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## AP®︎/College Physics 1

### Course: AP®︎/College Physics 1>Unit 4

Lesson 3: Centripetal forces

# Identifying force vectors for pendulum: Worked example

A worked example finding all force vectors acting on a pendulum moving in a horizontal circle.

## Want to join the conversation?

• Shouldn't the answer be D? He never mentioned that the air resistance was absent, and air resistance would push the ball in the opposite direction of its velocity as shown in the leftmost arrow of choice D.
• Usually for pendulums we ignore air resistance because the math would be too complicated.
• What about the normal force acting on the ball? Is it equal to the y-component of the force of tension or is the force of gravity equal to the sum of the magnitudes of the y-component of tension and the normal force?
(1 vote)
• The normal force itself is the force of tension. The ball wants to go in outwards direction so it actually pulls the string. The normal force that the string applies on the ball is the force of tension we are talking about.
• Why did not we choose D as there is a centrifugal force to the left?
• This is a very common misconception. There is no such thing as a centrifugal force. This is a fictitious force. The reason why it looks as if such a force exists is because of the objects inertia. When you are getting something to move in a circle, you are constantly changing its direction, but the object's inertia causes it to want to continue in a straight line. This resistance to a change in its motion is what looks like a centrifugal force but its really just inertia. Therefore, D cannot be the answer as no such force acting towards the left exists. Hope this helps!
• Is there centripetal force for pendulum-type structures? Why or why not?
• Yes, there is. When a pendulum is moving it is traveling at a speed in a path that is a section of a circle. Therefore, there must be a centripetal force. Note, that through the pendulum's path, the magnitude of the force changes due to the fact that the speed of the pendulum changes.
(1 vote)
• I have some doubts...when do we use centrifugal force? and what happens when there is friction...how does it affect the motion?
(1 vote)
• Wait... Why isn't the answer "D"? Won't there be Air Resistance? He never mentioned that the place is vacuum.
(1 vote)
• The question states constant speed meaning the ball isn't slowing down and therefore cannot be subjected to air resistance.
(1 vote)
• just.....but aren't components also forces? @
(1 vote)
• image is on 3D not 2D right? and ,the ball is moving is 3D right? yeah because you can see an arrow in the image that looks like rotation on the x and z

if so then why only 2D arrows....