Q161 · Practice questionComplex unfamiliar7 marks
QUESTION 161 (7 marks)
A bob moves in a horizontal circle on a string of length . The string maintains an angle of to the vertical. Ignore air resistance; use .
a)[2 marks]
Calculate the tension in the string.
b)[3 marks]
Determine the speed of the bob.
c)[2 marks]
Explain why a separate additional centripetal force must not be added to the force diagram.
WORKED SOLUTION
7 marksPractice marking scheme
ANSWER
(a) . (b) . (c) The inward net force is the horizontal component of tension; centripetal force is its role, not an extra interaction.
Worked solution
(a) The vertical component of tension balances weight: . Therefore .
(b) The radius is . The horizontal tension component is centripetal: . Equivalently, .
(c) The physical forces are tension and gravity. Their resultant points horizontally inward. The term centripetal describes this resultant, so adding an extra inward force would count the same force twice.
Equivalent physically justified methods and consistent equivalent units accepted.
Displayed decimals are model answers; accept appropriate significant figures and consistent rounding from stated constants.
Part a: Balance vertical tension with weight.
Part a: Calculate the tension.
Part b: Calculate the horizontal radius rather than using string length as radius.
Part b: Equate the inward tension component to centripetal force.
Part b: Calculate the speed.
Part c: Identify the real forces and their resultant.
Part c: Explain that centripetal force is the inward net force, not an additional force.
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusHow many marks did you earn?
Compare your working with the guide above.