Uniform circular motion vs. Orbital motion

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In TBR, it says that in uniform circular motion, velocity changes direction, speed is constant and thus there is a centripetal force. However, this force does no work on a circling object because F and v are perpendicular

However, with orbital motion, TBR says that KE, F, v, a and mv increase the closer an orbiting body is to the more massive object. If KE changes, doesn't this mean that work is done? How is this different from uniform circular motion?????


Thanks a lot!

 

[LoLCareerGoals]

Orbital motion is elliptical, not circular. And yes work IS being done. One of the TBR or EK1001 questions for that chapter in SN2ed schedule directly addresses that.

 

[Meredith92]

There is a greater force on the orbiting planet when it's close to the sun... Therefore it's acceleration/ velocity is greater when it's on that part of the orbit. The r is constant in regular uniform circular motion where's the r changes in elliptical motion which changes the force

 

[Meredith92]

Just came across a similar problem in TBR that got me confused #14 in chapter 4
The passage describes a situation in which a bullet is shot into a block that is hanging from a string.
Question 14. The work done on the block by the tension in the string is:
A 0
B T x L
C -T x L
D mg x L

The answer key says: at all points of the motion the tension is perpendicular to the direction of travel of the bullet-block system. a force can only do work on an object if some component of that force is parallel to the displacement. therefore the tension can do no work on the block

This seems to directly go against what we said about elliptical motion. the force is always perpendicular to the motion of the planet, so how can work be done??

 

[Meredith92]

There is a greater force on the orbiting planet when it's close to the sun... Therefore it's acceleration/ velocity is greater when it's on that part of the orbit. The r is constant in regular uniform circular motion where's the r changes in elliptical motion which changes the force

Okay I actually think I was wrong with this comment. In drawing out a picture, I realized that for most points in an elliptical orbit, the force is NOT perpendicular to the planet. So although F does change depending on radius.. which changes kinetic energy etc.. this isnt the main point.. the main point is that there IS a force parallel to motion so work can be done.

In the case with the pendulum motion i wrote about above, i got confused because there is a change in KE so you would think work would be done by the tension in the string. However, the string itself is always perpendicular to the motion. its GRAVITY that is doing the work because it has a force parallel to motion!

(love when the lightbulb goes of and things come together!)

 

[brood910]

Okay I actually think I was wrong with this comment. In drawing out a picture, I realized that for most points in an elliptical orbit, the force is NOT perpendicular to the planet. So although F does change depending on radius.. which changes kinetic energy etc.. this isnt the main point.. the main point is that there IS a force parallel to motion so work can be done.

In the case with the pendulum motion i wrote about above, i got confused because there is a change in KE so you would think work would be done by the tension in the string. However, the string itself is always perpendicular to the motion. its GRAVITY that is doing the work because it has a force parallel to motion!

(love when the lightbulb goes of and things come together!)

In that case, it is not elliptical. It is uniform circular motion. Think of a ball attached to a string that you spin. It doesnt spin in an elliptical path.

 

[Meredith92]

Right! Which means the force has to be perpendicular to the velocity ( which is not the case in elliptical motion) thanks!

 

[mehc012]

In TBR, it says that in uniform circular motion, velocity changes direction, speed is constant and thus there is a centripetal force. However, this force does no work on a circling object because F and v are perpendicular

However, with orbital motion, TBR says that KE, F, v, a and mv increase the closer an orbiting body is to the more massive object. If KE changes, doesn't this mean that work is done? How is this different from uniform circular motion?????


Thanks a lot!

Another way to think of it: KE increases the closer an orbiting body is to the more massive object...but in uniform circular motion, the orbiting body never gets any closer to the center object. Since KE never changes, work is not done for circular motion.

In an ellipse, the orbiting body gets closer and farther from the center mass as it travels around the orbit.