A 4-kg block on a horizontal surface experiences forces of 7 N to the left and 3 N to the right. What is the acceleration and its direction?

A 4-kg block on a horizontal surface experiences forces of 7 N to the left and 3 N to the right. What is the acceleration and its direction?

• A. 0 m/s^2
• B. 1.0 m/s^2 to the left
• C. 1.0 m/s^2 to the right
• D. 4.0 m/s^2 to the left

Answer: (B)

The main idea is that acceleration comes from the net force acting on an object, and its direction is the same as the net force. Here, the block has a 7 N force to the left and a 3 N force to the right, so the forces don’t cancel. The net force is 4 N to the left. Using Newton’s second law, acceleration equals net force divided by mass: a = F_net / m = 4 N / 4 kg = 1 m/s^2. The direction follows the net force, so the acceleration is 1.0 m/s^2 to the left. Note: zero acceleration would require equal and opposite forces, which isn’t the case here. An acceleration of 1.0 m/s^2 to the right would require the net force to act to the right, which it does not. An acceleration of 4.0 m/s^2 to the left would need a net force of 16 N, not 4 N.

The main idea is that acceleration comes from the net force acting on an object, and its direction is the same as the net force. Here, the block has a 7 N force to the left and a 3 N force to the right, so the forces don’t cancel. The net force is 4 N to the left. Using Newton’s second law, acceleration equals net force divided by mass: a = F_net / m = 4 N / 4 kg = 1 m/s^2. The direction follows the net force, so the acceleration is 1.0 m/s^2 to the left.

Note: zero acceleration would require equal and opposite forces, which isn’t the case here. An acceleration of 1.0 m/s^2 to the right would require the net force to act to the right, which it does not. An acceleration of 4.0 m/s^2 to the left would need a net force of 16 N, not 4 N.