Weight

Acceleration of Gravity Objects that fall to the Earth all experience an acceleration. The acceleration due to gravity is g = 9.8 m/s2. This acceleration must be due to a force.

Force of Gravity The acceleration of a falling mass m is -g. The force on the mass is found from F = ma (action). This gravitational force is F = -mg. Demonstration: drop ball and light object, compare acceleration to force Kinematic view Dynamic view

Gravity at Rest The force of gravity acts on all objects all the time. If an object is at rest, the law of inertia says that the net force is zero. There must be a force opposite to gravity that cancels it out. F = -Fgrav = mg Demonstration: set ball on table Fgrav = -mg

Normal Force The force that opposes gravity for objects on the ground is called the normal force. It is perpendicular (normal) to the plane of the ground. The force is a result of the law of reaction. FN = mg Fsb = -Fbs = mg Demonstration: push on table, push on wall.

Normal Force and Weight The normal force pushing up against gravity can be measured. We measure weight with a scale that measures normal force. Weight is a force, not a mass. Pounds measure weight, so force (not mass) can be measured in pounds. Equipment: spring scale

Accelerated Weight An elevator is accelerating downward at 2.0 m/s2. The person has a mass of 70 kg. What weight is on the scale? Add all the forces, but the net force is – ma = FN – mg. Solve for FN = m (g – a) The scale shows 56 kg.

Weightlessness If the elevator accelerated downward at g, the normal force would become 0. The person would feel weightless. An object in free fall is weightless, but not massless. Microgravity research at NASA Demonstration: link to NASA next