Hint
Choose and retain a positive direction; distinguish signed quantities from magnitudes. Identify the named recipient and the specified positive direction.
Worked solution
(a)
Identify the named recipient and the specified positive direction.
The specified force is 12 N to the right.
Answer: \(12\) N
(b)
The original force acts on B from A. Exchanging the bodies gives a force on A from B with the same magnitude and opposite direction. Its right-positive component is -12 N. These two forces belong in different single-body diagrams.
Answer: -12 N on the right-positive axis, i.e. 12 N left, acting on A and exerted by B.
(c)
Apply the supplied gravitational acceleration to the block mass.
The weight is \(\displaystyle Mg=30 N\) downward.
Answer: \(30\) N
Maths4U indicative marking guidance
Maths4U indicative marking guidance, not an official mark scheme. Credit complete valid alternative methods and consequential reasoning. An unexplained result does not earn a method mark.
(a) · 2 marks
- 1 mark: Identify the named recipient and the specified positive direction.
- 1 mark: Give the correct requested result, retaining signs, units and any domain restrictions.
Alternative methods: Any complete mathematically valid alternative method with the same restrictions and equivalent exact result is acceptable.
Common error: A third-law pair acts on different objects.
(b) · 3 marks
- 1 mark: Identify the force recipient and source in the reversed interaction.
- 1 mark: Give equal magnitude and opposite direction, retaining the signed component on the stated axis.
- 1 mark: Give the complete requested result, using the required units, precision and boundary inclusion.
Alternative methods: Any complete mathematically valid alternative method with the same restrictions and equivalent exact result is acceptable.
Common error: Weight and normal reaction on the same block are not an interaction pair.
(c) · 2 marks
- 1 mark: Apply the supplied gravitational acceleration to the block mass.
- 1 mark: Give the correct requested result, retaining signs, units and any domain restrictions.
Alternative methods: Any complete mathematically valid alternative method with the same restrictions and equivalent exact result is acceptable.
Common error: A third-law pair acts on different objects.
Total: 7 marks.