IB Physics HL A 2 4 Resultant Force From Diagrams Questions

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

Exam points

Resolve the forces shown on a free-body diagram into consistent components and directions.
Calculate and interpret the resultant force on the system from a complete force diagram and mark-scheme evidence.

[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.

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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: $F = T\sin 30$ .
$T = \frac{mg}{\cos 30}$ .
$F = mg\tan 30 = 0.025 \times 9.8 \times \tan 30 = 0.14\ \text{N}$ .

Allow $g=10\ \text{N kg}^{-1}$ .
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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IB Physics HL A 2 4 Resultant Force From Diagrams Questions question 1