IB Physics HL A 2 7 Frictional Force Questions

Practise analysing HL friction systems with changing surfaces, coupled blocks, circular motion, slopes and drag.

Exam points

Identify friction as a force parallel to the contact surface and distinguish static friction, Ff ≤ μsFN, from dynamic friction, Ff = μdFN.
Calculate limiting or kinetic friction and use its direction and magnitude in complete force-balance or motion evidence.

[Maximum number: 5]

A girl on a sledge is moving down a snow slope at a uniform speed.

Figure for Question IB Physics HL A 2 7 Frictional Force Questions question 1 — IB Physics HL

(a)

The sledge, without the girl on it, now travels up a snow slope that makes an angle of $6.5^{\circ}$ to the horizontal. At the start of the slope, the speed of the sledge is $4.2 \mathrm{~m} \mathrm{~s}^{-1}$ . The coefficient of dynamic friction of the sledge on the snow is 0.11 .

[ 3 ]

(i)

Show that the acceleration of the sledge is about $-2 \mathrm{~m} \mathrm{~s}^{-2}$ .

[ 3 ]

friction force down slope $=\mu mg\cos(6.5^\circ)=5.9\ \mathrm N$

component of weight down slope $=mg\sin(6.5^\circ)=6.1\ \mathrm N$

acceleration $=\frac{12}{5.5}=2.2\ \mathrm{m\,s^{-2}}$

(b)

The coefficient of static friction between the sledge and the snow is 0.14 . Outline, with a calculation, the subsequent motion of the sledge.

[ 2 ]

maximum frictional force $=\mu R=7.5\ \mathrm N$ ;
the sledge will not move because the maximum static friction force is greater than the component of weight down the slope

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IB Physics HL A 2 7 Frictional Force Questions question 1

Question (a)

Question (i)

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