3.3 Falling and turning Force is a vector quantity – it has direction as well as magnitude. That is why we represent forces using arrows. In this section, we will look at two situations where we have to think carefully about the directions of forces. ◆ When an object is falling, two forces act on it, one downwards and the other upwards. ◆ When an object is changing direction, a force acts on it sideways, pushing it round the corner.
Falling through the air
unbalanced, and he slows down. The idea is to reach a new, slower, terminal velocity of about 10 m/s, at which speed he can safely land. At this point, weight = drag, and so the forces on the parachutist are balanced. The graph in Figure 3.10 shows how the parachutist’s speed changes during a fall. ◆ When the graph is horizontal, speed is constant and forces are balanced. ◆ When the graph is sloping, speed is changing. The parachutist is accelerating or decelerating, and forces are unbalanced.
The Earth’s gravity is equally strong at all points close to the Earth’s surface. If you climb to the top of a tall building, your weight will stay the same. We say that there is a uniform gravitational field close to the Earth’s surface. This means that all objects fall with the same acceleration as the ball shown in Figure 3.6, provided there is no other force acting to reduce their acceleration. For many objects, the force of air resistance can affect their acceleration. Parachutists make use of air resistance. A free-fall parachutist (Figure 3.8) jumps out of an aircraft and accelerates downwards. Figure 3.9 shows the forces on a parachutist at different points in his fall. At first, air resistance has little effect. However, air resistance increases as he falls, and eventually this force balances his weight. Then the parachutist stops accelerating – he falls at a steady rate known as the terminal velocity. Opening the parachute greatly increases the area and hence the air resistance. Now there is a much bigger force upwards. The forces on the parachutist are again
air resistance
weight
Figure 3.9 The forces on a falling parachutist. Notice that his weight is constant. When air resistance equals weight, the forces are balanced and the parachutist reaches a steady speed. The parachutist is always falling (velocity downwards), although his acceleration is upwards when he opens his parachute.
parachute opens Speed
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Figure 3.8 Free-fall parachutists, before they open their parachutes. They can reach a terminal velocity of more than 50 m/s.
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Cambridge IGCSE Physics
Time Figure 3.10
A speed–time graph for a falling parachutist.
Original material © Cambridge University Press 2014
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