Forces Change Motion

1. Forces Change Motion Objective 4.02 Evaluate how pushing or pulling forces can change the position and motion of an object.

2. The Forces on an Airplane • The motion of any object can be understood by looking at all of the forces that are acting on that object. • A large jumbo aircraft can weight up to 350 tons! An exceptional upward force must be applied to overcome the downward pull of gravity. This force is called lift.

3. Notice how the top wing is curved and the bottom is flat. As the airplane speeds down the runway, air rushes up and over the curved part of the wing. The curving air moves more quickly than the air below the wing which travels in a straight line. The difference in the speed of the air moving along the top and bottom of the wing causes a difference in air pressure.

4. The slow-moving air below the wing has a higher pressure than the fast-moving air above the wing. This higher pressure pushes up on, or lifts, the airplane wing. The faster the airplane moves, the greater the lift underneath the wings. Eventually, the force of the lift becomes stronger than the force of gravity. At this point, the plane rises into the air.

5. As the plane flies through the air it encounters a force called air resistance, or drag. Drag is like friction, only in air. The plane constantly collides with air particles, and these collisions slow down the plane. The more surface area the air encounters, the greater the effect of drag.

6. Drag can be a useful force. For example, when a sky diver opens his parachute, he hopes that its wide surface encounters a great deal of drag. This drag slows his fall so he can drop gently to the ground.

7. For an airplane, drag is a force which must be overcome in order to move forward. Engineers can minimize drag by making the surfaces of the plane as smooth as possible. The surfaces of wings are sleek and slippery, not rough and bumpy. Also, the shape of the body is designed to minimize air resistance. The nose is pointed, and the body is rounded so that air slides easily over the top. Designs of this nature are called streamlined, or aerodynamic.

8. While a streamlined shape can reduce drag, a force is required to overwhelm it. This force is called thrust. Thrust is a forward force produced by an engine. In a jet engine, air is sucked in by a fan and then squeezed into a tiny space. Fuel is sprayed into the tiny space and then ignited. The heat from the burning fuel causes the air to expand very fast. When something expands very rapidly, it causes an explosion. In an engine, the explosion is very controlled. The hot gases shoot out the back of the engine. The force of their backward’s exit thrusts the plane forward.

9. Forces on a Shark • All of the forces in the direction of an object’s motion will speed up an object. Forces in the opposite direction of movement will slow an object down. Look at the forces at work on this shark.

10. Each arrow shows a different force. All of the forces work together to move the shark. Thrust is the force that moves the shark forward. A shark’s tail provides forward thrust. The faster the shark moves its tail, the more thrust it will have. The more thrust that the shark has, the faster it will move through the water.

11. As the shark moves through the water, drag acts on the shark to slow it down. Drag happens in the opposite direction of thrust. Drag is the force of the water as it is pushed out of the way of the moving shark. The faster the shark moves, the more drag will be acting against its forward motion.

12. The water also pushes against the shark’s fins. This force creates pressure under the fins. The force and pressure create lift. Lift pushes the shark up in the water. The shark can control lift and thrust by its motions. All three forces work together to determine how the shark moves underwater, and all three forces act to determine how fast the shark moves. The shark’s position will change quickly if the shark is moving quickly. The shark’s position will change slowly if the shark is moving slowly. • Forces act on all objects on Earth and in space. Pushing and pulling forces change the position and motion of all objects.

13. Review 1. The motion of an object can be understood by examining what? • the magnetic forces that are acting on that object • the gravity that is acting on that object • the object’s inertia • all of the forces that are acting on that object

14. 2. What is the push or pull on an object called? • drag • force • lift • motion

15. 3. The forward motion of a shark is caused by which force? • thrust • pull • drag • lift

16. 4. Which design do you think is more aerodynamic? • Stealth Bomber • World War II Bomber • Both are equally aerodynamic • Neither plane has an aerodynamic shape

17. 5. What is another name for air resistance? • lift • thrust • inertia • drag

18. Is it The End???

19. Now It Is!!! The End!!!