Coin on a rotating turntable: friction, centrifugal pseudo force and slipping
A coin sits on a turntable. Raise the speed or the radius until it slips and flies off along the tangent. See the same event from the ground (friction gives the centripetal force) and from the rotating turntable (centrifugal pseudo force).
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About this experiment
| Subject | Physics |
|---|---|
| Class | 11 |
| Chapter | 4. Laws of Motion |
| NCERT section | 4.9, 4.10 |
| Level | NCERT level |
| Time | about 15 minutes |
| Access | Included in a PractiSci plan |
Aim
To see what keeps a coin moving in a circle on a turntable, when it slips, and how the same situation is described by an observer on the ground and by an observer on the rotating turntable.
Theory
Ground frame (inertial): the coin moves in a circle, so it needs a centripetal force m ω² r towards the axis. The only horizontal force on it is static friction f, so f = m ω² r. Friction has a limit: f ≤ μ m g. The coin stays on the turntable as long as ω² r ≤ μ g.
When ω² r > μ g the friction cannot supply the centripetal force: the coin slips and, with no net force, flies off in a straight line along the tangent (Newton's first law). It does not fly out radially. The maximum speed is ω max = √(μ g / r), so a coin farther from the axis slips at a smaller ω.
Rotating frame (non-inertial): for an observer sitting on the turntable the coin is at rest, so the forces must add to zero. Friction points inwards, so a pseudo force of size m ω² r, the centrifugal force, must point outwards. It is not caused by any body and it exists only in the rotating frame. When it exceeds μ m g, the coin moves outwards in this frame, along a spiral.
Formulas used
- centripetal force F = m ω² r = m v² / r
- condition for not slipping: ω² r ≤ μ g
- ω max = √(μ g / r), v max = √(μ r g)
- centrifugal pseudo force F = m ω² r (outwards, rotating frame only)
Procedure: what to do in the virtual lab
- Set ω = 3 rad/s and r = 1 m with μ = 0.4. Check that ω² r = 9 is more than μg = 3.9, so the coin slips. Reduce ω until it just stays.
- Read ω max from the readouts and check ω max = √(μg / r).
- Increase r with ω fixed: see that the coin slips farther from the axis first.
- Switch to the rotating frame and compare the two arrows (friction and centrifugal) when the coin is at rest.
- Watch what happens to the coin when it slips, in the two frames.
Observations to make
- The coin leaves along the tangent, not along the radius.
- In the rotating frame the coin at rest has two equal and opposite arrows; in the ground frame there is only one, the friction.
- A smaller μ or a larger r makes it slip at a lower ω.
What you can change in this experiment
- Angular speed of the turntable ω (0 to 6 rad/s)
- Distance of the coin from the axis r (0.2 to 2.2 m)
- Coefficient of friction μ
- Observer: On the ground (inertial frame) / On the turntable (rotating frame)
Viva questions on Coin on a rotating turntable
Questions an examiner may ask in the Class 11 physics practical. Run the experiment in the lab to find the answers yourself, then check them in the Learn tab.
- What provides the centripetal force for a coin on a turntable?
- In which direction does the coin fly when it slips? Why?
- What is the centrifugal force? Is it a real force?
- What happens to the maximum speed if the radius is made four times as large?
Frequently asked questions
What is the aim of the Coin on a rotating turntable experiment?
To see what keeps a coin moving in a circle on a turntable, when it slips, and how the same situation is described by an observer on the ground and by an observer on the rotating turntable.
Which formulas are used in the Coin on a rotating turntable experiment?
centripetal force F = m ω² r = m v² / r; condition for not slipping: ω² r ≤ μ g; ω max = √(μ g / r), v max = √(μ r g); centrifugal pseudo force F = m ω² r (outwards, rotating frame only)
What viva questions are asked on Coin on a rotating turntable?
What provides the centripetal force for a coin on a turntable? In which direction does the coin fly when it slips? Why? What is the centrifugal force? Is it a real force? What happens to the maximum speed if the radius is made four times as large?
How do I do the centrifugal force class 11 experiment for Class 11 online?
Open the Coin on a rotating turntable virtual lab on PractiSci, follow the procedure above, change the values and read the live graph. It matches the Class 11 physics practical, with the aim, theory, observations and viva questions on this page.
At what angular speed does a coin slip off a turntable?
When ω² r exceeds μ g, so ω_max = √(μ g / r). A coin placed farther from the centre slips at a lower speed.
Does a coin fly off radially or along the tangent?
Along the tangent. Once friction can no longer supply the centripetal force, no force acts sideways, so by Newton's first law the coin moves in a straight line.
Is centrifugal force real?
No. It is a pseudo force seen only by an observer rotating with the turntable. From the ground only friction acts on the coin, and it is the centripetal force.
Can I do the Coin on a rotating turntable experiment online?
Yes. Open it as a 3D virtual lab in your browser or on your phone, change the values and read the results from live graphs. It is part of a PractiSci plan.
Syllabus: Class XI Physics, Chapter 4 — Laws of Motion, §4.10 (circular motion, motion of a car on a level road)
What you do in the lab
Change the controls and watch the physics behave in real time in 3D. Live readouts and graphs show the numbers behind it, and the Learn tab has the theory, procedure, observations and viva questions. Works on a phone or a laptop, in English and Hindi.
Topics: centrifugal force · pseudo force · rotating frame · circular motion · friction · banking · laws of motion
Read the full guide: Pseudo force.
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