On a cold day, Ming rubs her hands together to warm them up. She presses her hands together with a force of 4.1 N. Each time she rubs them back and forth they move a distance of 16 cm with a coefficient of kinetic friction of 0.44. Assuming no heat flow to the surroundings, after she has rubbed her hands back and forth eight times, by how much has the internal energy of her hands increased?