Question

1) A food server is holding a 2 kg food tray as shown. Estimate the torque with respect to an axis through his shoulder exerted by the force of the tray on his hand. Explain.

[Note that I am not asking you for the net torque, just the torque from one force (the tray pushing on his hand).]

2)You can probably stand flatfooted on the floor and then rise up and balance on your tiptoes. Why are you unable to do this if you stand so your toes are touching a wall? (Try it!) Use physics from this chapter to explain.

3)When a ladder leans against a wall (and stays put) there are at least 4 forces on the ladder.

a) Describe them, including the object causing the force and the object the force acts on.

b) If you lean the ladder down too low it will slide and fall. Describe why this occurs in good physics terms

Answer 1

(1) I need to see the figure to answer this question and you have not shared one. Share the figure and I'll try to answer your question.

(2) When you are standing flatfooted on the floor and then rise up and try to balance the weight of your body on your tiptoes, you are actually positioning the centre of gravity of your body so that the entire weight is balanced on the tiptoes. In this case, your tiptoes are the base of support and base of support is exactly below your center of gravity, which provides the balance to the system and you are able to stand on your tiptoes. On the other hand, when you are touching a wall, it moves your base of support out from under your center of gravity. In order to stand on your toes, you must transfer the center of gravity forward. To transfer the center forward, you must lean over. The wall prevents you from doing this. Hence you can't stand on your tiptoes if you are touching a wall.

(3) (a) Four forces acting on the ladder, along with other desired information are given below

          N_G = Normal force on the ladder exerted by the ground

          N_W = Normal force on the ladder exerted by the wall

          F_G = Frictional force acting on the ladder because of surface of ground when ladder slips

          F_W = Frictional force acting on the ladder because of the surface of wall when ladder slips

(b)      Moment of forces F_G & F_W about point O is zero (as perpendicular distance is zero). When you lean the ladder down too low, the moment due to force N_G increases (as distance OB increases) and moment due to force N_W decreases (as distance OA decreases).

The net moment about point O is N_G (OB) - N_W (OA), which causes the ladder to fall down.