Checkpoint: force from a graph and joined boxes

A 5 kg cart has a flat velocity–time line at 3 m/s. What is the net force?

• 15 N
• 1.67 N
• 0 N

A 3 kg object gains 12 m/s in 4 s, from rest. What is the net force?

• 4 N
• 9 N
• 36 N

A cart moves forward. Its velocity–time line stays above zero and slopes down. What is the net force?

• It is zero
• It points against the motion
• It points the way the cart moves

A light string ties two boxes, 4 kg and 6 kg. The floor is smooth. A 20 N pull acts on one. What is a?

• 3.3 m/s²
• 2 m/s²
• 5 m/s²

Which force is inside the two-box system?

• Nothing, since every force is outside
• The outside pull F
• The pull of the string on each box