# Help mechanics

1. Dec 2, 2011

### drabdallh

if a mass is hangig from ceiling of a car the car starts to accelerate forward with acceleration greater than g the mass will rotate and then starts to accelerate forward wheil its in the same position making an angle with the vertical

my question what caused the rotation?
i know there is the tension of the rope but this not enough for rotation
and for a person outside the car no other force is there
yes the one inside say there is a force butits only inertia.
so what causes rotation

Last edited by a moderator: Dec 9, 2011
2. Dec 2, 2011

### anigeo

from theory of motion in a vertical circle the mass will complete the rotation if the velocity
at the the topmost point is $\sqrt{gr}$ here r is the length of the wire.the car starting with some acceleration imparted some acceleration on the mass also such that its velo at the highest point became$\sqrt{gr}$ so it was able to complete the rotation.
in short the force needed for rotation was provided by the car itself as the mass remains attached to the car

3. Dec 9, 2011

### drabdallh

yes the car will impart some its acceleration to the mass but the acceleration of th car is forward while the upside rotation of the mass is backward
so the rotaion cant be due to car acceleration

4. Dec 9, 2011

### Staff: Mentor

Once equilibrium is reached, both car and mass will have the same acceleration.
While the mass initially moves backward with respect to the car, viewed from the ground it always moves forward.

5. Dec 9, 2011

### chrisbaird

You get rotations when the total force on an object is not directed at its center of mass. In this case, the rope from which the object hangs must not be attached at the object's center of mass. If it was, there would be no rotation.

6. Dec 9, 2011

### technician

If I read this question correctly a car is accelerating forward and a mass hanging from the ceiling of the car is at an angle to the vertical.
There are 2 relevant forces acting on the mass
1) its weight acting vertically downwards.
2) the tension in the string
The mass will be accelerating with the acceleration of the car and the horizontal component of the tension in the string provides the resultant forward force on the mass.
If the angle of the string to the vertical is ∅ then TSin∅ is the force responsible for the acceleration of the mass. TSin∅ = ma
The vertical component of the tension (TCos∅) will equal the weight (mg) of the mass
These 2 equations give Tan∅ = a/g

7. Dec 9, 2011

### drabdallh

all what you said is correct still no answer to my question the mass will move forward yes but in the same time will rotate anti clockwise what caused this rotation???

8. Dec 9, 2011

9. Dec 9, 2011

### drabdallh

for more specificity it will not rotate i.e completely but it will get higher in a circular patter.
and to make the question more simpler
this motion to occur need 2 things the object to be moving and a centripital fore
my whole problem that the motion is not true it is relative on tothe car yet the object starts to rotate how come???

10. Dec 9, 2011

### Staff: Mentor

So your mass is like a lump of concrete with a hole bored horizontally through its centre, and supported on a frictionless axle located through this hole? It is the axle itself that is supported by the rope dangling from the roof?

Is that the picture?

11. Dec 10, 2011

### drabdallh

yes it is like serway problem page 161
n.b the pic in serway shows that there would only be a horizontal movement but mathematically impossible as the the tangent of the triangle should be the same lenght of the rope and this wouldnt be possible if it stays on the same horizontal level

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Last edited: Dec 10, 2011
12. Dec 10, 2011

### Staff: Mentor

The car accelerates, yanking the string along with it. The string doesn't stretch (much), its tension increases, giving an upward (and forward) force to the mass at its end.