A lift hangs on a cable and moves upwards at a constant velocity. What is the resultant force on the lift?
AThe resultant acts upwards
BThe resultant acts downwards
CThe resultant keeps growing
DThe resultant force is zero
Answer & Solution
Correct answer: D. The resultant force is zero
1. Two vertical forces act on the lift, the tension in the cable upwards and gravity downwards.
2. Constant velocity means the acceleration of the lift is zero.
3. By Newton's second law, zero acceleration means the resultant force must also be zero.
4. The tension and the force of gravity are therefore equal in magnitude and opposite in direction.
5. It is a common mistake to think that a moving lift must have a net force on it; only an accelerating lift does.
6. A resultant that keeps growing would make the lift speed up more and more, which contradicts the constant velocity given.
_Source: Siyavula Physical Sciences Grade 11 (Everything Science, CC BY 4.0), Chapter 2: Newton's laws_
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