CONCEPTS AT A GLANCE The restoring force of a spring may contribute to the net external force F that acts on an object. According to Newtons second law, the resulting acceleration a is directly proportional to the net force. The springs on his feet, his weight, and the normal force from the ground provide the net force that causes this fellows bouncing motion. ( Windsor & Wiehahn/Stone/Getty Images)

Figure 10.4 CONCEPTS AT A GLANCE The restoring force of a spring may contribute to the net external force SF that acts on an object. According to Newton’s second law, the resulting acceleration a is directly proportional to the net force. The springs on his feet, his weight, and the normal force from the ground provide the net force that causes this fellow’s bouncing motion. (© Windsor & Wiehahn/Stone/Getty Images)