You are Here: Home >< Physics

# Torque and Rotations Question watch

1. The top end of a two-by-four piece of lumber that is 4.2 m long is leaned at a height of 2.4 m against a smooth wall so that the bottom end makes an angle of 35 ∘ with the floor. This board has an inertia of 4.2 kg .

What is the normal force exerted by the wall on the board?

Attempted solution:

The sum of the forces in both the x and y directions is zero. The gravitational force and the normal force from the floor are both exclusively in the y direction and the normal force from the wall and frictional force from the ground are in the x direction.

The friction force and the normal force from the wall on the board are in opposite directions, so we know
Fx=Fw-Ff=0
Ff=Fw and that they are acting in opposition to each other.

We also know that net torque is zero, since the system is not moving.
In the x direction
tao=Fxrsin(theta)

Since I do not have a coefficient of static friction for the floor or for the lumber, how am I supposed to solve this question?
2. Although I do not agree with your initial assumptions, you can always enter a new variable for the floor friction or create an assumption to provide a solution.
3. I haven't attempted it, but since I don't know the friction on the floor, I'd take moments about that point - meaning any force acting there is out of the equation.

Edited to add: and unless there's anything missing from your question (or unless I've misunderstood) then it becomes a very straightforward problem.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 20, 2018
The home of Results and Clearing

### 1,062

people online now

### 1,567,000

students helped last year
Today on TSR

### University open days

1. Sheffield Hallam University
Tue, 21 Aug '18
2. Bournemouth University
Wed, 22 Aug '18
3. University of Buckingham