# Taken from - Chapter 10, sec. 2

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Taken from - Chapter 10, sec. 2
Friction and Gravity Taken from - Chapter 10, sec. 2

January 31, 2011 Your word is Friction.

What is friction? When you ride a sled down a snowy hill, the sled’s motion changes at the top of the hill, down the side and then again at the bottom. What causes the change in motion? There are different forces acting on the sled – those forces are gravity which makes the sled speed up - and friction which brings the sled to a stop at the bottom of the hill. Let’s look more at these two forces.

Friction Remember - Force is a push or pull on an object that can result in changing that object’s motion. Remember - The strength of force is measured in the SI Unit called the newton. Since gravity is a force – it is also measured in newtons.

The causes of friction The strength of friction depends on how hard the surface of the object and the surface the object is on push together. It also depends on the type of surface the object has and the type surface the object is on. Try this – rub your hands together very lightly and then rub them together with more force – which causes more friction? When you rub your hands together more forcefully.

How does Friction work? Friction acts in the direction opposite of the direction of motion. So when your sled is sliding down the hill, friction is working in an direction up the hill. Without friction – moving objects would just keep going until they hit something.

4 Types of friction: Static Friction – friction that acts on an object that is not moving or is at rest. Example: Your desk is sitting on the floor right now and is not moving, so the only type of friction working on your desk is static friction. In your foldable : Put the term ‘static friction’ on the outside Put the definition on the inside flap Draw a picture on the inside on the part of the paper directly across from the flap.

4 Types of friction: Sliding Friction – friction that occurs when two surfaces slide over each other. Example: When you push your textbook across your desk you have sliding friction working on both the book and the desk. In your foldable : Put the term ‘sliding friction’ on the outside Put the definition on the inside flap Draw a picture on the inside on the part of the paper directly across from the flap.

4 Types of friction: Rolling Friction – friction that occurs when an object rolls across another surface. Example: When you ride a skateboard or bicycle on the road. In your foldable : Put the term ‘rolling friction’ on the outside Put the definition on the inside flap Draw a picture on the inside on the part of the paper directly across from the flap.

4 Types of friction: Fluid Friction – friction that occurs when a solid object moves through a fluid, such as water, air, or oil. Example: When you ride a surfboard through an ocean wave. In your foldable : Put the term ‘fluid friction’ on the outside Put the definition on the inside flap Draw a picture on the inside on the part of the paper directly across from the flap.

Exit Slip On a separate sheet of paper and without looking at your foldable/notes - List and describe the 4 types of friction.

February 1, 2011 Your word is Gravity.

Gravity Gravity is the force that pulls objects toward each other.
The Earth pulls on the moon which keeps the moon in orbit around the Earth. Gravity also pulls on us to keep us on the Earth. The two factors that affect gravity are mass and weight.

Mass and Weight Mass is the amount of matter in an object.
Weight is a measurement of the force of gravity on an object. Remember – mass does not change from place to place because the amount of matter in an object does not change with location. Remember – weight does change from place to place, such as here on Earth vs. on the moon, because the force of gravity changes. The moon has less gravitational pull than the Earth, so an object would weigh less on the moon.

Exit Slip In your notebooks – calculate the weight of the following objects based on their given mass: Notebook – 10 grams Pencil – 0.5 grams Desk – 10 kilograms Textbook – 20 grams Person – 50 kilograms You may use a calculator if you have one.

Your words are Mass and Weight.
February 2, 2011 Your words are Mass and Weight.

Free Fall Free fall occurs when the only force acting on an object is the force of gravity. During free fall, gravity is an unbalanced force which causes the object to accelerate. Because the only force acting on an object in free fall is gravity, all objects in free fall accelerate at the same rate - the rate of acceleration due to gravity. The rate of acceleration due to gravity is 9.8m/s2.

Air Resistance Objects falling through air experience a type of fluid friction called Air Resistance. Like all other types of friction – air resistance works in a direction opposite of the falling object. Therefore, air resistance works in an upward direction. Try this: Drop a flat piece of paper from a height equal to the height of your desk. Then drop a crumpled sheet of paper from the same height. Which one experiences the most air resistance? The flat sheet will experience the most air resistance because it has a larger surface area.

Terminal Velocity We know that falling objects accelerate at the rate due to gravity – 9.8m/s2. However, air resistance increases with velocity. At this point the forces on the falling object will be balanced, meaning there is no longer any acceleration. The object keeps falling but at the a constant rate = no acceleration. Terminal Velocity is the greatest velocity a falling object reaches. Terminal velocity is reached when the force of air resistance equals the force of gravity on the object, or the object’s weight.

Projectile Motion An object that is thrown, instead of dropped straight down, is called a projectile. Projectile motion describes the motion of a projectile. NOTE: A projectile and an object dropped straight down will reach the ground at the same time. This happens because both objects are acted on by the force of gravity in the same way. See the picture on p. 348 in the textbook.

Exit Slip On a separate sheet of paper do the ‘writing in science’ activity on page 348 in your textbook.