Exploring Forces · Answer guide

Lesson 1 · Recognise what a force can change

Identify interacting objects and describe changes of speed, direction or shape.

1. A sponge becomes flatter when gently pressed. The observed effect is…

   Answer: a change of shape. Pressing deforms the sponge, so the observed effect is a shape change.

2. A force can change…

   Answer: speed, direction or shape. Forces can change motion or deform an object. Not every force causes all of these effects.

3. A book rests on a table. Which statement is correct?

   Answer: Earth pulls down and the table pushes up. The stationary book has gravitational and contact interactions; the forces can balance.

Discuss: Choose a safe everyday example such as opening a drawer or gently pressing a sponge. Name the two interacting objects, the object studied and the change observed.

Teaching note: An unmoving book still experiences Earth’s downward pull and an upward contact force from the table. Stillness alone does not mean no forces exist. Open responses need discussion; they are not automatically marked.

Lesson 2 · Use friction evidence fairly

Identify friction on a sliding object and plan a controlled comparison.

1. A block slides right across a stationary table. Friction on the block acts…

   Answer: left, opposing the sliding. In this stated situation, friction opposes the block’s relative sliding over the table.

2. Which comparison is fairer for testing surface effects?

   Answer: Same block and comparable starts on each surface. Keeping the object and starting conditions similar helps isolate the effect of the surface.

3. Why repeat stopping-distance measurements?

   Answer: To see variation and improve the evidence. Repeated trials show how variable the results are and reduce reliance on an unusual trial.

Discuss: Plan the records needed to compare two safe flat surfaces using a lightweight toy. State how you would keep starting conditions similar and repeat trials. A plan and supplied data are sufficient; do not use roads or high-speed objects.

Teaching note: Friction is not always opposite an object’s overall travel in every situation. Here we study a block sliding across a stationary table, where friction on the block opposes its sliding direction. Open responses need discussion; they are not automatically marked.

Lesson 3 · Distinguish forces that act across a gap

Use examples to separate magnetic, gravitational and electrostatic interactions.

1. Which interaction can occur without contact?

   Answer: A magnet attracting iron across a small gap. Magnetic attraction can act across a gap. The other listed interactions involve contact.

2. Two like magnetic poles facing each other tend to…

   Answer: repel. Like poles repel; unlike poles attract.

3. Which pairing correctly distinguishes mass and weight?

   Answer: Mass in kg; weight in N. Mass is measured in kilograms; weight is a force measured in newtons.

Discuss: Sort these situations and justify each: a hand pressing clay, Earth pulling a falling leaf, two magnets attracting across a gap, and a charged comb attracting paper. Use the descriptions; no electrical source is needed.

Teaching note: A charged plastic comb attracting paper is not the same interaction as a magnet attracting iron. Similar movement can have different causes. Open responses need discussion; they are not automatically marked.

Final check · Set 1

Try one set after the lessons. Use the other set after reviewing any errors.

1. A soft ball changes direction after touching a guide. Evidence supports…

   a force interaction with the guide. A direction change is an effect of a force interaction; it does not require a mass change.

2. Which is the symbol for the SI unit of force?

   N. The force unit is the newton, symbol N.

3. A block slides left on a stationary table. Friction on the block acts…

   right. Friction opposes relative sliding, so it points right in this case.

4. Changing block mass and surface together makes a comparison…

   harder to interpret because two factors changed. Two changing factors make it harder to identify the cause of a difference.

5. A charged comb attracts paper across a gap. The interaction is…

   electrostatic. The interaction involves electric charge and is electrostatic.

6. Which conclusion follows from one iron clip being attracted to a magnet?

   This clip shows magnetic attraction in the test. The observation supports a statement about the tested clip, not every material.

Final check · Set 2

Try one set after the lessons. Use the other set after reviewing any errors.

1. An unmoving book on a table…

   can experience balanced forces. Earth and the table exert forces even while the book stays still.

2. Pressing soft clay mainly demonstrates a possible force effect on…

   shape. The clay deforms: its shape changes under the applied interaction.

3. Which trial record is most useful?

   Surface, starting method, repeated distances and units. This record includes conditions and measurable repeated outcomes.

4. Friction on a sliding block is classified as…

   a contact force. The block and surface touch, so the friction interaction is a contact force.

5. Unlike magnetic poles facing each other tend to…

   attract. Unlike poles attract; like poles repel.

6. Weight describes…

   the gravitational force on an object. Weight is a force due to gravity and is measured in newtons.