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Forces - Forces and Terminal Velocity

This GCSE Physics quiz challenges you on forces and terminal velocity. When you run as hard as you can, you soon reach a point where you can run no faster. This is your terminal velocity (or terminal speed if you are changing direction). Terminal velocity is the maximum velocity an object can reach and it depends on the magnitude of the resultant force - when it becomes zero, terminal velocity is reached. But what is it that reduces a resultant force to zero? When you are running, it is air resistance.

For the GCSE, you are expected to know that the faster an object moves through a fluid, the greater the frictional force that acts on it, as well as the speed changes of an object falling through a fluid due to the force of gravity.

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The latter could cover all sorts of things like dropping a marble into a tube filled with golden syrup, how birds of prey increase or decrease their terminal velocity or free-fall skydiving! One of the general skills you need for GCSE physics is the ability to interpret graphs, including velocity-time graphs for objects that reach terminal velocity. On such a graph, a gradient of zero following a period of acceleration indicates that terminal velocity has been reached.

In everyday language, people usually use the word fluid to mean something that is a liquid. In science, it just means a substance capable of flowing, therefore gases (including the air) are fluids too. So, back to running. The faster that you run, the greater the friction of your body with the air (we call this friction air resistance). When the force of air resistance matches the force that your legs can create, the resultant force is zero, so you don't accelerate any more.

Similar thinking can be applied to vehicles. One of the facts about a car that interests some people is the top speed. This is limited by the driving force that the motor can create. When the speed of the car reaches the point where air resistance is equal to the driving force, the car will go no faster. Manufacturers of fast cars keep the mass as low as possible and make the engine generate as much power as it can.

When falling towards earth, parachutists reduce terminal velocity by greatly increasing their surface area. When they jump out of a plane or helicopter, they experience a period of free-fall. During this time, their parachute is not deployed and they simply fall through the air, pulled towards the surface of the Earth by gravity. At the start of the jump, the force of gravity is much larger than the air resistance, so the parachutist accelerates rapidly. As air resistance increases with the increase in speed, the acceleration gradually slows to zero as air resistance and gravitational forces become equal. The parachutist has reached terminal velocity at that point. When the parachute is opened, this presents a greater surface area to the air and therefore the air resistance suddenly increases. This causes a rapid deceleration and so the air resistance force falls again (remember slower speeds mean less air resistance). When it has dropped to the point where it equals the pull of gravity on the parachutist, deceleration stops and the parachutist descends with a much reduced terminal velocity.

For a free-falling human being approaching the Earth's surface, terminal velocity is about 120 miles per hour. In 2012, the first human to free-fall faster than the speed of sound was Felix Baumgartner. He jumped from a balloon at a height of 39,000 metres and his highest free-fall speed was about 880 miles per hour. The air is much thinner at such heights so air resistance is much lower.

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  1. If an object is placed into a fluid and released on Earth, what will initially happen to the object?
    Initially, the force of gravity is greater than the resistance force from the fluid
  2. What is weight measured in?
    Weight is measured in newtons, whilst mass is measured in kilograms
  3. Which statement is true for an object which is travelling through a fluid?
    Fluids are liquids and gases
  4. An astronaut travels to another planet. On this planet she records that she has a weight of 400 N. She knows from measurements back at the space centre that she has a mass of 72 kg. What is the strength of gravity on this new planet?
    Rearranging the equation to g = W/m and substituting with the figures gives you the correct answer
  5. What is the speed of an object known as when the force due to gravity and the frictional forces acting on the object are equal?
    All objects travelling through a fluid have a terminal velocity
  6. What is the weight of a person on Earth if their mass is 65 kg?
    Unless you are told otherwise, assuming that the Earth's gravitational field strength is 10 N/kg is acceptable for the GCSE. In everyday language, people often refer to their weight in kilograms - in science, that is incorrect, the SI unit for weight is the newton
  7. Why does a parachute reduce a parachutist's terminal velocity?
    This is achieved by increasing the surface area of the parachutist
  8. The gravity on the moon is 1/6 of the force due to gravity on the Earth. What is the weight of a person on the moon who has a mass of 65 kg?
    The mass of a person remains the same no matter where they are in the universe. The weight of an object is created by the force of gravity acting on its mass
  9. What could a horizontal line on a velocity-time graph indicate?
    A velocity time graph can show you when an object is at rest, whether it is accelerating or if it is travelling at a constant speed. The gradient gives you the acceleration - a horizontal line has zero gradient which means zero acceleration. An object travelling at terminal velocity is no longer accelerating through the fluid. The area underneath a velocity-time graph will tell you the distance travelled by an object
  10. Which formula correctly relates the weight of an object and the force exerted on it by gravity?
    This is a variation of the equation from Newton's second law

Author: Martin Moore

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