push
It matches it, up to a maximum. Push with 20 N and friction is 20 N. Push with 50 N and friction is 50 N. Push past the maximum and the box moves. A friction force that was genuinely larger than the push would accelerate the box backwards, which nothing has ever done.
2. Tension does not always oppose motion
Tension pulls along the rope, inwards on whatever is attached at each end. A tow truck pulling a car puts the car under tension in the direction it is travelling. The rule that "tension always opposes motion" fails on the first example most textbooks print.
3. A compass needle is not attracted to Earth's magnetic north pole
Like poles repel. If the needle's north end were attracted to a magnetic north pole, magnetism would not work.
The magnetic pole sitting near the geographic North Pole is a magnetic south pole. That is why the needle's north end points at it, and it is why the two names disagree.
4. Positive and negative are unlike charges
A common sentence in this chapter describes "the attraction between like charges" while discussing a positive and a negative object. Like charges repel. Unlike charges attract. The situation described is right, the label is wrong.
A force pair is not a balanced pair
Textbooks often use one drawing for both, and they are different ideas.
| Balanced forces | A force pair (Newton's third law) | |
|---|---|---|
| How many objects | One | Two |
| Book on a table | Gravity pulls the book down, the table pushes the book up. Both on the book | The book pushes down on the table, the table pushes up on the book. One force on each |
| Can they cancel out? | Yes. That is why the book stays still | Never. They act on different objects |
The question that separates them: if the table pushes up on the book exactly as hard as the book pushes down, why does the book not fly upwards? Because those two forces are not acting on the same thing.
The field forces, as four quick stations
| Station | What to do | What it shows |
|---|---|---|
| Magnets | Two bar magnets, ends together both ways. A compass near one end | Force without contact, and a field with direction |
| Magnetic materials | Test iron, steel, brass, aluminium, copper, plastic | Only iron, nickel, cobalt and steel. Not all metals |
| Balloon and paper | Rub a balloon on hair, hold it over torn paper | Electrostatic force, acting through air |
| Drop test | A stone and a crumpled paper ball together, then a flat sheet | Gravity pulls both the same. The flat sheet is slowed by air resistance, not by weaker gravity |
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| The graph misses the origin | The force meter was not zeroed | Hang it empty, adjust to zero, rerun. This is the commonest fault |
| 1 kg reads 1 N | They read the gram scale | Both scales are printed on the tube. Use the newton side |
| The line bends at the top | The mass is past the meter's range | A 10 N meter tops out near 1 kg. Use the 20 N |
| The meter no longer returns to zero | It was overloaded once | The spring is stretched permanently. Replace it and do not let anyone test its limits |
| The balloon lifts nothing | Humid day | Rub harder and use smaller pieces of paper |
| The compass points at the magnet | Correct. The magnet is far closer than the Earth | Move the magnet away and it swings back to north |
Safety
- Slotted masses over the bench, never over feet
- Alnico magnets snap together hard enough to pinch fingers and chip the magnets. Use the keepers
- Keep magnets away from phones, laptops and bank cards
- If you use iron filings, keep them in a sealed bag or use a field demonstrator plate. Filings and eyes do not mix, and they stick to the magnets for ever
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks. Part A is contact and field forces, mass and weight, and the compass question. Part B is the weight against mass investigation with the graph
- Marking memorandum, including the three answers that look right and score nothing
Related practicals
- Static and kinetic friction, measured. Friction in full, with a proper investigation
- Magnetism and magnetic fields. Field lines, neutral points and how to plot them
- Electrostatic charge. Charging by friction, in detail
- Newton's second law, Grade 11. Where this leads: force, mass and acceleration together
What you need to run it
Force meters. That is the purchase, and a class needs six of them.
Our tubular force meters are R70 each in five ranges, with 229 on the shelf. The 10 N is the right one for 100 g to 1 kg, which is exactly what this practical uses, and the 20 N covers anything heavier. Six of them is R420 and it turns a demonstration into a class practical.
Do not buy a spring balance marked only in grams. The whole point is reading newtons, and a gram scale quietly turns the practical back into a definition.
Alnico bar magnets start at R195 a pair, and small compasses are R2 each, which is the cheapest useful thing in our catalogue. Six of them lets every group see a field direction instead of watching the teacher.
For the field pattern, a magnetic field demonstrator plate at R113 replaces iron filings. No mess, nothing stuck to the magnets afterwards, and it works every time. We do not stock iron filings, and having used both, the plate is the better answer.