# Getting wrong answer on physics problem

#### srhly

A bead slides without friction around a loop-the-loop. The bead is released from height of 25.1m from the bottom of the loop-the-loop which has a radius of 9m. THe acceleration due to gravity is 9.8 m/s^2. There is a point A at the very top of the loop. I need to know the normal force(Fn) at that point if the mass is 3 g. I converted the 3 g to kg and then tried to solve. I set Fn-mg=0 so Fn=mg. So I took 0.003 and multiplied by 9.8m/s^2 but got the answer wrong.

Related Introductory Physics Homework Help News on Phys.org

#### gnome

srhly said:
I set Fn-mg=0 ...
Why?

F = mg gives you the force that gravity exerts on the bead. Why would you think that would be equal to the force that the loop exerts on the bead after the bead has accelerated down the track and halfway around the loop?

Use energy considerations to figure out the speed of the bead when it reaches point A.

Then use circular motion (radial acceleration) and gravity to determine the normal force necessary to keep the bead moving along its circular path.

Look at Logistics' question for more ideas: