# heeeeeeeeeeeeeelp Physics

A 15 kg, 1m wide door which has frictionless hinges is closed but unlocked. A 400 g ball hits the exact middle of the door at a velocity of 35 m/s and bounces off elastically, thereby causing the door to slowly swing open. How long in seconds does it take for the door to fully open (rotate 90 degrees)?

1. The force exerted by the hinges has a torque of zero relative at the hinges. This means that angular momentum is conserved w.r.t. all point on the line between the two hinges. We also have conservation of energy. The moment of inertia of the door w.r.t. the rotation axis is:

I = 1/3 m L^2

where M = 15 kg is the mass of the door and L = 1 m is the width of the door.

Let's denoting the initial velocity of the ball in the forward direction (the direction where the door opens) by Vi, the velocity after it bpuncess off the door in the opposite direction as Vf. Let's put Mb = 400 g = mass of the ball and we put R = 1/2 m = radius at which the ball hits the door.

We then have:

Conservation of angular momentum:

Mb Vi R = I omega - Mb Vf R (1)

Conservation of energy:

1/2 Mb Vi^2 =
1/2 I omega^2 + 1/2 Mb Vf^2 (2)

From (1) it follows that:

Vf = I omega/(Mb R) - Vi

Inserting in (2) gives:

1/2 I Omega^2 +
1/2 Mb (I omega/(Mb R) - Vi)^2 =
1/2 Mb Vi^2

If you expand this out, you can divide bvy Omega (omega = 0 is an unphysical solution corresponding to the ball moving through the door without interacting with it). You then get:

Omega = 2 Vi/[R(1 + I/(Mb R^2))]

The time to rotate through 90 degrees is pi/(2 Omega).

posted by Count Iblis
2. hey,@ count ibis, don't u think your omega answers value is too big to open the door slowly,

posted by montes

posted by Anubhav
4. 1.68?

posted by steven lou
5. 0.57222??

posted by Anonymous

@- Count Iblis do you know the answer?

posted by pyhsiclover
7. I got the distance the door travels is pi/2 x 1m which is the radius since the door will travel in a circle, then since the momentum is conserved, the velocity of the door will be 14/15, and then pi/2 divided by 14/15 gives 1.68

posted by Anonymous
8. but 1.68 is wrong answer

posted by hemant
9. Finally 0.572 is correct

posted by Tony
10. its not

posted by Mathslover

## Similar Questions

1. ### Physics

A 15 kg, 1m wide door which has frictionless hinges is closed but unlocked. A 400 g ball hits the exact middle of the door at a velocity of 35 m/s and bounces off elastically, thereby causing the door to slowly swing open. How
2. ### Physics

A 15 kg, 1m wide door which has frictionless hinges is closed but unlocked. A 400 g ball hits the exact middle of the door at a velocity of 35 m/s and bounces off elastically, thereby causing the door to slowly swing open. How
3. ### Physics

A 15 kg, 1m wide door which has frictionless hinges is closed but unlocked. A 400 g ball hits the exact middle of the door at a velocity of 35 m/s and bounces off elastically, thereby causing the door to slowly swing open. How
4. ### Science

A 15 kg, 1m wide door which has frictionless hinges is closed but unlocked. A 400 g ball hits the exact middle of the door at a velocity of 35 m/s and bounces off elastically, thereby causing the door to slowly swing open. How

A 15 kg, 1 m wide door which has frictionless hinges is closed but unlocked. A 400 g ball hits the exact middle of the door at a velocity of 35 m/s and bounces off elastically, thereby causing the door to slowly swing open. How
6. ### pyhsic question for count iblis

PLEASE GIVE THE ANSWER,I ALREADY INPUT TO YOUR FORMULA AND IT IS FALSE... Is your answer 0.57? A 15 kg, 1m wide door which has frictionless hinges is closed but unlocked. A 400 g ball hits the exact middle of the door at a
7. ### Physics

There is a door that is 1.0 m wide and can rotate about a vertical axis without friction. The doorâ€™s mass is 15 kg and is free to swing open. Someone fires a 10 g bullet at 400 m/s into the exact center of the door,
8. ### Physics

A 10 g dart traveling at 400 m/s sticks into a 10 kg, 1 m wide door at the edge opposite the hinge causing the door to swing open. What is the angular velocity of the door immediately after impact?
9. ### Physics

A 10 g dart traveling at 400 m/s sticks into a 10 kg, 1 m wide door at the edge opposite the hinge causing the door to swing open. What is the angular velocity of the door immediately after impact?
10. ### Physics

A uniform 22 kg door that is 2.5 m high by 0.80 m wide is hung from two hinges that are 20 cm from the top and 20 cm from the bottom. If each hinge supports half the weight of the door, find the magnitude and direction of the

More Similar Questions