Lesson 6

Objectives Observe the properties of sliding friction. Measure the force of friction on a wooden block pulled across different surfaces. Measure the force of friction on loads of different weights Measure the force of friction on a wooden block with different base areas in contact with a surface.

Getting Started What do you know about friction? Identify a situation where friction works against you. Identify one in which friction works for you. Identify some situations where friction is very low or almost zero. What would happen if there were no friction?

Vocabulary – Lesson 6 Friction: The force that resists motion between two surfaces in contact with each other Sliding Friction: The friction produced when two objects are in contact and are rubbing against each other Air Friction: Friction that opposes the motion of objects moving in air. Static Friction: Friction that acts on objects that are not moving. L6

Vocabulary Con’t Rolling Friction: Friction needed to make a wheel or tire turn. Fluid Friction: Friction that occurs when objects move through fluids.

Vocabulary – Lesson 6 Surface area: The force of friction does not change when the base area of an object in contact with a surface changes. The area in contact is called the surface area. Pressure: When the surface area changes, the force per unit area changes. This is known as changes in pressure. L6

Vocabulary – Lesson 6 Newton’s 1 st Law of Motion (Inertia): An object at rest will remain at rest and an object in motion will move at constant speed in a straight line if no unbalanced forces act on it. Velocity: The speed at which an object is traveling in a single direction

Notes When two objects slide against each other, there are two forces involved, one on each object. EXAMPLE: When a block moves across a table, the sliding block exerts a force on the table because the blocks bottom surface is in contact with the table. The table pushes on the block with an equal and opposite force. The opposite force is called frictional force and it opposes the motion of the block.

Notes If friction is the only force acting on the block, the block will slow down and stop. If a 2 nd force, equal to the 1 st force, is applied in the direction of motion, the block will keep on moving at a constant speed.

Notes The size of the frictional force depends on: 1. the surface of the objects 2. the size of the force pushing the objects together.

Notes Two smooth surfaces rubbing against each other may have greater friction than two rough surfaces. This can occur because friction is the result of atomic interactions between surfaces in contact. Very smooth surfaces sometimes allow for stronger interactions at the atomic level, which produces greater friction.

Lesson 6-1

Question – Lesson 6.1 Does friction depend on the type of surfaces in contact? Do different surfaces produce different amounts of friction?

Reflection The result of forces between the stationary block and the surface area is static friction. It takes a certain amount of force to overcome static friction and start the block moving (until the bonds between the block and the surface area are broken). The force needed to put the block in motion is greater than the force needed to keep the block moving. L6

Lesson 6-1 If _________________,then __________ because _________________.

Lesson 6-1 Materials

Lesson 6-1

Force Needed to Move a Block.27 m (27 cm.) Over Different Surfaces L 6-1

Lesson 6 L 6-1

Lesson 6-1

Lesson 6-2

Question – Lesson 6.2 Does changing the load change the force of friction across a surface? If you add a larger load, will the force of friction change?

Lesson 6-2 If _________________,then __________ because _________________.

Lesson 6-2 Materials

Lesson 6-2

Force Needed to Move Two and Three Blocks 0.27 m (27 cm.) Over Different Surfaces

Lesson 6-2 L 6-2

Lesson 6-2

Lesson 6-3

Question 6.3 Does changing the surface areas in contact change the amount of friction? Does surface area affect friction? What is the relationship between surface area and friction ?

Lesson 6-3 If _________________,then __________ because _________________.

Lesson 6-3 Materials

Lesson 6-3

Changing the Surface Area L 6-3

Changing the Surface Area (summary) 6-3

Lesson 6-3