A box with mass of 50 kg is at rest on a horizontal surface. The coefficient of static friction is 0.8. (a) How much force do you need to apply horizontally to make it move? (b) If someone comes to help by applying an upward force of 50 N on the box, how much force do you need to apply horizontally to make it move? [Answer: (a) 400 N, (b) 360 N]