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## AP®︎/College Physics 1

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

Lesson 3: Centripetal forces

# Identifying centripetal force for ball on string

Identifying forces or force components acting as the centripetal force for a ball on a string moving in a horizontal circle.

## Want to join the conversation?

• What about friction in the first example?
• I'm not sure if this is right, so take this as a guess.
Friction acts opposite to the motion of direction. I believe that in this case, the force of friction would be acting perpendicular to the tension force. Because the force of friction acts perpendicularly to the tension force, friction would not be part of the net centripetal force as the force of friction does not act towards the centre of the circle but rather tangentially to the circle, in the direction opposite to the object's motion. Thus, since there is no force acting opposite to the force of friction, the ball would eventually come to a stop.
• At , what happens if the mass on a string is spun so fast that the force of tension only has the x component? What would counter mg?
• since Tension is always parallel to the rope, if you want to spin it faster, you increase the force you apply to the rope F_t. while you're interested in the F_tx increasing, you also increase F_ty which makes your ball go higher (since it is stronger gravity).
the tricky part though: mg does never change on the ball; the ball has the same mass and g is a constant, so what changes? Well, I did say that you increase the ball up, but doing so LOWERS the F_ty force (due to the angle between Ft and F_ty going up 90, which makes F_ty literally 0). A summary is that while you spin it faster, you need to spin it, even more faster to counter the constant mg force.
so this is one of the main reasons you can NEVER spin a ball mid-air keeping the ball as high as your hand, because then the force that counters mg is 0, and then drops down then until F_ty gets some angle
(1 vote)
• second example, shouldn't there also be a Fn counteracting Fg, not just the tension force?
• The object is hanging from the ceiling. It is not in contact with the ground. Therefore, there is no normal force.
• I have a question. Please help me understand why don't we have a force that would go in opposite direction from centripetal force? Is it because that one would be applied to the string and not to the object on it?
• Are you asking why we don’t have a centrifugal force? Think about Newton’s first law of motion: an object in motion stays in motion unless acted upon by an outside force. This includes direction; an object will not want to change direction unless it is acted upon by an outside force. So when you have a ball on a string, the ball wants to go straight. It’s the string providing the centripetal force that causes the ball to go in a circular path. We don’t need another force to go in the opposite direction, because that would cancel out the centripetal force, causing no acceleration. That doesn’t make sense, because the ball is accelerating (changing direction, therefore changing velocity)
Let me know if you need more help!
• At how do we know that the vertical component of the force of tension completely cancels the gravitational force? Their might be an additional normal force (which obviously is smaller than the gravitational force in magnitude)!
(1 vote)
• We know that the vertical component of the tension “cancels” the force of gravity because there is no acceleration in the vertical (y) direction. Since according to Newton’s second law, ΣF=ma, F_ty + F_g = m*a_y.

a_y = 0 m/s^2

F_ty + F_g = m*0
F_ty + F_g = 0
F_ty = -F_g
That last equation shows that the vertical component of the force of tension is equal in magnitude, but opposite in direction, to the force of gravity.

There isn’t a normal force because normal force requires contact with a surface. In that second example, the ball on the string is just moving through the air. It’s not on a table.

Does that help?
• 1) In the second example at of a string attached to the ceiling and a ball in the other end of it, in order for the ball to do a circular motion shouldn't we apply an external force?

2) Had we left the ball from that point, wouldn't it move like a pendulum (left and right)?

3) Also, is the force of tension in the horizontal axis called a centripetal force in a pendulum?
(1 vote)
• 1) When started studying the motion, the ball was already doing a circular motion so no external force was required due to inertia
2) If the ball is at rest and when released then it will move like a pendulum. However as it undergoes circular motion, the tension which would cause it to move left to right is used as centripetal force
3) In circular motion it is a centripetal force
• Isn’t gravity pulling the ball at an angle instead of straight down?
(1 vote)
• Are you talking about the second example? If so, the force of gravity always acts downward on an object, toward the earth.