Final answer:
When a ball made of iron is held at rest under water, it experiences a buoyant force that helps support its weight, resulting in less force needed compared to holding it in rest in air.
Step-by-step explanation:
When a ball made of iron is held at rest under water, it experiences a buoyant force that helps support its weight. This buoyant force is equal to the weight of the water that the ball displaces. The force needed to hold the ball in rest under water is therefore less compared to holding it in rest in air.
To calculate the force needed to hold the ball in rest under water compared to holding it in rest in air, we can use Archimedes' principle. The formula to calculate the buoyant force is:
Buoyant force = density of fluid * volume of fluid displaced * acceleration due to gravity
Since the density of water is 1000 kg/m³, we can calculate the volume of water displaced by the iron ball and then use the formula to find the buoyant force.