In unity physics plays a very important role .So to make the object rotate real and affected by force. We have to set the physics of unity 4 engine manually.If you are using unity 5 then things goes someway different.
Unity 4 Users:
To achieve the affect of rotating the object we need to set maxAngularVelocity of the objects with rigid body to 80 or above which is 7 by default.By Doing it all objects in a project , with rigid bodies gets maximum angular velocity to 80. Following are the steps to set max angular velocity in the project.
The Angular velocity of rigidbody is maintained by maxAngularVelocity in order to avoid numerical unstability with fast moving objects. As it may prevent from fast rotations, we can also override the angular velocity of individual rigid body. maxAngularVelocity is (0 to infinity) and default is 7.
Following are the steps to set maxAngularVelocity:
1.Go to the Menu Bar and select Edit.
2.Go to the Project Settings and select Physics option.
3.On the Inspector tab set Max Angular Velcoity to 80 or above which is 7 by default.
After doing this save the changes.Now you are good to go by applying torque to the rigid body.