An object stays motionless in the water of a pool, a metre beneath the surface, with nothing else touching it. Water pushes up on it with upthrust, and its weight pulls it down.
Pick the statement that describes the vertical forces on the object while it stays still.
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A still object has no acceleration, so the resultant force is zero. The upward force, upthrust, must therefore equal the downward force, its weight. That is the third option. Water resistance only acts while something moves through the water, so for a still object it is zero, and the first two options compare upthrust with the wrong force. Upthrust greater than weight would leave a resultant force upwards, and the object would rise instead of staying still.
How the marks are earned
- Choose upthrust equals weight: the object is not moving, so the upward and downward forces balance.
- One answer only: two answers earn nothing, and a circled or underlined right option counts if the box is blank.