Forces gravity, weight, free fall, friction
Forces can affect motion in several ways: They can make objects start moving They can make objects move faster They can make objects move slower They can make objects stop moving They can make objects change direction They can make objects change shape
WEIGHT GRAVITY and FREE FALL
Gravity Gravity is a basic property of all matter
Every object exerts a gravitational force on every other object. So why doesn t everything fly together? demonstration
Law of Universal gravitation: Newton: the force of gravity between two objects depends on the objects masses and the distance between their centers.
remember: WEIGHT THE MAGNITUDE OF THE FORCE OF GRAVITY ON AN OBJECT Weight is caused by gravitational acceleration. scientifically speaking, mass isn t related to size. Mass is related to how much an object resists changes to its state of motion (its inertia). versitarium
Demonstrating acceleration based on mass in microgravity. Accelerating Mass - NASA Using tools in space Without the force of gravity Tools such as drills are designed to make work easier. Sometimes, the environment in which a person works dictates whether or not a certain tool will get the job done. In the microgravity environment of space, astronauts are often required to do simple repairs. On Earth, gravity helps anchor a person to the ground, making tightening a bolt a rather simple activity. Engineers have had to develop different types of bracing systems a whole new set of tools to assist astronauts working in space.
Gravitational force All masses near Earth feel a gravitational force proportional to their mass: the bigger the mass, the bigger the gravitational force. The equation for gravitational force is: F G = Mass x Gravity = mg. Or W = mg The value of g (the strength of the gravitational field) is unique to each planet. While g here on Earth may be ~9.8 m/s 2, on Jupiter, g ~25 m/s 2 and on the Moon, g is only ~ 1.6 m/s 2.
W = mg Weight = mass acceleration due to gravity. This follows directly from F = m a. F g = mg Near the surface of the Earth, g = 9.8 m/s 2.
Gravitational force is associated with acceleration in the direction of that force. Simply put, an object subject to a gravitation force will fall in the same direction in which the force is acting.
What is free fall? Free fall is motion under the influence of gravity only - no friction or air resistance. When things fall they constantly accelerate. If we can ignore air resistance, all objects accelerate at the same rate. 9.8 m/s 2 = g (acceleration due to gravity on Earth at sea level)
Things that increase air resistance. shape- more surface area means more air resistance. velocity- the faster you go, the more air resistance (this is why meteors burn up in the atmosphere) The thickness of the air you go through (there is less air resistance higher up in the atmosphere)
Galileo did experiments to convince others that the acceleration caused by gravity would be the same for all freely falling objects if there was no air to retard their motion. He dropped two heavy metal balls together from the leaning tower. Although one weighed much more than the other, they reached the ground almost at the same time. demonstration 14
When most of the air is removed from a container, two objects fall at almost the same rate. If all the air was removed, they would accelerate downward at exactly the same rate. misconceptions more misconceptions Apollo 15 1971 Hammer and feather
Terminal Velocity Terminal velocity- the velocity at which the upward force of air resistance equals the downward force of gravity. Once you reach this velocity you will no longer accelerate. (just stay at the same velocity) Parachutes increase your surface area to increase your air resistance in order to reduce your terminal velocity so you don t die when you hit the ground.
How Fast? The velocity of a free-falling object which has been dropped from a position of rest is dependent on the length of time it has fallen. The formula for determining the velocity of a falling object after a time of t seconds is: v f = gt An object s downward velocity increases 9.8 m/s for every second of free fall.
v f = gt t = 6 s How FAST? Example v f = (9.8 m/s 2 ) (6 s) = 58.8 m/s t = 8 s v f = (9.8 m/s 2 )(8 s) = 78.4 m/s
Knowing how fast and object is falling allows you to calculate how far it has fallen in a given amount of time. How Far? The distance fallen after a time of t seconds is given by the formula below: x = ( 1 / 2 ) g t 2
x = ( 1 / 2 ) g t 2 t = 1 s How FAR? Example x = ( 1 / 2 ) (-9.8 m/s 2 ) (1 s) 2 = -4.9 m t = 2 s x = ( 1 / 2 ) (-9.8 m/s 2 ) (2 s) 2 = -19.6 m t = 5 s x = ( 1 / 2 ) (-9.8 m/s 2 ) (5 s) 2 = -122.5 m The NEGATIVE displacement, indicates that the object is falling DOWN
weightlessness What is it according to the physics of forces?
International Space Station A joint project between the US and Russia. In 1998 began construction on the Space Station. The first crew arrived in 2000. It is a lab, orbiting the planet where crews work and do research. It is the 4 th Space Station to orbit the Earth.
Weightlessness People aboard the space station appear to be weightless (They float around like there is no gravity). However, they are not out of Earth s gravitational field (gravity is still pulling on them). When astronauts are in the space station, their mass is the same as it is on Earth. The gravitational force on the space station is only slightly less than the gravitational force on Earth.
The space station is actually in freefall around the planet. That is what orbit is (constant free fall around the planet). It is moving so quickly forward, it clears the planet because the earth curves.
They never fall to Earth, since the curvature of Earth exactly matches the shape of the orbit, but they are constantly falling, nonetheless. The station and its contents are weightless since no force is exerted to counterbalance the gravitational force. Based on what it means for something to have weight, this explains why despite having mass and despite being subject to a gravitational force the astronauts are weightless. Why astronauts are weightless
Frame of Reference The weightlessness has to do with the frame of reference. If everything is falling at the same rate, it appears as though it is floating with no gravity. Similar to being on an amusement park ride that drops you straight down. NASA - mass vs weight
If you put a penny on your knee and ride something that drops you, like the Demon Drop, it will float in front of you, at least from your frame of reference. Really it is falling at the same rate you are. In the case of the space station everything is falling with you (air included), so you can t tell you are falling. Weightless sensation
Friction is a force that always exists between any two surfaces in contact with each other.
Cause of friction
The sliding soap experiment
Frictional Forces Occur When Materials are in Contact N Spring Scale F M 1 W f s Surfaces in Contact F = Force Causing Motion (Pull on Scale) F s = Force of Static Friction (Resists Motion) N = Force Normal Holds Surfaces in Contact W = Weight of Object ( Mass x Gravity)
There are two main kinds of friction, based on how the two surfaces are moving relative to each other: Static friction The friction that exists between two surfaces that are not moving relative to each other. Kinetic friction The friction that exists between two surfaces that are moving relative to each other.
Friction is a Force That Resists Motion N Spring Scale F M 1 W f s Surfaces in Contact The pink block M 1 will not move until the force, F (pull on the scale ) exceeds the force of Static Friction f s.