2.1.4Interactions Over Small and Large Distances
Resolving Forces (HT Only) — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 9 parts Lesson 5 of 26
Part 1 of 7Why resolve a force? (HT only)
Forces at angles
- Forces at angles A force acting at an angle has an effect in two directions at once.
Components
- Components It can be replaced by two perpendicular forces, usually horizontal and vertical, which together have the same effect.
- Putting the two components back together as a right-angled triangle gives the original force.
Independence
- Independence The horizontal and vertical components act independently, so each can be dealt with separately.
- The vertical forces decide whether an object lifts or sinks, and the horizontal forces decide whether it speeds up or slows down.
Part 2 of 7Resolving by scale drawing (HT only)
1
Choose a scale, such as 1 cm for 10 N.
2
Draw the force as a line at the correct angle with the correct length.
3
Draw a horizontal line from the start and a vertical line from the end to form a right-angled triangle.
4
Measure the two shorter sides and convert using the scale.
5
The sides are the horizontal and vertical components.
6
Label each component with its size in newtons and its direction.
Part 3 of 7Worked example (HT only)
| Step | Working |
|---|---|
| Situation | A 50 N force pulls a sledge at 30° above the horizontal |
| Scale | 1 cm represents 10 N, so the line is 5.0 cm long |
| Draw | Line at 30° to the horizontal; complete the right-angled triangle |
| Measure | Horizontal side 4.3 cm, vertical side 2.5 cm |
| Convert | Horizontal component about 43 N, vertical component about 25 N |
| Meaning | 43 N drags the sledge forwards while 25 N lifts it slightly, reducing friction |
Part 4 of 7Adding forces at an angle (HT only)
1Draw forces head to tail
- Draw forces head to tail Place each arrow at the end of the one before, using scale.
2Close the triangle
- Close the triangle Draw from the start of the first arrow to the end of the last.
3Measure resultant
- Measure resultant Its length gives the size and the angle gives the direction.
Part 5 of 7Typical examples (HT only)
| Situation | How resolving helps |
|---|---|
| Pulling a suitcase with a handle at an angle | Only the horizontal part moves it forward; the vertical part lifts it |
| Object on a slope | Weight splits into a part along the slope and a part into the slope |
| Equilibrium at a point | Components cancel so the forces form a closed triangle |
| Boat pulled by two ropes | Add the force arrows head to tail to find the single force that would have the same effect |
Part 6 of 7Checking a scale drawing (HT only)
Accurate drawings
Sharp pencil and ruler
- Sharp pencil and ruler Thin, accurate lines reduce measuring errors.
Protractor angles
- Protractor angles Measure angles from the correct line and read the correct scale on the protractor.
Sensible answer
- Sensible answer A component can never be larger than the original force, so check the numbers make sense.
Part 7 of 7Exam technique for resolving forces (HT only)
1
Read the scale given in the question and write it next to your drawing.
2
Draw the longest line you can, within the space, because bigger drawings are measured with smaller percentage errors.
3
Mark each angle and each component clearly so the examiner can follow your working.
4
Give the final answer in newtons with a direction, allowing a small tolerance either way.
Key terms
| Term | Meaning |
|---|---|
| Resolving | Splitting one force into two perpendicular components. |
| Component | One of the perpendicular parts of a force. |
| Scale drawing | A drawing in which lengths represent forces in proportion. |
| Equilibrium | A state in which the resultant force is zero. |
| Resultant | The single force that replaces several forces acting together. |
Avoid these mistakes
1
Choosing a scale so large or small that measurements become inaccurate.
2
Forgetting to convert the measured length back to newtons.
3
Adding the two components arithmetically to get the original force; they are at right angles.
4
Drawing arrows tail to tail instead of head to tail when combining forces.
5
Measuring the angle from the wrong line so the components are swapped.
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