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My son and I were discussing tension in a handheld rope with a 10 lbm weight on the end. The hand exerts an upward force of 10 lbf and gravity exerts a 10 lbf downward (net = zero). With these vector forces acting in opposite directions they would appear to ADD to 20 lbf of tension in the rope rather than 10 lbf. Where is the conceptual fallacy? How can I explain this clearly? Asked Hecket Answer When you consider the two points where force vectors are drawn 1. At hand and 2. On the hanging weight Tension of 10 lbf is produced throughout the rope. See the diagram ## Relativistic travel and ageing

Today’s doubt- If i am moving at constant speed of 0.5 times speed of light away from another person, it means both mine and his frame of reference are inertial (since both are not accelerated), then in my frame that person is moving away from me at 0.5 times speed of light and by special theory of relativity, he would be younger than me. But in his frame of reference I am going away and I should be younger. Actually who is getting younger and why?

## Rotation of a ball in mid air

Hi, i’m working for a company where we use gyroscopes to measure the rotation of a ball in 3 dimensions. The output from the sensor is sent 25 times a second and is then converted to deg/s in each of the three axis’s. My task is to analyze the data and then create a program using it. I have some questions regarding this.

1. How do i find a good approximate of the total rotation of the ball in one number. The rotation change (not a lot but it makes things hard) in each of the axis’s while it’s airbourne, but when i use the following formula: sqrt((rotationX)^2 + (rotationY)^2 + (rotationZ)^2) i get almost zero total change of rotation. Is it correct, and why?

2. If i want to measure the total change of rotation between two consecutive samples, is it then correct to use this formula:
sqrt((rotationX.now-rotationX.last)^2 + (rotationY.now-rotationY.last)^2 + (rotationZ.now – rotationZ.last)^2). Why or why not?

Bonus: Does anyone know what or how the gyroscope measures?
Is it torque, just rotation in general or something else?

## Need help with this Physics question from electromagnetism

A 20.0 m long wire is carrying a 150.0 mA current parallel to a second wire that is extremely long which carries a current of 250.0 mA in the same direction. These wires are placed 40.0 cm apart in a 5.5 T magnetic field that is directed into the page. Calculate the magnetic force on each wire and indicate the direction of the force. Each of these wires would generate a smaller magnetic field, indicate the direction this magnetic field would travel between the wires.

Mark Dreeson

## Motion on a frictionless surface

Suppose a body is kept on frictionless surface(but gravity os there). If we give a little push to the body , will it continue in state of motion with constant velocity or not..?….if yes then why…?

If the surfaces are totally frictionless theñ on giving a slight push it would be put to motion with uniform velocity. Newtons first kaw is the reason. Since there is no net force acts, velocity should remain constant.
The force due to grsvity acts vrtically downwards and os canceled by the normal reaction

## Ideal gas laws and Physics of the atmosphere

How can you use the ideal gas law as a model to describe the physics of the atmosphere?

and

How does a environment satellite work? How correct are they? Normal Human Eyes can sense the visible spectrum only.