# Does the ideal Pendulum slow down?

#### RetailBoy

I understand when wind resistance is involved it loses KE to the air molecules (also damping force and pulley friction).

But for ideal...
KE and PE are conserved (PEtoKEtoPE)
Momentum is not.

Does it slow down? and what causes it to? gravity?

#### chiddler

5+ Year Member
negatory. if ideal, oscillations last forever because there is nothing to stop it, as you wrote.

the following i'd like to be fact checked: momentum is not conserved if the system we are considering is the spring only.

#### MedPR

Removed
My guess is no. Gravity is what gives it PE = what makes it fall and accelerate and also what makes it slow as KE converts to PE. So everything gravity "takes" on the way up, it "adds" on the way down.

OP

#### RetailBoy

negatory. if ideal, oscillations last forever because there is nothing to stop it, as you wrote.

the following i'd like to be fact checked: momentum is not conserved if the system we are considering is the spring only.
p=mv

From /my/ understanding, when something stops, momentum cannot be considered conserved.

Like the pendulum and spring both stop at at max displacement/height and then change directions.

#### HipChick

Momentum of the SYSTEM is conserved unless some outside force acts upon the system (friction, air "resistance", etc) in absence of any outside force/torque, the ideal pendulum shouldnt slow down. Because there is conservation of energy, as you described, from PE to KE and back...

Doesnt whether it "slows down" depends on the distance from equilibrium(spring)? In a free swing pendulum, as the bob reaches the highest angle (max displacment) velocity slows to zero, thus the change from KE to PE. I'll break out real thinking in the morning.

At least that's my rationalization. But I'm so burned out I could be completely BSing.

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OP

#### RetailBoy

But there is a restoring force on the spring.

Not sure if it would be considered an external force of the system though

5+ Year Member

#### HipChick

Well thanks chiddler! I hate wording...

#### milski

##### 1K member
5+ Year Member
Provided there is no friction in the hinge and no air resistance, it will not slow done nor stop. As you said, energy is conserved and there is nothing to stop the continuous exchange KE<->PE.

Momentum is also preserved - you need to look at the correct system. In that case the force on the pendulum is exerted by the Earth, via the gravity force - you'll have to include it in the system when calculating the total momentum. There is a tension force from the attachment point as well, so that needs to part of the system too. At the end, you cannot calculate anything practical this way but you can rest assured that no momentum was lost (or harmed) during that experiment.

#### dmf2682

##### Membership Revoked
Removed
Well, if you mean slow down as in velocity decreases, yes. It slows down every oscillation. At Max height, v is zero.

But agree with everyone else in saying that in an ideal pendulum energy is conserved.

#### MedPR

Removed
Well, if you mean slow down as in velocity decreases, yes. It slows down every oscillation. At Max height, v is zero.

But agree with everyone else in saying that in an ideal pendulum energy is conserved.
Tricky, but good point.

#### Morsetlis

##### I wish I were a dentist
7+ Year Member
e = mc^2

The equation is conserved. Who cares about momentum? ENERGY is what makes the universe.