# Resultant force in vertical circular motion

• B
• Goliatbagge
In summary, the resultant force in a circular motion is always pointing towards the center if the motion is at a constant speed, but in non-uniform circular motion, the resultant force can point diagonally downwards in the leftmost and rightmost positions. This is because the component of the force perpendicular to the instantaneous direction of motion only changes the direction, not the speed. In this case, the component of the force parallel to the direction of motion changes the speed. This is different from uniform circular motion, where the resultant force always points towards the center at all times.

#### Goliatbagge

Suppose we have a vertical circular motion with gravity according to the image below.

In the leftmost and rightmost positions the resultant force is pointing diagonally down. Isn't the resultant force supposed to be pointing at the center at all times in a circular motion? What am I getting wrong?

Goliatbagge said:
Suppose we have a vertical circular motion with gravity according to the image below.

View attachment 293265

In the leftmost and rightmost positions the resultant force is pointing diagonally down. Isn't the resultant force supposed to be pointing at the center at all times in a circular motion? What am I getting wrong?
There's a difference between circular motion in this case and uniform circular motion (i.e. at constant speed).

vanhees71, Lnewqban and Goliatbagge
PeroK said:
There's a difference between circular motion in this case and uniform circular motion (i.e. at constant speed).
Ok, so let me get this straight. The resultant force in a circular motion is always pointing to the center if, and only if, the motion is at a constant speed. For example, if we are spinning a weight attached in a string vertically in a gravity field it will NOT have constant speed and therefore the rule does not apply.

Is this correct?

Goliatbagge said:
Ok, so let me get this straight. The resultant force in a circular motion is always pointing to the center if, and only if, the motion is at a constant speed. For example, if we are spinning a weight attached in a string vertically in a gravity field it will NOT have constant speed and therefore the rule does not apply.

Is this correct?
Yes. In general (this applies to any motion, in fact), the component of the force perpendicular to the instantaneous direction of motion changes only the direction (not the speed) and the component parallel to the direction of motion changes the speed.

Lnewqban and Goliatbagge

Goliatbagge
Love this animation! Thank you!

Lnewqban
Goliatbagge said:
Ok, so let me get this straight. The resultant force in a circular motion is always pointing to the center if, and only if, the motion is at a constant speed. For example, if we are spinning a weight attached in a string vertically in a gravity field it will NOT have constant speed and therefore the rule does not apply.

Is this correct?