Question

forces that change motion quick check
during a game of pool, a cue ball travels to the left with 70 n of force and collides with the four ball moving with a force of 50 n to the right. if you assign a negative value to the force moving to the right, what is the net force of this system? (1 point)
○ 120 n
○ 20 n
○ 20 m/s2
○ -20 n

Explanation:

Step1: Define forces with signs

Left force (cue ball): \( +70\,\text{N} \) (assuming left is positive, right is negative as per problem)
Right force (four ball): \( -50\,\text{N} \)

Step2: Calculate net force

Net force \( F_{\text{net}} = F_1 + F_2 = 70 + (-50) \)
\( F_{\text{net}} = 70 - 50 = 20\,\text{N} \) (Wait, no—wait, the problem says "assign negative to right". Wait, cue ball is left (positive?), four ball is right (negative). Wait, let's recheck:

Wait, the cue ball is moving left with 70 N force. The four ball is moving right with 50 N force. We assign negative to right, so four ball's force is -50 N. Cue ball's force: direction left—if right is negative, left is positive? Wait, no—maybe the coordinate system: let's say right is negative, so left is positive. So cue ball force: +70 N (left), four ball force: -50 N (right). Then net force is 70 + (-50) = 20 N? Wait, but wait, maybe I mixed up. Wait, the problem says "assign a negative value to the force moving to the right". So force to the right is negative, force to the left is positive. So cue ball (left) is +70 N, four ball (right) is -50 N. Net force: 70 + (-50) = 20 N? But wait, the options have 20 N and -20 N. Wait, maybe I got the direction wrong. Wait, maybe the question is about the direction: if right is negative, then left is positive. Wait, no—maybe the forces are acting on the system. Wait, when two objects collide, the forces are the forces each exerts on the other? Wait, no, the problem says "the net force of this system". The system is the two balls. So the forces on the system: the cue ball has a force to the left (70 N), the four ball has a force to the right (50 N). But when considering the system, the internal forces cancel? Wait, no, maybe the problem is considering the forces as the applied forces? Wait, the problem statement is: "a cue ball travels to the left with 70 N of force and collides with the four ball moving with a force of 50 N to the right". So maybe these are the forces of their motion (i.e., the forces causing their motion). So to find net force, we sum the forces, with right as negative. So left is positive: 70 N (left), right is -50 N (right). So net force: 70 + (-50) = 20 N? But wait, the options include 20 N and -20 N. Wait, maybe I had the sign of the left force wrong. Wait, maybe the problem considers right as positive? No, the problem says "assign a negative value to the force moving to the right". So force to the right is negative, force to the left is positive. So cue ball (left) is +70, four ball (right) is -50. Sum: 70 - 50 = 20 N. So the net force is 20 N? But wait, let's check the options. The options are 120 N, 20 N, 20 m/s², -20 N. 20 m/s² is acceleration, so eliminate. 120 N is sum of magnitudes, but we need to consider direction. So 70 (left) and 50 (right): net force is 70 - 50 = 20 N to the left (since positive is left). So the net force is 20 N. Wait, but maybe the problem has a different sign convention. Wait, maybe "assign negative to right" means that forces to the right are negative, so the four ball's force is -50 N, and the cue ball's force: if left is opposite to right, maybe left is positive? Wait, no—maybe the force on the cue ball is to the left (70 N), and the force on the four ball is to the right (50 N). But when calculating net force on the system, we need to consider the forces acting on each object. Wait, no, the system is both balls, so the internal forces (the forces they exert on each other) cancel, but the problem is probably considering the external forces? Wait, the problem statement is a bit unclear, but the way it's phrased: "a cue ball travels to the left with 70 N of force and collides with the four ball moving with a force of 50 N to the right". So maybe these are the forces of their motion (i.e., the forces causing their velocity). So to find net force, we sum the forces, with right as negative. So left is positive: 70 N (left), right is -50 N (right). Net force: 70 + (-50) = 20 N. So the answer is 20 N.

Answer:

20 N (corresponding to the option "20 N")