That's because the hand is only exerting a touching force - and it is not touching block B. Note the interesting result that the force exerted by the hand does NOT appear in this diagram. The result is the diagram shown at the right. It also has a weight and the table must be holding it up. Since it's touching the table there could be (must be) a friction force acting on it from the table. So there must be another force acting on it to balance it. It feels object A pushing it from the left to the right but it isn't changing its velocity from 0. The net force on object A is the sum of all these forces and this is what goes into N2. That's because in a free-body diagram for A only forces felt by object A belong. Note the very useful and important point!Įvery force that appears in the free-body diagram for A has its "cause-feels" label pair end in A. The final free-body diagram for A is shown above. Since it is pushing into the box perpendicular to the surface, it is a normal force. Since it isn't changing its vertical velocity, there has to be an upward force from the table to balance it. We only know that the normal force from box B and the friction force from the table that push us to the left must balance the force of the hand pushing us to the right, since our velocity is not changing (from 0).Īs for up-down forces, we know that the object has a weight - it's being pulled down by the earth. Note that we don't know how big the friction force is - it might even be 0. Since that force would be parallel to the surfaces touching (box A and table) it is a friction force. So there might be a force from the table holding us back. So even if there were not a box B we might not start to slide. We know that has to be the case from N2! If there were no force back on box A from box B, there would be an unbalanced horizontal force pushing right and box A would begin to speed up. (In physics and math, "normal" just means "perpendicular", so a "normal force" is a contact force that is perpendicular to the surface.) Because we are being pushed into box B, box B is pushing back on us from the right toward the left. "Thinking like the box", we can see that the hand is pushing into the box from the left toward the right - a normal force. It's being touched by the hand, box B, and the table. So both boxes are at rest.įirst consider box A. The hand is pushing but not hard enough so that the boxes begin to slide. Suppose a hand is pushing a box that is sitting next to another box. label each force with our labeling convention so we can keep track of which force is which.draw arrows starting at the dot in the direction of the forces, with lengths indicating the approximate relative size of the forces.identify all the non-touching forces acting on our object ( W, E, M).decide what kind of touching forces these objects exert on our object ( N, T, f).Explicitly, for each object in the system being considered, we In this, we isolate one object at a time and look for the forces acting on it. Each object satisfies its own N2 response equation.Ī process that helps us disentangle all the different forces in a complex situation is the free-body diagram.Each object only feels forces acting on itself (N0).This allows us to keep careful track of both Newton's Zeroth Law ("N0") and Newton's Second Law ("N2"). In order not to get confused about which forces go where, we introduce a free-body diagram. We will be considering many cases that include many objects.
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