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CAIE A-Level Physics 3.1.2 Resultant Force and Acceleration

Practise resolving forces before applying F = ma, using signs, resultant direction, units and physical checks for calculated acceleration or force.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • Apply F=ma to calculate resultant force, mass or acceleration in a stated system.
  • Use a sign convention and identify that acceleration and resultant force have the same direction.
  • Resolve forces or combine them to find the resultant before applying F=ma.
  • Check units, direction and physical feasibility of the calculated acceleration or force.

3.1.2—F = ma and solve problems using it, understanding that acceleration and question 1

[Maximum number: 4]

A child on a sledge slides down a steep hill and then travels in a straight line up an ice-covered slope, as illustrated in Fig. 3.1.

Fig. 3.1 (not to scale)

Fig. 3.1 (not to scale)

The sledge passes point A with speed 18 ms−118 \mathrm{~ms}^{-1} at time t=0 and then comes to rest at point B. The child applies a brake to the sledge at point B. The brake does not keep the sledge stationary and it immediately slides back down the slope towards A .

The variation with time t of the velocity v of the sledge from t=0 to t=24 st=24 \mathrm{~s} is shown in Fig. 3.2.

Fig. 3.2

Fig. 3.2

Question (a)

(a)

The child and sledge have a total mass of 70 kg . The component of the total weight of the child and sledge that acts down the slope is 80 N .

Determine

[ 4 ]

Question (i)

(i)

the frictional force on the sledge as it moves from B towards A,

frictional force =
[ 2 ]

Question (ii)

(ii)

the angle θ\theta of the slope to the horizontal.

θ=\theta=
[ 2 ]
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