10-2: Friction, Gravity, and Elastic Forces. 8 th Grade Physical Sciences

Transcription

1 8 th Grade Physical Sciences

2 Purpose Students will be able to: 1. Identify factors that determine the friction force between two objects. 2. Identify the factors that affect the gravitational force between two objects. 3. Explain why objects accelerate during free fall. 4. Explain when matter is considered to be elastic.

3 What is a Friction? Friction is a force that two surfaces exert on each other when they rub against each other.

4 What is a Friction? Surfaces are not as smooth as you think. Under a microscope you can see they are quite rough. Polished Aluminum The rougher the surface, the more friction there will be.

5 What is a Friction? The following are types of friction: 1. Static friction 2. Sliding friction 3. Rolling friction 4. Fluid friction

6 Static Friction Static friction is the friction between objects that are not moving. When an object is at rest, static friction keeps the object from sliding around.

7 Static Friction When you first get an object moving, you must push with extra force to overcome static friction.

8 Sliding Friction Once the object starts sliding, static friction is gone now there is sliding friction. Sliding friction occurs as two objects slide past each other.

9 Sliding Friction Sliding friction can be controlled by adding substances like sand to ice, rosin powder to a gymnast s palms or a dancer s shoes.

10 Rolling Friction Rolling friction happens when something rolls across a surface. Rolling friction is usually less than sliding friction for similar materials.

11 Rolling Friction Engineers use bearings, which roll, to reduce friction between metal parts. Link to.gif file in teacher folder This is important for cars, skate boards, and bicycles.

12 Fluid Friction Fluid friction happens when a solid object moves through a fluid. Air, water, and oil are examples of fluids.

13 Fluid Friction Fluid friction is usually less than sliding friction. Engines have oil in them so metal doesn t slide on metal, instead there is fluid friction.

14 Gravity Gravity is a force that pulls objects toward each other. Isaac Newton thought about gravity and concluded gravity affects all matter in the universe.

15 Gravity It is gravity that keeps the moon going around the Earth. Gravity keeps the planets going around the sun. The larger an object is, the more gravity it has.

16 Gravity This idea is the Universal Law of Gravitation. A person has mass, so the Earth is attracted to them!! But the Earth s mass is so much greater, we don t notice it.

17 Gravity Note that if the distance increases, the force decreases. But if the mass increases, the force also increases.

18 Weight Weight is the gravitational force exerted on an object on the surface of a planet. We use a bathroom scale to measure it.

19 Weight It is calculated by multiplying the mass (in kg) and the acceleration due to gravity (on Earth, 9.8 m/s 2 ). The formula is: f = ma So a 120 kg man on Earth would weigh: f = 120kg x 9.8 m/s 2 f = 1176 N

20 Free Fall Free fall is when the only force acting on an object is gravity. A leaf falling from a tree is NOT in free fall since friction with the air causes it to drift back and forth.

21 Free Fall A skydiver starts in freefall, but then it too has friction with the air.

22 Free Fall When a ball free falls, it accelerates at 9.8 m/s 2. This means every second the velocity increases by 9.8 m/s.

23 Free Fall A projectile, or an object thrown sideways, still drops at the same rate, 9.8 m/s 2. Different masses fall at the SAME rate, 9.8 m/s 2.

24 Air Resistance Why does a feather float down then? Because of air resistance a type of fluid friction. In a vacuum it will not float down.

25 Elastic Forces A material is considered elastic if after being stretched or squeezed, it returns to its original shape. The two types of elastic forces are compression (squeezed) and tension (stretched).

26 Elastic Forces Don t Copy How could you find the tension on the fishing string?

27 Summary Friction is a force that two objects exert on each other. Types of friction include static, sliding, rolling, and fluid. All matter attracts each other, called gravity. The larger the object, the greater the force. The closer objects are, the greater the force. All objects free fall at the same rate IF there is no air resistance. Two elastic forces are compression and tension.