Contextual Problem: Friction

Elicitation

1. What makes a surface rough?

2. Why does sanding a the surface of a wooden desk make it smoother?

3. What happens if you are standing on a sheet of ice and push against a heavy sled?

4. What would you do to increase traction on a slippery sidewalk?

5. What is your definition of friction?

The Situation:

The prime concern of the operators of seniors' complexes is the mental and physical health of the occupants. In our climate, for up to four months, ice and snow pose major obstacles to the safe mobility of seniors outside the complex.

A contractor updating a seniors complex has designed a network of walkways wandering across the grounds. Your company has been hired to advise the contractor their design. Your recommendations may include design changes that might cost the contractor time and money. To convince her of your credibility and to satisfy her concerns, you must be able to demonstrate a complete understanding of friction.

Questions:

1. How do the following aspects of walkway design affect safety?

a) the degree of curvature

b) slope

c) surface texture

2. What texture would be suitable for the walkways' surface? Balance the need for:

i) winter traction

ii) an overly rough surface that would catch at the soles of seniors' shoes in the summer

iii) ease of snow removal

3. What is the effect of the coefficient of friction on walkway safety?

4. A person starts to move along one of the walkways. Create a sketch showing the balance of the following vectors: weight, surface support, push-off, friction.

Calculations

1. a) Calculate the force required to start a 30 N crate moving along a walkway ( µ = 1.2).

b) What would happen to the crate if 125 N were applied to it?

2. In winter, a 20 N wooden block just slides down a 17⁰ ice coated grass slope at constant speed. Find the coefficient of friction between the two surfaces

3. a) A 45 N crate sits on a 10⁰ walkway ramp. What force is required to just push it down the ramp?

b) What force is required to push the crate up the slope at constant speed?

4. A 27 N sled is pulled across the snow (µ = 0.2) by a child who pulls up on its tow rope with 50 N. If the rope angles up at 38⁰ and the child's force is just sufficient to move the sled, what weight can the child drag on the sled?

5. A gardener bends low and pushes with 190 N into a 12 N hand plow to move it across a patch of wet soil, µ = 2.3. If the handle dips at 12⁰, what weight of soil (cargo) can the hand plow carry?