Name:______Date:______Period: _____
Unit 3: Forces
Section 2: Friction
Learning Goals
· Define friction.
· Identify causes of friction.
· Distinguish among various types of friction.
What is fiction?
Friction is a force that resists the motion of objects on surfaces. Many kinds of friction exist.
Friction depends on both of the surfaces in contact.
Example: When the hockey puck slides on ice, a thin layer of water between the rubber and the ice allows the puck to slide easily.
Friction is a force, measured in newtons just like any other force.
Identifying Friction Forces
· Static friction keeps an object at rest from moving.
· Sliding friction is a force that resists the motion of an object moving across a surface.
A Model for Friction
Friction depends on a material’s properties such as roughness, how clean the surfaces are, and other factors.
The greater the force squeezing two surfaces together, the greater the friction force.
Reducing the Force of Friction
· Unless a force is constantly applied, friction will slow all motion to a stop eventually.
· It is impossible to completely get rid of friction, but it can be reduced.
The friction between a shaft (the long pole in the picture) and an outer part of a machine produces a lot of heat.
Friction can be reduced by placing ball bearings between the shaft and the outer part.
Using Friction
Friction is also important to anyone driving a car.
Grooved tire treads allow space for water to be channeled away from the road-tire contact point, allowing for more friction in wet conditions.
Shoes are designed to increase the friction between their soles and the ground.
Friction and Energy
· Friction changes energy of motion into heat energy.
· Friction is always present in any machine with moving parts.
If the machine is small, or the forces are low, the amount of heat produced by friction may also be small.
Each time two moving surfaces touch each other, tiny bits of material are broken off by friction. Breaking off bits of material uses energy.
Vocabulary
friction – a force that resists motion.
sliding friction – the friction force that resists the motion of an object moving across a surface.
static friction – the friction force that resists the motion between two surfaces that are not moving.
Review Questions
Use the word bank below to complete the fill-in-the-blank questions.
vector friction resists
heat weight surfaces
1. Friction is a force that ______motion.
2. Friction depends on both ______in contact.
3. You can represent the direction of force by using a ______diagram.
4. Sliding friction increases with ______.
5. It is impossible to totally eliminate ______.
6. Friction changes energy of motion into ______energy.
7. List two ways you can reduce friction.
a. ______
b. ______
8. List two ways you can increase friction.
a. ______
b. ______
9. Why does rubbing your hands together make them warmer?
______
______
Name: ______Date: ______Period: ______
Unit 3 Section 2: Friction
Just about every move we make involves friction of some sort. This skill sheet will provide you with the opportunity to practice identifying the friction force(s) involved in real-world situations.
Example
Marco and his dad are unloading cinder blocks from the back of their pickup truck. They need to haul the blocks across the grass to their backyard, where they are going to make a sandbox for Marco’s younger sister. Marco would like to haul a bunch of blocks at once. In the garage, he finds a plastic sled and his sister’s red wagon.
Question: Which type of friction would resist Marco’s motion if he pulled the blocks in the sled? Solution: Sliding friction.
1. Answer these additional questions about Marco’s sandbox building project.
a. Which type of friction would resist Marco’s motion if he pulled the blocks in the wagon?
______
b. Do you think it would take more force to transport five blocks in the sled or in the wagon? Why?
______
______
c. Would the friction force increase, decrease, or stay the same if Marco added two more blocks to the sled or wagon? Explain your answer. ______
______
d. Marco tries piling twelve cinder blocks into the wagon. He pulls and pulls but the wagon doesn’t move. What type of force is resisting motion now? ______
2. Brianna is rowing a small boat across a pond. The air is calm; there is no wind blowing.
a. What type of friction is resisting her motion? ______
b. If two friends join her in the boat, will the friction force change? Why or why not?
______
______
3. A freight train speeds along the railroad tracks at 150 km/hr.
a. Name two types of friction resisting this motion. ______
b. If this train were replaced with a mag-lev train, like the one described in the notes, which type of
friction would be eliminated? ______
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