# How Fast Must a Grasshopper Jump to Reach the Other End of a Straw?

• wrobel
In summary: If ##m<2M##, then the grasshopper must jump along the straw; if ##m>2M##, then the grasshopper must jump aside.
wrobel
Recently I encountered a funny thing. Consider the following problem from high school course.

A small grasshopper of mass ##m## sits on a tip of a straw. The straw is a thin homogeneous rod of mass ##M## and of length ##2l##. The straw lies on a smooth horizontal floor.
What smallest initial speed must have the grasshopper to jump to other tip of the straw?

As a rule, people solve this problem assuming that the grasshopper jumps along the straw. Nevertheless, for some range of ##m/M## this is not true: the smallest speed is attained when the grasshopper jumps sideways from the straw and the straw rotates under him.

:)

atyy and mfb
Jumping along the straw is nice and simple, but jumping to make the straw rotate brings in three coordinates that are all relevant. An interesting case.

bhobba
I got ##v0=\sqrt {2lg}##.

I felt compelled to solve it since I now teach at a high school.

osilmag said:
got v0=√2lgv0=\sqrt {2lg}.
Nope. The correct answer contains ##m,M## and for some range of ##m/M## it also contains nontrivial root of equation ## const\cdot\varphi=\sin\varphi##

LOL what?

Oh well, you didn't give an angle in your statement.

wrobel said:
the smallest speed is attained when the grasshopper jumps sideways from the straw and the straw rotates under him
Sure? If it can make the straw rotate, then it can make the straw slip underneath it, too, which shortens the necessary length of the jump.

Steelwolf
fresh_42 said:
Sure? If it can make the straw rotate, then it can make the straw slip underneath it, too, which shortens the necessary length of the jump.
Obviously. But the OP's point is that for some m/M values jumping non-parallel to the initial straw allows smaller speeds.

wrobel said:
2l2l2l. The straw lies on a smooth horizontal floor.
I assume this means friction-less?

wrobel said:
Nevertheless, for some range of m/Mm/Mm/M this is not true: the smallest speed is attained when the grasshopper jumps sideways from the straw and the straw rotates under him.
By "sideways" do you mean perpendicular to the straw, or at some non-zero angle? In either case the straw would rotate and translate.

A.T. said:
Obviously. But the OP's point is that for some m/M values jumping non-parallel to the initial straw allows smaller speeds.
I'm not convinced, i.e. I do not see how the question can be solved on the given data. How much speed is needed for a rotation in comparison to a straight slip?

fresh_42 said:
How much speed is needed for a rotation in comparison to a straight slip?
The OP states:
wrobel said:
The straw is a thin homogeneous rod
You can get the moment of inertial from that.

A.T. said:
I assume this means friction-less?
yes
A.T. said:
do you mean perpendicular to the straw, or at some non-zero angle?
I mean some non-zero angle, this angle is a root of equation above
A.T. said:
You can get the moment of inertial from that.
exactly
I have not kept my draft, if I remember correctly, for ##m\ge 2M## the grasshopper must jump aside, if ##m<2M## he must jump along the straw

A.T. said:
In either case the straw would rotate and translate.
yes it will rotate and translate

wrobel said:
if I remember correctly, for m≥2M the grasshopper must jump aside,
Interesting. Do you remember if the horizontal jump angle approaches some value like 90° when m/M approaches infinity?

pardon, ##\varphi## is an angle of straw's turn at the moment when the grasshopper landed on the tip.
I will restore my solution and put it here

here it is

#### Attachments

• thesolution.pdf
143 KB · Views: 328
Andy Resnick

## 1. What is the purpose of a grasshopper being on a straw?

The grasshopper may be using the straw as a resting place or as a way to elevate itself for better visibility or protection from predators.

## 2. Why do grasshoppers often perch on straws?

Grasshoppers are known for their ability to jump and fly, so perching on a straw allows them to have a quick escape route if needed.

## 3. Can grasshoppers survive on a diet of straw?

No, grasshoppers are herbivores and primarily feed on plants and grasses. They may occasionally nibble on a straw, but it does not provide enough nutrients for them to survive on.

## 4. Are grasshoppers harmful to straws?

Grasshoppers do not intentionally harm straws, but their sharp mandibles may accidentally cut or damage the straw while they are perching on it.

## 5. How long can a grasshopper stay on a straw?

This can vary depending on the individual grasshopper, but they can typically stay on a straw for a few minutes to several hours.

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