Pseudo force in an accelerating train: hanging bob, sliding block and balloon
A train compartment accelerates forwards. A hanging bob swings back, a block on the floor may slip and a helium balloon leans forward. Switch between the ground frame and the train frame and see the real forces and the pseudo force −ma.
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About this experiment
| Subject | Physics |
|---|---|
| Class | 11 |
| Chapter | 4. Laws of Motion |
| NCERT section | 4.9 |
| Level | NCERT level |
| Time | about 15 minutes |
| Access | Free, no card needed |
Aim
To see why an observer inside an accelerating train feels a backward "pseudo force", and to use it to find the angle of a hanging bob, the slipping of a block and the lean of a balloon.
Theory
Newton's laws hold only in an inertial frame (the ground, or a train moving at constant velocity). In a frame that is itself accelerating with acceleration a, a free body appears to accelerate backwards with −a. To use F = ma inside that frame we add an imaginary force, the pseudo force: F = −ma, directed opposite to the acceleration of the frame.
Bob: seen from the ground the bob accelerates with the train, so the string tension has a horizontal part: T sin θ = ma and T cos θ = mg, giving tan θ = a/g. Seen from the train the bob is at rest, so the pseudo force −ma balances T sin θ. The answer is the same in both frames.
Block: on the ground the only horizontal force on the block is friction f = ma forwards, which is possible up to μmg. In the train frame f balances the pseudo force. If a > μg the block slips backwards relative to the floor.
Formulas used
- pseudo force F = − m a (opposite to the acceleration of the frame)
- tan θ = a / g
- tension T = m √(g² + a²)
- g_eff = √(g² + a²)
- block does not slip if a ≤ μ g
- friction on the block f = m a (static)
Procedure: what to do in the virtual lab
- Set a = 3 m/s² and the observer on the ground. Find the real forces on the bob (tension and weight) and check that their sum is m a forwards.
- Switch to the train frame. A new arrow appears: the pseudo force −ma. Check that all forces on the bob now add up to zero.
- Increase the acceleration until the block slips (a > μg). Reduce μ and see it slip earlier.
- Make a negative (braking): what happens to the bob, the block and the balloon?
- Read θ from the readouts and check tan θ = a/g.
Observations to make
- The bob swings backwards, the balloon leans forwards: they are mirror images.
- The angle depends only on a/g, not on the mass of the bob.
- The pseudo force exists only in the train frame: it has no reaction partner and no source body.
- The block slips when the required friction ma is more than the largest friction μmg.
What you can change in this experiment
- Acceleration of the train (+ forwards) (-6 to 6 m/s²)
- Friction coefficient between block and floor
- Observer: On the ground (inertial frame) / Inside the train (non-inertial frame)
Viva questions on Pseudo force in an accelerating train
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 is a pseudo force? Who exerts it?
- Why does a passenger lurch forwards when the bus brakes suddenly?
- Why does a helium balloon move forwards when a car accelerates?
- In what frames is Newton's second law valid?
Frequently asked questions
What is the aim of the Pseudo force in an accelerating train experiment?
To see why an observer inside an accelerating train feels a backward "pseudo force", and to use it to find the angle of a hanging bob, the slipping of a block and the lean of a balloon.
Which formulas are used in the Pseudo force in an accelerating train experiment?
pseudo force F = − m a (opposite to the acceleration of the frame); tan θ = a / g; tension T = m √(g² + a²); g_eff = √(g² + a²); block does not slip if a ≤ μ g
What viva questions are asked on Pseudo force in an accelerating train?
What is a pseudo force? Who exerts it? Why does a passenger lurch forwards when the bus brakes suddenly? Why does a helium balloon move forwards when a car accelerates? In what frames is Newton's second law valid?
How do I do the pseudo force class 11 experiment for Class 11 online?
Open the Pseudo force in an accelerating train 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.
Why does a pendulum tilt backwards in an accelerating train?
Seen from the ground, the bob must accelerate forward with the train, so the string tilts until the horizontal part of the tension equals m a. Seen from the train, a pseudo force −ma pushes the bob backwards. Both views give tan θ = a / g.
Why does a helium balloon move forward when a car accelerates?
The air in the car is pushed backwards by the pseudo force and piles up at the rear, so the pressure is higher there. The light balloon is pushed to the lower-pressure front, the opposite of what a heavy bob does.
When does a block slip on the floor of an accelerating cart?
When the pseudo force m a is larger than the maximum friction μ m g, that is when a > μ g.
Can I do the Pseudo force in an accelerating train 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. This lab is free.
Syllabus: Class XI Physics, Chapter 4 — Laws of Motion, §4.9 (Newton's laws and non-inertial frames; pseudo force in the exercises)
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: pseudo force · non-inertial frame · accelerating train · plumb bob · friction · balloon in a car · laws of motion
Read the full guide: Pseudo force.
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