Horizontal component of Gravity???

Hi everyone. I'm currently studying physics and I'm using PMT notes. On the oscillations topic, I came across a weird quote that says:
"In a simple pendulum, the restoring force is provided by the horizontal component of gravity acting on the pendulum bob."

Can someone give me an explanation? Thank you!!

Reply 1

jnsmith

3

Original post by jia_jie0728

Hi everyone. I'm currently studying physics and I'm using PMT notes. On the oscillations topic, I came across a weird quote that says:
"In a simple pendulum, the restoring force is provided by the horizontal component of gravity acting on the pendulum bob."
Can someone give me an explanation? Thank you!!

it means the tangential component. the gravity force may be resolved into a force perpendicular to the string/rod (tangential), which is the 'restoring force', and a force parallel to the string/rod, which results in tension in the string/rod

Reply 2

Harik0

9

Original post by jia_jie0728

Hi everyone. I'm currently studying physics and I'm using PMT notes. On the oscillations topic, I came across a weird quote that says:
"In a simple pendulum, the restoring force is provided by the horizontal component of gravity acting on the pendulum bob."
Can someone give me an explanation? Thank you!!

surely this is referring to the horizontal component of tension. there are 2 (probably more, but this is afaik) derivations of pendulum motion.
(1). Suppose (up to a first order approximation for the amplitude being small), the pendulum is constrained to move horizontally. Then you can talk about the vertical component of tension balancing weight and the horizontal component being mg tan(theta), which is mg theta as an approximation, and hence show that omega = sqrt(g/l) (by subbing in x_horizontal = l sin theta = l theta to a first order approximation and getting a=g/l theta)
(2). You can also resolve perpendicular and parallel to the string, and say the parallel component of gravity is providing the restoring force. This is what is actually happening, but unless you do rotational dynamics, it is impossible (without deriving rotational dynamics results) to do actual derivations/calculations here.

So, if you have a qualitative question, either explanation will probably get the marks. (2) is more correct in that the explanation does not need a small angle approximation, but (1) will work.
If it is anything quantitative, unless you're confident enough about rotational dynamics and have faith in the examiner also knowing rotational dynamics, just go for the explanation in (1).

Reply 3

Eimmanuel

Study Forum Helper

15

Original post by jia_jie0728

Hi everyone. I'm currently studying physics and I'm using PMT notes. On the oscillations topic, I came across a weird quote that says:
"In a simple pendulum, the restoring force is provided by the horizontal component of gravity acting on the pendulum bob."

Can someone give me an explanation? Thank you!!

Not sure why they consider the restoring force to be the horizontal component of gravity.
You can watch how an ex-MIT lecturer "derives" the restoring force for a simple pendulum.
The "derivation" starts around 27:40. The lecturer also "explains" using a small angle approximation.

To aid you in understanding what other users have described, I show a diagram below.

Reply 4

Harik0

9

Original post by Eimmanuel

Not sure why they consider the restoring force to be the horizontal component of gravity.
You can watch how an ex-MIT lecturer "derives" the restoring force for a simple pendulum.
The "derivation" starts around 27:40. The lecturer also "explains" using a small angle approximation.

To aid you in understanding what other users have described, I show a diagram below.

yep, the video is the derivation i explained in (1)

This might confuse you further but I need to include it for completeness

Horizontal component of Gravity???

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