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What is Friction?
Why is ice so slippery? Why is it so difficult to push or pull a heavy box across the carpet?
It’s the friction that makes the box hard to push or the lack of friction that makes ice slippery. Friction is a force between two surfaces that resists motion. But friction is necessary for walking and for vehicles to move! Friction comes in many forms:
Static Friction – the force that prevents a stationary object from moving.
Sliding Friction – the force that hinders the motion of an object after it has overcome static friction and is sliding on the surface.
Rolling Friction – the friction that a rolling object experiences against a surface. For example, when a car is moving, the tires experience rolling friction. Rolling friction can be 100 to 1000 times less than static or kinetic friction.
Understanding Friction: Part 1
Objective: To understand how friction and weight are related.
Figure 1
Prediction: Which do you think will have more sliding friction, one book or two?
Procedure:
- Tie a string around a textbook. Hang the textbook from a spring scale. How heavy is it? ________Newtons
- Holding the spring scale, pull the textbook and record the force that you have to apply to get the book to start moving (the reading on the spring scale in Newtons). This force is equal to the force of static friction.
- What is the frictional force when the book is moving? This is the force of sliding friction.
- Now, repeat steps 1-2 with two textbooks instead of one.
- Record all results in the table below.
Results:
Number of Books Weight (N) Static Friction Force (N)
Sliding Friction Force (N) 1 2
Discussion:
How do your results compare to your prediction?
How does an object’s weight affect the friction force?
Understanding Friction: Part 2
Objective: To understand that different materials have different frictional properties.
Prediction: Which tires have the most sliding friction? (Hint: You need sliding friction to keep from sliding!) Procedure:
- You have three sets of tires and each set is different from the other. How are these tires different? Using the little Lego with the cross hatch opening, build connect two tires so that the tires do not roll.
- Hang the plastic cup on a string from the axle of the two tires.
- Drop Unifix cubes into the cup to see how much force is necessary to get the tires to start moving Remember, 10 cubes = 10 grams = 0.1 Newtons.
- Determine which set of tires has the greatest frictional force. Remember, without sliding friction, your car will slide!
