Occupants in a free-falling plane can also experience near weightlessness. Because the plane and occupants are falling at the same rate, the occupants appear to float inside the plane. This is sometimes used to simulate space conditions.
FIG. 4.25 An astronaut is in a constant free-fall when weightless. (The feeling you get on a rollercoaster as you drop is the feeling they have all the time!)
Forces in lifts
You can have greater or lesser weight due to external forces. Imagine you are standing on weighing scales calibrated in newtons in a lift and have a mass of 50 kg (Fig. 4.26).
2 m s–2 2 m s–2 3 m s–1
a bc FIG. 4.26 External forces can cause you to have greater or less weight (a) When the lift is stopped, you have normal weight: 50 kg × 9.8 m s−2 = 490 N
(b) When the lift accelerates downwards, you have less weight: 50 kg × (9.8 − 2) m s−2 = 390 N
(c) When the lift accelerates upwards, you are pushed down into the floor and experience greater weight: 50 kg × (9.8 + 2) m s−2 = 590 N
(d) When the lift is moving with constant velocity, you have normal weight: 50 kg × 9.8 m s−2 = 490 N
Note that F = ma only applies to net forces. Before you calculate acceleration, make sure you have added or subtracted the vector forces to find the resultant net force. This is the F you use in the formula F = ma.
