General ScienceMadrid 2026Question 2A
X-linked inheritance
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Problem statement
A person pushes a 20 kg box along a horizontal surface. The coefficient of kinetic friction between the box and the floor is $\mu = 0.2$. The person applies a constant horizontal force of 60 N. Take $g = 9.8\ \text{m/s}^2$.
a) Draw the free-body diagram of the forces acting on the box (0.4 marks).
b) Calculate the friction force acting on the box (0.8 marks).
c) Calculate the net force, and explain what would happen to the motion of the box if the person stopped pushing it (0.8 marks).
d) Calculate the deceleration the box would have if the person stopped pushing it (0.5 marks).
Source: Madrid university entrance exam (PAU, Spain) · 2025-2026 · Ordinary session · General Science · Question 3B. Official exam set by the public universities of the Community of Madrid, to whom it belongs; translated from Spanish and reproduced with its source in order to explain it for teaching purposes.What this lesson answers
- What does the statement tell us before we calculate anything?
- a) Which forces act on the box, and why only those?
- b) Why is the normal force equal to the weight here?
- b) Why is the friction μ·N and independent of the push?
- c) What is left when we add the four forces?
- c) What happens to the box if the person stops pushing?
- d) How fast does it brake, and why does it not depend on the mass?
- How far does it travel before stopping? Two ways, with an example
- The answers and how to check them
- Common mistakes to avoid
- Exam tips
- If you want top marks
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