# Confused about which forces are external when Newton's Second Law is used

Thread moved from the technical forums, so no Homework Template is shown.
So I have a trolley of mass m that moves on a straight line.
A sphere of mass m, is attached on the trolley with a light string of length a and it is left to oscillate.

Just to give some idea of their positions:
r_trolley = xi
r_sphere = (x-asinθ)i - acosθj (θ is the angle between the string and the vertical - j axis)

Are the external forces the weight of the trolley, mass and the reaction betweeen the trolley and the rack/ground?

Lnewqban
Gold Member
Welcome!
Is the sphere oscillating under the trolley?
You determine the limits of the system and which forces are external or internal.
If the system to study is trolley-string-sphere, the weights and the reaction forces are external, while the tension of the string is internal.
If the system is only the trolley, then the string pulls externally from it.

Welcome!
Is the sphere oscillating under the trolley?
You determine the limits of the system and which forces are external or internal.
If the system to study is trolley-string-sphere, the weights and the reaction forces are external, while the tension of the string is internal.
If the system is only the trolley, then the string pulls externally from it.
Yes the sphere is hanging below the trolley.
I see, I have been trying to calcualte the rate of change of angular momentum (torque) using:
##\dot{\vec{h_{a}} + \dot{\vec{r_{a}}x p = Q,##

##\Q = sum_{n=i}^\n (r_{i} - r_{p}) x F_{i} ,##
PS:sorry for the equations, if someone can fix it , that wouldb ereally helpful (thank you n advance!)

F_i are the external forces of the system.
In the case that I take p as the combined momentum of the trolley and the sphere, does it mean that the exetrnal forces that I consider are the weights and the reaction force of the trolley?

Thank you!

• Lnewqban
haruspex
Homework Helper
Gold Member
2020 Award
In the case that I take p as the combined momentum of the trolley and the sphere, does it mean that the exetrnal forces that I consider are the weights and the reaction force of the trolley?

Thank you!
Weights and reaction force from the ground, yes. The latter may include a horizontal component.

• curiousPep and Lnewqban
We really need a system diagram and a Free Body Diagram (FBD) to see what is happening.

• Lnewqban
A.T.
Are the external forces the weight of the trolley, mass and the reaction betweeen the trolley and the rack/ground?
Which forces are external/internal depends on how you define the bodies for analysis. Which definition of bodies is sensible depends on the specific question being asked.

• jack action, curiousPep, Lnewqban and 1 other person
By considering the trolley, string and mass, the external forces are the weights and the reaction force but the weight and reaction force for the trolley have no effect in the cross product to callculate Q, since r_a - r_p (p is the centre of mass of trolley) is parallel to the direction of F_i (external forces)

haruspex