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Forces and Motion - Effects of air resistance and water resistance

Grade 5IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Air resistance and water resistance are types of friction called 'drag' that act in the opposite direction to an object's motion.

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Air resistance occurs when an object moves through the air and collides with air particles. The force is represented as FairF_{air}.

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Water resistance occurs when an object moves through water. Because water is more dense than air, the resistance force (FwaterF_{water}) is significantly stronger than air resistance.

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The magnitude of these forces depends on the object's surface area. A larger surface area results in more collisions with particles, increasing the resistance force.

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Streamlining is the process of shaping an object (like a fish or a boat) to have a small frontal surface area and smooth curves to reduce the effect of resistance.

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The net force FnetF_{net} on a falling object is the difference between gravity WW and air resistance RR.

📐Formulae

Fnet=Fthrust−FresistanceF_{net} = F_{thrust} - F_{resistance}

Weight(W)=mass(m)×gravity(g)Weight (W) = mass (m) \times gravity (g)

Resistance∝Surface AreaResistance \propto Surface \ Area

💡Examples

Problem 1:

A skydiver with a mass of 70 kg70\text{ kg} is falling. When they open their parachute, they slow down. Explain the change in forces.

Solution:

The surface area increases, which increases the air resistance (FairF_{air}).

Explanation:

Before opening the parachute, the downward force of gravity WW is much larger than the upward air resistance RR. Once the parachute opens, the surface area increases dramatically, making R>WR > W for a short time, causing the skydiver to decelerate.

Problem 2:

Why is a shark's body shaped like a teardrop (pointed at the front and smooth)?

Solution:

To minimize FwaterF_{water} through streamlining.

Explanation:

The shape allows water particles to flow smoothly around the body rather than hitting a flat surface. This reduction in water resistance allows the shark to move at high speeds using less energy.

Problem 3:

Compare two balls of the same mass: Ball A is a solid metal sphere, and Ball B is a hollow plastic sphere of the same size. Which one experiences more air resistance at the same speed?

Solution:

Both experience the same air resistance FairF_{air}.

Explanation:

Air resistance depends on the shape, speed, and surface area of the object. Since both balls have the same size and shape, the number of air particles they collide with at a specific speed is identical, regardless of their mass.