The correct answer is: A. A point.
Kennedy’s theorem states that the instantaneous centers of rotation of three bodies in plane motion lie on a straight line. This line is called the instantaneous center of the system.
The instantaneous center of rotation of a body is the point about which the body appears to be rotating at a given instant. It can be found by drawing a line through the centers of mass of the two bodies and perpendicular to the line of action of the force between them.
The instantaneous center of the system is the point about which the entire system appears to be rotating at a given instant. It can be found by drawing a line through the instantaneous centers of rotation of the three bodies.
The instantaneous center of rotation is a useful concept in physics and engineering. It can be used to analyze the motion of complex systems, such as machines and vehicles. It can also be used to design machines and vehicles that are more efficient and safe.
Here are some additional details about each option:
- Option B: A straight line. This is the correct answer. Kennedy’s theorem states that the instantaneous centers of rotation of three bodies in plane motion lie on a straight line.
- Option C: Two straight lines. This is not the correct answer. Kennedy’s theorem states that the instantaneous centers of rotation of three bodies in plane motion lie on a straight line, not two straight lines.
- Option D: A triangle. This is not the correct answer. Kennedy’s theorem states that the instantaneous centers of rotation of three bodies in plane motion lie on a straight line, not a triangle.