Recognising forces and drawing diagrams

Weight is not caused by the table.

The Earth exerts the gravitational force on the book. In the usual near-surface model it acts vertically downwards.

Use W=mg.

W=2.5×9.8=24.5 N.

Separate gravity from support.

No. Gravity still acts. The supporting normal contact force can be absent while the weight remains.

The book has no vertical acceleration.

N=W in this specified situation. The forces are opposite and their resultant is zero.

Use the surface geometry.

Perpendicular to the plane, directed away from the surface. It is not generally vertical.

Balance the vertical forces.

N=W+P, provided vertical acceleration is zero and these are the only vertical forces. Normal reaction is not automatically W.

Use relative sliding at the contact.

To the left along the floor.

Is there a tendency to slide?

No. With no horizontal sliding tendency in this simple situation, static friction can be zero. “Rough” means friction is available, not that it is always nonzero.

The relevant motion is at the contact.

Friction opposes relative slipping or its tendency between surfaces. For a foot pushing backwards on the ground, static friction from the ground can act forwards on the person.

Choose the lamp as the body.

Weight downward and tension upward. In this two-force stationary model their magnitudes are equal.

Consider compression.

A rod can transmit a pushing compressive force, often called thrust in this setting, as well as tension if the model permits. An ideal flexible string cannot push.

Use motion relative to the fluid.

Opposite the relative motion through the fluid. The relevant velocity is not always velocity relative to the ground.

Use the stationary two-force model.

The upward buoyant force equals the downward weight. This equality depends on the stated equilibrium conditions.

Separate propulsion from aerodynamic support.

Weight downward, lift upward, thrust forward and drag backward. With constant velocity in this idealised arrangement, opposite totals balance; lift and thrust are distinct.

Identify the receiver of that force.

No. It acts on the table. The corresponding contact force from the table on the book belongs on the book’s diagram.

Motion need not require a resultant in its direction.

No. Draw only identified interactions. A body can move at constant velocity with zero resultant force, or move right while a leftward resultant slows it.