Victory Glide

What are the physical forces that decides how fast a sled moves?

Physics, science

The sled is packed and now we need to move it down the trail. Moving something requires a FORCE, but there are actually many forces that come in to play with getting the sled to move. So how do we figure out how much the dog team needs to pull? The first thing to remember is Newton’s First Law of Motion, or the Law of Inertia. This First Law states that any object that is not moving has balanced forces (all the forces acting on that object cancel each other out). If you remember from Activity 3.1, forces have both a magnitude and a direction. For the sake of illustration, you can draw a Force Diagram using the object’s center of mass as a point to show how the forces act on it. So if you sled is sitting on flat ground, not moving: What are the physical forces that decides how fast a sled moves?

Scroll down for activity! Or click to download Victory Glide Activity (PDF)

ACTIVITY

Procedure

1) Have students split into teams of 2-3 students. Each team should have a several rubber bands, a pair of scissors, a marker, duct tape, their own shoes, a ruler, and various testing surfaces (wax paper, aluminum foil, paper, etc.).

2) Have students compare the bottoms of their shoes and make notes on the differences in texture, materials, surface area, and general conditions for each shoe.

3) Cut the rubber band to make a long piece of rubber. Mark inch intervals on the relaxed rubber band.

4) Using the duct tape, tape one end of the cut rubber band to the toe of one of the shoes, trying to keep it close to the sole of the shoe.

5) Put the shoe on the ground. Note what sort of ground surface (carpet, tile, etc) the shoe is on.

6) Have one student pull the free end of the rubber band parallel to the ground surface with a gentle, consistent pressure. Continue pulling with the same gentle consistent pressure until the shoe begins to move.

7) At the moment the shoe moves, the second student should measure the distance between the inch marks on the rubber band with the ruler. Record this number as Static Friction.

8) After the shoe is moving, continue pulling with the same gentle, consistent pressure. Measure and record the distance between the inch marks on the rubber band with the ruler while the shoe is in consistent movement. This number becomes Kinetic Friction.

9) Repeat the experiment two more times (same shoe, same surface). Record your findings for all three tests.

10) Do the same test for the other shoes. You should have three Static tests and three Kinetic tests for each shoe recorded.

11) Experiment using different surfaces (paper? Aluminum foil? Sand paper?) Record your findings for each surface.

12) Have the students discuss the following questions:

a. what shoe surface had the least amount of static and kinetic friction? The most? Was this consistent through all the different testing scenarios?
b. What floor surface had the least amount of static and kinetic friction? The most? Was this consistent with all the shoes tested?
c. What were your controls in this experiment (things that did not change, ie the weight of the shoe, the elastic properties of the rubber band, the flatness of the surface, etc.)
d. What were your possible sources of error for this experiment?
e. How would the results of this experiment change if you were pulling the shoe up an incline? Down an incline?

13) Students do further experimentation:

When you introduce inclines (angle ?) into the testing system, you are also introducing vector properties for your forces. While the Weight of the sled will always have a direction directly downward, the Normal Force holding the sled on the surface always acts perpendicular to the surface:

Our force diagram then becomes:

And each of the force components then has x and y vector values based on the incline angle (?).

Let’s assume that the sled load has a weight (W) of 200 lbs (we will make the musher run next to the sled for this climb), that the incline angle is 10° from horizontal, and the static coefficient of friction (µs) for the wet snow on the blue plastic runners is 0.14.

a) Draw the force diagram for this scenario.
b) Translate the system into x-y coordinates (either break the Fn and Ffk forces into x-y components, OR break W into components of your revised system, based on your incline angle)
c) Using your new x-y components, determine the values of your normal force and the kinetic frictional force (Ffk) that needs to be overcome in order for the sled to move.
d) What is the minimum amount of force up the incline that the dogs will need to exert in order to move the sled uphill?

Rex Runner Plastic Codes

Theory

So how do we figure out how much the dog team needs to pull?

The first thing to remember is Newton’s First Law of Motion, or the Law of Inertia. This First Law states that any object that is not moving has balanced forces (all the forces acting on that object cancel each other out). If you remember from Activity 3.1, forces have both a magnitude and a direction. For the sake of illustration, you can draw a Force Diagram using the object’s center of mass as a point to show how the forces act on it. So if you sled is sitting on flat ground, not moving:

What are the physical forces that decides how fast a sled moves?

The Force Diagram becomes represented as:

The Normal Force and the Weight of the object exactly match, only with different directions, so the sled is not moving, or it is in a state of equilibrium.

To move the sled forward, what forces will have to be overcome first?

Any time you drag something across a surface, you have to overcome the force of Friction. Friction is a function of the type of surfaces rubbing together and the weight of the object. Lighter objects and smoother surfaces have less friction than heavy objects and rough surfaces. The Coefficient of Friction, ?, is used to depict the relative differences in roughness in surfaces. There are two different kinds of friction, static friction and kinetic friction. Static friction (Ffs) is the frictional force needed to overcome to get an object to start to move, and usually it is bigger than the kinetic friction (Ffk), which is the frictional force applied to keep an object moving at a constant speed. Let’s look at the Force Diagram for getting the sled to move on a flat surface:

For each different surface, there is a specific frictional coefficient, ?. For sleds moving on snow, the coefficient of friction can vary depending on what kind of snow (fresh, wet, dry, icy, rutted, deep, etc) and what kind of surface is on the sled runners gliding over the snow.

Modern dog sled runners typically use plastic to cover the runners. This plastic offers some protection for the runner itself, and are also easily replaceable so they can be changed out depending on the trail conditions or the wear on the plastics. Sled manufacturers offer different plastic types (usually with different colors) based on snow temperature and trail conditions, similar to the wax used on cross country skis.

The Frictional Forces are calculated with the formula Ff = ? * Fn.

The activity today is going to look at how frictional forces vary depending on the coefficient of friction of different surfaces.

Student Analysis

a. What shoe surface had the least amount of static and kinetic friction? The most? Was this consistent through all the different testing scenarios?

b. What floor surface had the least amount of static and kinetic friction? The most? Was this consistent with all the shoes tested?

c. What were your controls in this experiment (things that did not change, ie the weight of the shoe, the elastic properties of the rubber band, the flatness of the surface, etc.)

d. What were your possible sources of error for this experiment?

e. How would the results of this experiment change if you were pulling the shoe up an incline? Down an incline?

Do further experimentation:

When you introduce inclines (angle ?) into the testing system, you are also introducing vector properties for your forces. While the Weight of the sled will always have a direction directly downward, the Normal Force holding the sled on the surface always acts perpendicular to the surface:

Our force diagram then becomes:

And each of the force components then has x and y vector values based on the incline angle (?).

Let’s assume that the sled load has a weight (W) of 200 lbs (we will make the musher run next to the sled for this climb), that the incline angle is 10° from horizontal, and the static coefficient of friction (µs) for the wet snow on the blue plastic runners is 0.14.

a) Draw the force diagram for this scenario.

b) Translate the system into x-y coordinates (either break the Fn and Ffk forces into x-y components, OR break W into components of your revised system, based on your incline angle)

c) Using your new x-y components, determine the values of your normal force and the kinetic frictional force (Ffk) that needs to be overcome in order for the sled to move.

d) What is the minimum amount of force up the incline that the dogs will need to exert in order to move the sled uphill?

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