Section 2: Friction, Gravity, and Elastic Forces

Transcription

1 Chapter 10, Section 2 Friction, Gravity, & Elastic Forces Section 2: Friction, Gravity, and Elastic Forces What factors determine the strength of the friction force between two surfaces? What factors affect the gravitational force between two objects? Why do objects accelerate during free fall? When is matter considered to be elastic? 1

2 Friction A force that two surfaces exert on each other when they rub against each other Smooth surfaces produce less friction than rough surfaces Table Talk: Share with a partner when friction is good, and when it is bad. Without friction, a moving object will not stop until it strikes another object Causes of Friction Try It! Rub both of your hands together, first very lightly, then harder. Voila! Friction! The strength of friction depends on 2 things: Types of surfaces How hard the surfaces push together 2

3 Gravity A force that pulls objects toward each other Not just on Earth, it s also the force that keeps all planets orbiting the sun The Universal Law of Gravitation states that the force of gravity acts between all objects in the universe Gravity Between Objects The force of gravity between objects increases with greater mass and decreases with greater distance. 3

4 Mass and Distance The force of gravity between objects increases with greater mass and decreases with greater distance Table Talk: Now that you know this.why does the sun exert such a large gravitational force on the planets? Mass and Weight The gravitational force exerted on a person or object at the surface of a planet is known as weight. 4

5 Gravity & Weight Try It! How much would a 50 kg person weigh? 50kg X 9.8 m/s 2 = 490 N Table Talk: Would you weigh more or less on the moon? Would your mass change? Weight = the gravitational force exerted on a person or object at the surface of the planet. It varies with the strength of the gravitational force Weight = mass x acceleration (due to gravity) Weight is measured in Newtons (N) Mass is measured in kilograms (kg) Acceleration due to gravity = 9.8 m/s 2 Gravity & Motion Free Fall = when the only force acting on an object is gravity Acceleration is in the downward direction ALL objects free fall accelerate at the same rate regardless of their masses Table Talk: If this is true, why would the ball hit the ground faster than the feather when they are dropped from the same height at the same time? 5

6 Free Fall The graph shows how the speed of an object in free fall changes with time. Use the graph to answer the following questions. Free Fall Interpreting Graphs: What is the speed of the object at 1 second? At 3 seconds? 9.8 m/s; 29.4 m/s. 6

7 Free Fall Calculating: Calculate the slope of the graph. What does this number represent? The slope is 9.8. The speed increases by 9.8 m/s each second. Free Fall Predicting: Use the slope that you calculated in Step 2 to predict the object s speed at 6 seconds m/s 7

8 Free Fall Drawing Conclusions: The graph has a constant slope. What does the slope tell you about the object s motion? The object s speed increases at a constant rate. Air Resistance Falling objects with a greater surface area experience more air resistance. 8

9 Air Resistance Try It! Take two pieces of paper, exactly the same size. Leave one flat and crumple one into a ball. Drop them from the same height at the same time. What happens? A type of fluid friction. Acts on objects falling through the air. Objects with greater surface area experience more air resistance as they fall. In looking at this picture, what else could affect how quickly the papers fall? Elastic Forces Table Talk: Can you think of some other things that may involve compression and tension forces? Matter is elastic if it returns to its original shape after squeezed or stretched. Compression = An elastic force that squeezes or pushes matter together. Tension = An elastic force that stretches or pulls matter. The upward force of tension balances the shoe s weight See BrainPop Gravity end 9

10 End of Section: Friction, Gravity, and Elastic Forces 10