# Help Please - Ball swinging from a rope

Help Please -- Ball swinging from a rope

## Homework Statement

A ball of mass 30 kg is hanging from a rope of 3 meter tied to the roof of a truck. The truck is moving at a constant speed of 29.6 m/s. As you look forward at the ball hanging from the roof of the truck, you see the ball hanging to the right at an angle 45 from the horizontial.

1-What is the net force acting on the ball?

2-What is the radius of the curved path of the ball if one viewed the ball from overhead?

## The Attempt at a Solution

For part 1, I think you would just solve for the net force using marad but I just wanted confrim.

For part two, I have no idea where to even begin.

## The Attempt at a Solution

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Simon Bridge
Homework Helper

## Homework Statement

A ball of mass 30 kg is hanging from a rope of 3 meter tied to the roof of a truck. The truck is moving at a constant speed of 29.6 m/s. As you look forward at the ball hanging from the roof of the truck, you see the ball hanging to the right at an angle 45 from the horizontial.

1-What is the net force acting on the ball?

2-What is the radius of the curved path of the ball if one viewed the ball from overhead?
There appears to be information missing.
i.e. is the truck moving to the left or the right? Is the ball on a rope set up as a linear pendulum or circular pendulum or something else?

Since the truck is moving at a constant speed, there is no special reason for the rope to be hanging at any particular angle except 0deg (equilibrium due to gravity). In question 2, viewed from overhead, the path of the ball would not be curved at all.

The problem is mainly geometry - but you need to describe the situation.
Is there supposed to be a diagram?

haruspex
Homework Helper
Gold Member
There appears to be information missing.
i.e. is the truck moving to the left or the right? Is the ball on a rope set up as a linear pendulum or circular pendulum or something else?

Since the truck is moving at a constant speed, there is no special reason for the rope to be hanging at any particular angle except 0deg (equilibrium due to gravity). In question 2, viewed from overhead, the path of the ball would not be curved at all.

The problem is mainly geometry - but you need to describe the situation.
Is there supposed to be a diagram?
The truck is moving at constant speed, not constant velocity. If the rope hangs at a steady angle to one side, that tells us the trajectory of the truck.

Jbow615, I don't know what "marad" stands for, but yes, centripetal force is what you have to think about. What are the forces on the ball, and what is the relationship between their magnitudes?

Simon Bridge
Homework Helper
Oh that's the information missing... ?! <facepalm>
So the truck goes "forward" and the ball hangs "to the right" of the direction of travel.

I figured marad means $\small ma_{rad}$ ... i.e. the mass times the centripetal acceleration.

"Where to begin" would be a free-body diagram of the ball ... what are the forces? What do they have to sum up to?

Ok I finally figured it out last night. Thank you guys for your help.