Two bodies of mass M each are placed R distance apart. In another system, two bodies of mass 2M each are placed $${R \over 2}$$ distance apart. If F be the gravitational force between the bodies in the first system, then the gravitational force between the bodies in the second system will be

Light rays move in straight lines. But through an optical fibre, they can move in any type of zig-zag path because

Compared to audible sound waves, ultrasound waves have

A rigid body of mass 2 kg is dropped from a stationary balloon kept at a height of 50 m from the ground. The speed of the body when it just touches the ground and the total energy when it is dropped from the balloon are respectively. (Take, acceleration due to gravity, g = 9.8 m/s²)