EduNinja

Weight is a force

Weight is the gravitational force on a mass:

$F_g=mg$

The direction is toward the local gravitational field source.

Use local $g$

The value of $g$ depends on location. Use the value stated or the local field strength appropriate to the body’s position; mass does not change when the object is moved.

Common trap

Mass is measured in kilograms and is not a force. Weight is measured in newtons and can change when $g$ changes.

[ 1 ]

The probe is carried to the asteroid on board a spacecraft.

Calculate the weight of the probe when close to the surface of the asteroid.

Build the force model

Choose the system, draw a labelled free-body diagram, classify the interactions and resolve components. Apply Newton’s laws with the correct boundary: contact forces, field forces, friction, tension, buoyancy and restoring forces each have their own direction and conditions.

Track momentum

Use $ec p=m ec v$ , $ec J=\Delta ec p$ and momentum conservation only after checking external impulse. Distinguish elastic and inelastic collisions, explosions and energy transfer.

Track circular motion

The inward resultant provides $a_c=v^2/r=\omega^2r$ . It may come from tension, gravity, normal, friction or a field force. Angular and linear descriptions are linked by $v=\omega r=2\pi r/T$ .

Final checks

Ask: Which body is the system? Which forces are external? Is mass constant? Is acceleration uniform or radial? Is kinetic energy conserved, transferred or increased?

A.2.13—Weight

Calculate Weight from Mass

Retrieve the A.2 Forces and Momentum Model

A.2.13 Exam Analysis