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Falling objects and terminal velocity
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A falling object speeds up, but air resistance grows until the forces balance. Then it falls at a steady terminal velocity.
Forces on a falling object
Weight acts downwards. Air resistance (drag), a kind of friction, acts upwards, opposing the motion.
Friction is a force that always opposes motion.
Resultant force = weight − air resistance.
Friction is a force that always opposes motion.
Resultant force = weight − air resistance.
Reaching terminal velocity
1. At the start, speed is zero so there is no air resistance. The object accelerates at g.
2. As speed increases, air resistance increases, so the resultant force and the acceleration decrease.
3. When air resistance = weight, the resultant force is zero. The object falls at a constant speed: its terminal velocity.
2. As speed increases, air resistance increases, so the resultant force and the acceleration decrease.
3. When air resistance = weight, the resultant force is zero. The object falls at a constant speed: its terminal velocity.
Opening a parachute
Opening the parachute greatly increases the air resistance. It is now bigger than the weight, so the resultant force is upwards and the skydiver slows down.
As speed falls, air resistance falls, until it equals the weight again: a new, lower terminal velocity.
As speed falls, air resistance falls, until it equals the weight again: a new, lower terminal velocity.
A skydiver has a mass of 80 kg (weight 800 N). Air resistance is 300 N. What is her acceleration?
- Resultant force = 800 − 300 = 500 N downwards
- a = F ÷ m = 500 ÷ 80
Answer: 6.25 m/s2
Weight acts down; air resistance acts up and grows with speed. Acceleration falls as air resistance rises. At terminal velocity, air resistance equals weight and the resultant force is zero. A parachute raises air resistance, giving a lower terminal velocity.
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