A mother spins her son in a circle of radius R at angular speed wi. Then the mother pulls in her arms and spins
1
him in a circle of radius R with the same angular speed.
2
How does the magnitude of the child's centripetal
acceleration change when the mother pulls her arms
in?
Choose 1

A body has centripetal acceleration with magnitude a such that

a = v ² / R

where v is the body's tangential speed and R is the radius of the circular path the body takes.

Convert the child's angular speed ω into linear/tangential speed. Assume angular speed is measured in rad/s and tangential speed in m/s. For every 2π rad that he revolves around his mother, the child travels a distance of 2πR m, so that

ω = (ω rad/s) • (2πR/(2π) m/rad) = Rω = v

Then the child's acceleration is

a = (Rω)² / R = Rω ²

When the mother pulls her arms in, the distance R gets halved and changes to R/2, so that the child's new acceleration is

a = (R/2 • ω)² / (R/2) = (1/4 • (Rω)²) / (1/2 • R) = 1/2 Rω ²

so the child's centripetal acceleration decreases by a factor of 2.

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia