IB Physics HL A.2.4 Translational Equilibrium

Practise analysing HL equilibrium systems with coplanar forces, tension, circular paths, slopes and coupled bodies.

Exam points

Analyse coplanar force systems and solve for unknown resultants or tensions.
Apply equilibrium reasoning to circular paths, ladders, slopes and coupled bodies.

[Maximum number: 3]

A vertical wall carries a uniform positive charge on its surface. This produces a uniform horizontal electric field perpendicular to the wall. A small, positively-charged ball is suspended in equilibrium from the vertical wall by a thread of negligible mass.

Figure for Question A.2.4—Resultant force from diagrams question 1 — IB Physics HL

The thread makes an angle of $30^{\circ}$ with the vertical wall. The ball has a mass of 0.025 kg .

Determine the horizontal force that acts on the ball.

Horizontal force on ball: $F = T\sin 30$ .
$T = \frac{mg}{\cos 30}$ .
$F = mg an 30 = 0.025 imes 9.8 imes an 30 = 0.14\ ext{N}$ .

Allow $g=10\ ext{N kg}^{-1}$ .
Award [3] marks for a bald correct answer.
Award [1 max] for an answer of zero if the horizontal force is interpreted as the horizontal component of the net force.

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A.2.4—Resultant force from diagrams question 1