Unit 2:Motion

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Center of Mass

The centre of gravity of an object is that point at which the entire mass may be considered to be concentrated . It is a point fixed relative to that object,but does not necesarily lie inside tha material of the object.If the object is located within the gravitational field created by a star,planet or other astronomical body,it also hasa center of gravity at which the entire weight of the body may be considered to be concentrated. The centre of gravity and centre of mass are the same point. The centre of mass helps us undestans when object will be stable or unstable.. An object has greatest stabiity when:-

  • It centre of mass is as low as possible
  • Its base is as wide as possible
  • its center of mass(gravity) is directly above or below the base of support.

By contrast,a high center of mass, a center of mass beyond the mass of support and a small base of support all contribute to the instability of the body.Many bodies such as a aball or meter stick,have sufficiet symmetry so there is an abvious geometrical center at which the center of mass lies .If the body is supported at the cetre of mass,the body will be in equilibrium and will balance. For example the center of mass of of a metre stick is at the 50-cm mark. If you support the stick at this mark,it will balance.If you support the stick at any other point,to the left or the right of the center of mass,it will rotate and fall.
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Linear inertia
A girl standing in a crowded bus grabs the rail as she lurches forward when the driver makes a sudden stop.It takes considerable effort to put in motion a stationary automobile. A book sits on your desk unread. A door stands ajar untill closed by someone or the wind. All these ojects possess linear inertia. If an object is not moving,a force is required to set it in motion.
The property of an object to resist change in its linear state of motion is called is called linear inertia
Rotational inertia A raw egg spun on the table top wobbles and soon stops while a hard-boiled egg spins rapidily. A top spins and remains upright untill its rotational speed decreases and finally topples . All these objects possess rotational inertia. If an object is rotating,a torque is required to stop it.If an object is not rotating, a torque is required to set it rotating.
The property of an object to resist change in its rotaional state of motion is called is called rotational inertia
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Velocity and Accelaration

Periodic Motion