An object falling through a fluid initially accelerates due to the force…

The idea

When an object is released its speed is zero, so drag is zero. Falling through air, upthrust is negligible; the resultant force is downwards and equal to weight, so it accelerates at about 9.8 m/s². In a liquid, upthrust is not negligible, so the initial resultant force is weight minus upthrust, making initial acceleration less than g. As it speeds up, fluid resistance grows. The resultant force is now weight minus drag and upthrust (if in a liquid). This resultant force is still downwards but smaller, so the object speeds up at a steadily decreasing rate. On a velocity-time graph, this is a curve that starts steep and gradually flattens. In a denser fluid, drag builds up faster, so acceleration falls away sooner.

What earns the marks

• The weight of the object acting downwards.
• The drag or air resistance being zero, or very small, at the start.
• Identifying that in a liquid, the initial resultant force is weight minus upthrust.
• A resultant downward force, so the object accelerates.
• The acceleration decreasing as the resistance from the fluid grows.

Common mistakes and how to put them right

• Saying the object falls at a constant speed from the moment it is released.

• Claiming air resistance acts downwards as well as the weight.

• Ignoring upthrust when calculating the resultant force of an object falling through a liquid.

• Stating that the weight increases as the object falls faster.

Exam tips

• Name all forces and compare them, because a resultant answer needs the comparison.
• Remember to include upthrust when an object falls through a liquid, as it is not negligible.