# Pendulum measuring acceleration of car

This was originally posted in a non-homework forum and does not use the template.
I came across this problem which says" A pendulum is hung by roof of a train having mass 1 kg . The train then accelerates and pendulum makes an angle with vertical. What is acceleration of train? " Is it equal to gsinθ?

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Bystander
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See, you're understanding vectors, and which components mean what.

See, you're understanding vectors, and which components mean what.
So was I right. I searched for it over internet which says about using tan instead of sin but it seemed unreasonable to me
Thank you.

Bystander
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Gold Member
My bad. What you found about tan is correct. The gravitational force is perpendicular to the train's acceleration. The force acting on the pendulum mass is the vector sum of the gravitational force and the train's acceleration.
Mea culpa.

My bad. What you found about tan is correct. The gravitational force is perpendicular to the train's acceleration. The force acting on the pendulum mass is the vector sum of the gravitational force and the train's acceleration.
Mea culpa.
What did you mean by that. I want to learn which is correct gsintheta or gtantheta.

Bystander
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gtantheta.
The vector triangle to be examined has three sides: 1) the hypotenuse, which is the resultant of gravitational force on the pendulum, and the train's acceleration (pulling the mass toward rear of train); 2) the vertical leg, which is mg of the pendulum; and 3) the horizontal leg, which is atrainm. The relationship between the two legs of the triangle is "opposite" (horizontal) over "adjacent" (vertical), or tangent, since the angle of the resultant is measured from the vertical.

The vector triangle to be examined has three sides: 1) the hypotenuse, which is the resultant of gravitational force on the pendulum, and the train's acceleration (pulling the mass toward rear of train); 2) the vertical leg, which is mg of the pendulum; and 3) the horizontal leg, which is atrainm. The relationship between the two legs of the triangle is "opposite" (horizontal) over "adjacent" (vertical), or tangent, since the angle of the resultant is measured from the vertical.
Oh sorry, I have got your point. You are right. Thanks for correction.

The vector triangle to be examined has three sides: 1) the hypotenuse, which is the resultant of gravitational force on the pendulum, and the train's acceleration (pulling the mass toward rear of train); 2) the vertical leg, which is mg of the pendulum; and 3) the horizontal leg, which is atrainm. The relationship between the two legs of the triangle is "opposite" (horizontal) over "adjacent" (vertical), or tangent, since the angle of the resultant is measured from the vertical.
I mistook that first. g is not the resultant. It is component.

Bystander