Forces: Friction and Gravity (Alex Williams)

Title: Forces: Friction and Gravity

Principles Investigated:

-Force (friction, gravity), air resistance Extension activities: kinetic energy, potential energy, acceleration, velocity, momentum.

Standards:

8.1.a: Students know a force has both direction and magnitude.

8.2.b: Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.

8.2.e: Students know that when the forces of an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction).

Extension Activity: 8.9.f: Apply simple mathematical relationships to determine a missing quantity in a mathematical expression, given the two remaining terms (including speed = distance/time, density = mass/volume, force = pressure × area, volume = area × height).

Materials:

-Eggs (obtained at a grocery story)

-Packing materials such as mini fruit crates, paper, tissue paper, string, small boxes, bubble wrap, rice, beans, scissors, tape, rubber bands, magazine pages, newspaper, paper bowls, paper cups, plastic bags, and other recycled materials (obtained at a grocery store or office supply store).

Procedure:

Problem or Question: How can you use air resistance to engineer a contraption that will safely land a raw egg on the ground, without cracking.

Procedure:

1. Observe materials (posted on a PowerPoint), and draw or write down how you would engineer your apparatus.

2. Meet with your group, and brainstorm one way to build an apparatus for your egg.

3. Gather materials provided, and construct your device.

4. Using your notecard, write the name of your contraption, and take a picture of the contraption and notecard with your smart phone. Label your photo with your group name and "before." Upload to the dropbox folder Egg Drop Photos_AW_4.30.13: https://www.dropbox.com/home/525s13/Egg%20Drop%20Photos_AW_4.30.12

5. Go outside, and drop your egg! (with the class). Assign one person to time the egg drop, also using their smartphone.

6. Record your groups' egg drop time on your notecard.

7. Take an "after" picture of the contraption (showing the egg) and the notecard with your smartphone. Upload to the same dropbox folder (labeling with your group name and "after").

Extension Activities: This egg drop experiment has multiple extensions that you could use to challenge students needing additional challenges (GATE students), or use as additional lessons in a unit.

This egg drop activity works great to also explain to students the concepts of Potential Energy and Kinetic Energy:

Potential Energy: Potential Energy is the stored energy possessed by an object. In this case, think of your egg in your contraption, just as it is about to fall.

Kinetic Energy: Kinetic Energy is the energy of motion. In this case, think of your egg as it is falling.

This activity can also be used as a unit to further explain velocity, acceleration, and momentum. In this case, have students follow the following steps in creating their egg contraptions:

1. Find materials and build contraption to protect your egg from a fall of (know exact distance. For example, 5 meters).

2. Find the mass of the entire contraption, including the egg

3. Drop the contraption and calculate time of fall with a timer or stopwatch

4. Using your collected data, you can now calculate velocity (v=d/t), acceleration (a=v/t), and momentum (p=mv).

Student prior knowledge:

-Students should have basic understanding of what a force is (a push or a pull).

-Students should have a basic understanding of what friction is.

-Students will have an understanding of gravity; that it is pulling things towards the center of the earth.

Explanation:

Friction: a force that two surfaces exert on each other when they rub against each other. The strength of friction is determined by two things: type of surface and how hard the surfaces are pushed together.

Example: rub your hands together. That's creating friction! Now, rub them together lightly - that creates less friction. Rub them together by pressing harder - that creates more friction. What happens? (the harder you press, the warmer your hands will get, due to increased friction).

Types of Friction:

Static Friction: acts on objects not moving.

Because of static friction, you have to have extra force to start a stationary object in motion. Take this child below, for example. Because his car was at rest, he needs to exert a force greater than the stationary friction acting on his car between the wheels and the grass, in order to move it.

-Possible misconception: Static friction is not the same as static electricity. In this case, static means "not moving."

Sliding Friction: when two solid surfaces slide over each other.

Look at the photo below. This boy exerts sliding friction against the slide, as he slides down it.

Rolling Friction: when an object rolls across a surface.

This little girl riding her bike is a perfect example of rolling friction. The wheels of her bicycle create rolling friction against the pavement. Rolling friction is usually less than sliding friction.

Fluid Friction: when a solid object moves through a fluid. Usually less than sliding friction. Fluids are any material that flows, which would include liquids and gasses (such as water or air).

Fluid friction is created as this little boy's surfboard glides through the water. Fluid friction is also created as this little girl drops back down into the man's arms (remember, air is considered a fluid). This is known as air resistance.

Gravity: gravity is a force that pulls objects towards each other, and in our case here on earth, everything is being pulled toward the center of the earth. The force of gravity depends on the mass of objects, and the distance. The greater the mass, the greater the force of gravity. If distance increases, the force of gravity decreases.

Questions & Answers:

1. Give an example of how you could reduce friction when moving your refrigerator across your kitchen.

Answer: Because rolling friction is less than both static friction and sliding friction, it would be easier to move your refrigerator across your kitchen if it were on wheels.

2. Compare the direction in which the force of friction acts as compared to the direction of motion. Also, provide a real-life example.

Answer: the force of friction acts in the direction opposite of the object's motion. For example, when you move a desk across the room. If you slide the desk forward, sliding friction acts in the opposite direction (in this case, backwards).

3. What force changes when a skydiver's parachute opens? What force stays the same?

Answer: The force of fluid friction (or air resistance), will increase with the opening of a parachute. Surface area also plays a part in air resistance, because the greater the surface area, the greater the air resistance. So, the air resistance will increase depending on the size of the parachute. Gravity is the force that will stay the same.

Applications to Everyday Life:

1. A meteor, a Space Shuttle, Felix Baumgartner

As a meteor or a Space Shuttle enter the earth's atmosphere, fluid friction between the object and the air creates so much heat that the meteor or space craft glows as it falls through the atmosphere. See the "Russian Meteor" video below for an amazing look at what friction can do. In creating Space Shuttles, special materials had to be manufactured that could withstand the heat created by this friction. In the case of Endeavour, silica tiles were used to cover the shuttle, because silica tiles are made of sand, which could withstand the heat created by the fluid friction. In Felix Baumgartner's fall (see video below), he uses a parachute to increase his air resistance which helps him land safely.

2. Greasing a gear (as in a bicycle chain)

When you add oil or grease to a chain, like on a bicycle, you decrease the sliding friction between the gears and the chain, allowing you to ride faster. Also, reducing this friction would make it easier to shift gears and extend the life of the gears on a bicycle. This same concept also applies to other types of machinery, like a car, where friction is reduced by adding oil to the engine.

3. Adding sand to an icy road

Though it seems that you typically decrease friction, sometimes increasing friction is important. In cold locations, ice often forms on roads during the winter months. Without friction, cars could potentially slide dangerously around on the icy roads. To increase friction, one method used is adding sand to the road. This increases the rolling friction between the wheels of a car and the road, making it easier to control a car on the road, so you do not slide. Additionally, if your car gets stuck in a snowbank, your wheels can spin and you might not be able to get out of the snow. Adding sand or dirt gives your tires something to grip, and this additional friction allows you to get out of the snow bank.

Photographs: **Pending: will add "before" and "after" photos of egg drop, from dropbox once experiment is complete.

Videos:

Felix Baumgartner's fall from the stratosphere: