Momentum carries a running history of parameter updates into later steps. In mini-batch SGD, where each batch gives a noisy estimate of the downhill direction, averaging recent steps can reduce sideways jitter and help progress across shallow plateaus. section 2.5.4
A hiker in fog
A hiker can see only a small patch of a mountain and may misjudge the downhill direction. Momentum acts like inertia: instead of reacting sharply to each new patch, the hiker carries recent steps forward, gliding through plateaus and avoiding some sideways jitter. section 2.5.4
AlexNet used momentum of 0.9 with mini-batch SGD and batches of 128 images. It was one part of a larger set of training choices—including ReLU activations, weight initialization, dropout, and data augmentation—that helped make deep learning work at scale. section 2.5.4 section 2.5.5





