Final answer:
The direction of vector a⃗ when the pendulum is at position 1 depends on how the pendulum is oriented. Without more information, we cannot determine the exact direction of a⃗. However, considering the cross-product formula for angular momentum, we know that the vector will be perpendicular to the plane containing the position vector r and linear momentum vector p.
Step-by-step explanation:
The direction of the acceleration vector a when a pendulum is at position 1 can be deduced from understanding how pendulums move. At the highest point in its swing, which is presumably position 1, the pendulum is momentarily at rest before it begins to swing back down. At this point, the acceleration due to gravity acts directly downwards. Therefore, the direction of the acceleration vector a will be vertically downwards, towards the center of the Earth. This is because pendulums are acted upon by gravity, which provides the restoring force causing the pendulum to swing. Therefore, the direction of a⃗ when the pendulum is at position 1 will be perpendicular to the plane containing r and p.