Transcript Forces Change Motion

Forces Change Motion
Objective 4.02
Evaluate how pushing or pulling forces can change
the position and motion of an object.
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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
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.
• 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.
• 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.
• 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.
Review
1. The motion of an object can be understood by examining what?
A. the magnetic forces that are acting on that object
B. the gravity that is acting on that object
C. the object’s inertia
D. all of the forces that are acting on that object
2. What is the push or pull on an object called?
A. drag
B. force
C. lift
D. motion
3. The forward motion of a shark is caused by which force?
A. thrust
B. pull
C. drag
D. lift
4. Which design do you think is more aerodynamic?
A. Stealth Bomber
B. World War II Bomber
C. Both are equally aerodynamic
D. Neither plane has an aerodynamic shape
5. What is another name for air resistance?
A. lift
B. thrust
C. inertia
D. drag
Is it The End???
Now It Is!!!
The End!!!