The Book on the Table — the Force You Cannot See
A pile of heavy books lies at rest on a table. Gravity is pulling on it the whole time, and it is not moving, so something must be pushing back exactly as hard. That something is the table, and the force it exerts is the normal force — perpendicular to the surface, and easy to forget precisely because nothing ever happens. Lean on the pile and watch the table match you newton for newton, then slide the table away and see what a body with a resultant force on it actually does.
Cara memakai simulasi ini
- Slide Books in the pile from one volume to three and watch both forces follow the weight
- Lean on the pile with up to 60 N and see the table's push grow by exactly that much
- Press Slide the table away to remove the support entirely
- Switch between eye level and a close view of the books with the camera bar
Yang perlu diamati
- The free-body diagram carries only external forces, each one labelled by its source
- With nothing else acting vertically the table's push is exactly the weight of the pile
- Press down and F_N rises above mg — the normal force is not a synonym for weight
- The resultant stays pinned at zero however the two forces change, and the pile stays still
- With the table gone the normal force disappears and the resultant is the whole weight
- The floor then takes over the table's job, and the pile is in equilibrium again
Fisika di baliknya
A body is in translational equilibrium when it stays at rest, or moves at constant velocity, and that happens exactly when the resultant of all the forces on it is zero: ΣF = 0. For the pile there are two external forces while it rests on the table — its weight F_g = mg acting downward through its centre of gravity, and the normal force F_N acting upward, at right angles to the surface. Setting ΣF_y = 0 gives F_N = mg. That equality is a consequence, not a definition: press down on the pile with an extra force and the balance becomes F_N = mg + F_hand, so the table now pushes harder than the books weigh. The normal force is whatever the contact has to supply to keep the sum at zero. Remove the contact and there is no normal force at all: the resultant becomes the whole weight, and the pile accelerates downward at a = g.