Circular Motion Practice

Sample questions

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1. Sample 1difficulty 1/5

   An astronaut has mass $70~\text{kg}$ on Earth. On the Moon ($g \approx 1.6~\text{m/s}^2$), her <strong>mass</strong> is

   • A

     $112~\text{kg}$

   • B

     $70~\text{kg}$

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   • C

     $11~\text{kg}$

   • D

     $700~\text{kg}$

   Why

   Mass is a property of the object and does not change with gravity. Only weight ($m g$) changes.

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2. Sample 2difficulty 1/5

   A force is applied to a moving object opposite to its motion. Which must be true?

   • A

     Object speeds up.

   • B

     Object reverses immediately.

   • C

     Net force is zero.

   • D

     Object slows down.

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   Why

   Force opposite to motion produces deceleration; the object slows down (and may eventually reverse if force persists).

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3. Sample 3difficulty 2/5

   On a frictionless surface, blocks $A$ ($2~\text{kg}$) and $B$ ($3~\text{kg}$) are in contact. You push $A$ with a $20~\text{N}$ horizontal force so both accelerate together. What is the contact force that $A$ exerts on $B$?

   • A

     $20~\text{N}$

   • B

     $12~\text{N}$

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   • C

     $8~\text{N}$

   • D

     $4~\text{N}$

   Why

   System: $a = 20/5 = 4~\text{m/s}^2$. Isolating $B$: contact force $= m_B a = (3)(4) = 12~\text{N}$.

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4. Sample 4difficulty 2/5

   Two children pull on opposite ends of a rope, each with $200~\text{N}$. The rope is in equilibrium. What is the tension in the rope?

   • A

     $200~\text{N}$

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   • B

     $100~\text{N}$

   • C

     $0$

   • D

     $400~\text{N}$

   Why

   Tension at any cross-section equals the force pulling at either end: $T = 200~\text{N}$. (You don't add the two pulls.)

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5. Sample 5difficulty 2/5

   A car of mass $1200~\text{kg}$ rounds a horizontal curve of radius $40~\text{m}$ at $10~\text{m/s}$. What centripetal force does friction provide?

   • A

     $12{,}000~\text{N}$

   • B

     $1500~\text{N}$

   • C

     $3000~\text{N}$

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   • D

     $300~\text{N}$

   Why

   $F_c = m v^2/r = (1200)(100)/40 = 3000~\text{N}$.

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