If two equal external forces act on a block in opposite directions, what is the net external force and the resulting acceleration?

If two equal external forces act on a block in opposite directions, what is the net external force and the resulting acceleration?

• A. Net external force is zero; acceleration is zero.
• B. Net external force is 2F; acceleration is 2F/m.
• C. Net external force equals F; acceleration equals F/m.
• D. Net external force is undefined.

Answer: (A)

When two equal forces act on a block in opposite directions, they cancel each other out. The net external force is the vector sum of all forces, and equal forces in opposite directions add to zero. Newton’s second law says acceleration equals net force divided by mass, so a = F_net / m. With F_net = 0, the acceleration is zero. That means the block doesn’t change its motion: if it was at rest, it stays at rest; if it was moving, it continues moving with the same velocity and direction. The idea that the net force could be 2F or F would only arise if the forces didn’t cancel, and undefined would not apply here since the forces are well-defined and opposite.

When two equal forces act on a block in opposite directions, they cancel each other out. The net external force is the vector sum of all forces, and equal forces in opposite directions add to zero. Newton’s second law says acceleration equals net force divided by mass, so a = F_net / m. With F_net = 0, the acceleration is zero. That means the block doesn’t change its motion: if it was at rest, it stays at rest; if it was moving, it continues moving with the same velocity and direction. The idea that the net force could be 2F or F would only arise if the forces didn’t cancel, and undefined would not apply here since the forces are well-defined and opposite.