Olly_s15's Physics Question Thread

[olly_s15]

Okay, a bit stuck on this one.

A 4.0kg magpie flies towards a very tight plastic wire on a clothes line. The wire is perfectly horizontal and is stretched between poles 4.0m apart. The magpie lands on the center of the wire, depressing it by a vertical distance of 4.0cm. What is the magnitude of the tension in the string?

Thanks.

[QuantumJG]

The best I can think of is that the shape of the wire can be expressed as:



c, tension at the lowest point is represented as:

, note: a = 50 here!

I thought that λ could be represented as

c = 1 x 50 x 9.8 = 490N

Something tells me that my resoning is flawed!

Btw: Where did you get this question? 

[mark_alec]

The best I can think of is that the shape of the wire can be expressed as:

The wire will not be curved, it will be made up of straight line segments.

In order to calculate the tension, resolve the forces in the vertical direction.

[Mao]

All the forces acting on the magpie: there is gravity force downwards, and normal reaction force acting upwards, that is equal and opposite to gravity (40N)

Hence, the net force exerted by the string (tension) must be a 40N upwards force. Tension force must travel in the direction of the rope, hence, a force diagram can be constructed.

The rest should be fairly straight-forward, application of trigonometry. [1000.2N is the answer I got]

[/0]

The best I can think of is that the shape of the wire can be expressed as:

The wire will not be curved, it will be made up of straight line segments.

In order to calculate the tension, resolve the forces in the vertical direction.

I think (dunno for sure) that cosh models a wire hanging under only its own weight

[mark_alec]

http://en.wikipedia.org/wiki/Catenary

[olly_s15]

The correct answer is 1000N so Mao's method was correct. Thanks for the help.

[QuantumJG]

All the forces acting on the magpie: there is gravity force downwards, and normal reaction force acting upwards, that is equal and opposite to gravity (40N)

Hence, the net force exerted by the string (tension) must be a 40N upwards force. Tension force must travel in the direction of the rope, hence, a force diagram can be constructed.

The rest should be fairly straight-forward, application of trigonometry. [1000.2N is the answer I got]

So it doesn't curve but basically make that shape you showed?

It becomes much simpler now and getting 1000N is easy to find.

[Cthulhu]

All the forces acting on the magpie: there is gravity force downwards, and normal reaction force acting upwards, that is equal and opposite to gravity (40N)

Hence, the net force exerted by the string (tension) must be a 40N upwards force. Tension force must travel in the direction of the rope, hence, a force diagram can be constructed.

The rest should be fairly straight-forward, application of trigonometry. [1000.2N is the answer I got]

So it doesn't curve but basically make that shape you showed?

It becomes much simpler now and getting 1000N is easy to find.

When have you ever come across hyperbolic functions in 3/4 physics? and as that link from mark_alec explained, the first equation you gave was for a Catenary hanging under its own weight and doesn't really apply in this case.

[olly_s15]

A car of mass 1500kg traveling due west at a speed of 20m/s on an icy road collides with a truck of mass 2000kg traveling at the same speed in the opposite direction. The vehicles lock together after impact. Which vehicle experiences the greatest force.

Now from just looking at this, it seems the car experiences the biggest force (truck hitting car = owned car) but, I find that the truck experiences a greater impulse than the car.

Using the formula , i am assuming that the change in time is fixed for both vehicles and thus the vehicle experiencing the greatest impulse will experience the greatest force. Because:

Can anyone tell me if my reasoning is wrong  . I have a feeling it is. But it seems to make sense to me. Lol.

[superflya]

another way to look at it,
by finding the momentum of both the car and truck before and after the collision, u can work out which of the 2 has experienced a greater change in momentum. The one that has experienced the greater change in momentum will also experience a greater impulse.

correct me if im mistaken

[QuantumJG]

A car of mass 1500kg traveling due west at a speed of 20m/s on an icy road collides with a truck of mass 2000kg traveling at the same speed in the opposite direction. The vehicles lock together after impact. Which vehicle experiences the greatest force.

Now from just looking at this, it seems the car experiences the biggest force (truck hitting car = owned car) but, I find that the truck experiences a greater impulse than the car.

Using the formula , i am assuming that the change in time is fixed for both vehicles and thus the vehicle experiencing the greatest impulse will experience the greatest force. Because:

Can anyone tell me if my reasoning is wrong  . I have a feeling it is. But it seems to make sense to me. Lol.

Doesn't Newton's third law say that both experience the same force? Just an idea I threw out there.

[olly_s15]

another way to look at it,
by finding the momentum of both the car and truck before and after the collision, u can work it out which of the 2 has experienced a greater change in momentum. The one that has experienced the greater change in momentum will also experience a greater impulse.

correct me if im mistaken

Yes I've found the impulse of both car and truck and . But we're looking at which experiences a greater force. Not greater impulse.

[olly_s15]

A car of mass 1500kg traveling due west at a speed of 20m/s on an icy road collides with a truck of mass 2000kg traveling at the same speed in the opposite direction. The vehicles lock together after impact. Which vehicle experiences the greatest force.

Now from just looking at this, it seems the car experiences the biggest force (truck hitting car = owned car) but, I find that the truck experiences a greater impulse than the car.

Using the formula , i am assuming that the change in time is fixed for both vehicles and thus the vehicle experiencing the greatest impulse will experience the greatest force. Because:

Can anyone tell me if my reasoning is wrong  . I have a feeling it is. But it seems to make sense to me. Lol.

Doesn't Newton's third law say that both experience the same force? Just an idea I threw out there.

Hmm.. in a collision.. I'm not sure.

[Cthulhu]

.. dunno.