---
title: "Friction force; tipping and sliding of block"
subject: "Applied Mechanics"
chapter: 5
key: "semester_1/applied_mechanics:friction-force-tipping-sliding-block"
source: https://www.bctengineer.com/q/semester_1/applied_mechanics/friction-force-tipping-sliding-block
site: BCT Engineering
---

# Friction force; tipping and sliding of block

Applied Mechanics · Chapter 5: Friction

- Asked in: 2072 Kartik, 1+3 marks
- Video: [Statics: Lesson 63 - Friction Slipping Tipping Problem](https://www.youtube.com/watch?v=IXbIYBtJWNc) — Jeff Hanson
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/applied_mechanics/friction-force-tipping-sliding-block
- Whole chapter: https://www.bctengineer.com/md/semester_1/applied_mechanics/chapter-5

## Question

Define friction force and explain condition of tipping and sliding of a block.

## Short answer

Friction force is the resisting force along contact surfaces opposing relative motion, with limiting value μN. For a block of weight W, base width b, pushed at height a: sliding occurs if P > μW; tipping about the toe occurs if P > Wb/(2a). The smaller P decides the first failure mode.

## Answer

#### Friction force
Friction force is the resisting force that acts along the common tangent of two surfaces in contact and opposes the relative motion, or the tendency of motion, of one surface over the other. Its maximum (limiting) value is F = μN, where N is the normal reaction.

#### Sliding and tipping of a block
Take a block of weight W, base width b, acted on by a horizontal force P at height a above the base, with coefficient of friction μ.
- **Sliding:** the block slides when P exceeds the limiting friction: P > μW (since N = W).
- **Tipping:** the block tips about its lower toe when the overturning moment exceeds the restoring moment: P a > W b/2, i.e. P > Wb/(2a).

The smaller of the two values of P decides what happens first: if μW < Wb/(2a) the block slides; if Wb/(2a) < μW it tips. When tipping starts, the normal reaction moves to the toe of the block.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Friction force acts:
   A. Only when moving
   B. Normal to the surfaces
   C. Along the common tangent, opposing relative motion
   D. Along the motion
   Answer: C. It resists relative motion or tendency. (Key point: Friction force)

2. Tipping occurs when:
   A. P a > W b/2
   B. P > μW
   C. P a < W b/2
   D. P b > W a
   Answer: A. Overturning moment exceeds restoring moment. (Key point: Tipping)

3. If μW < Wb/(2a):
   A. Tips first
   B. Both
   C. Slides first
   D. Neither
   Answer: C. Smaller P governs. (Key point: Which first)

## Related in Applied Mechanics

- [Friction conditions; sliding or overturning of block](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Afriction-conditions-sliding-overturning) — Applied Mechanics, chapter 5, asked 2×
- [Laws of dry friction; overturning and sliding](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Alaws-dry-friction-overturning-sliding) — Applied Mechanics, chapter 5
- [Force P to prevent block sliding down incline](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Ablock-50kg-force-p-prevent-sliding-down) — Applied Mechanics, chapter 5
- [Force P to accelerate 600 N block](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Ablock-600n-force-p-acceleration-dynamic-equilibrium) — Applied Mechanics, chapter 9
- [Force P to give block acceleration of 10 m/s²](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Aforce-p-block-2500n-acceleration-10) — Applied Mechanics, chapter 9
- [Impending motion; friction force at different stages](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Aimpending-motion-friction-stages) — Applied Mechanics, chapter 5

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