6 (a) Figure 11 shows a person doing a push-up exercise - Edexcel - GCSE Physics - Question 6 - 2023 - Paper 2

According to the principle of moments, for the system to be in equilibrium, the total moments about the pivot (elbow joint) must be equal. The moment due to the weight of the forearm is calculated as:

$ext{Moment} = ext{Force} imes ext{Distance} = 23 ext{ N} imes 0.16 ext{ m} = 3.68 ext{ Nm}$

The moment due to the force from the biceps muscle is:

$ext{Moment} = 92 ext{ N} imes 0.04 ext{ m} = 3.68 ext{ Nm}$

Therefore, both moments are equal, proving that the system is in equilibrium.

To find the force from the muscle, we consider the moments again with the ball included. The moment due to the ball is:

$ext{Moment} = 15 ext{ N} imes 0.32 ext{ m} = 4.8 ext{ Nm}$

The total clockwise moment is:

$ext{Total Moment} = 3.68 ext{ Nm} + 4.8 ext{ Nm} = 8.48 ext{ Nm}$

To maintain equilibrium, the moment from the bicep must equal this:

$ext{Force} imes 0.04 ext{ m} = 8.48 ext{ Nm}$

Solving for the force gives:

$ext{Force} = \frac{8.48 ext{ Nm}}{0.04 ext{ m}} = 212 ext{ N}$

The upthrust on the ball in seawater is greater than the upthrust on the same ball in fresh water because seawater is denser than fresh water. This means that for the same volume of water displaced, the upward buoyant force (upthrust) is higher in seawater.

Since seawater is denser than fresh water, it exerts more buoyant force on the ball. As a result, the ball displaces a smaller volume of seawater to achieve the same upward force. This causes it to float higher on the surface, which results in less of the ball being submerged in seawater compared to fresh water.