Lecture 4 Force and Motion Gravitation. Chapter 2.7 2.15. Outline. Laws of Motion Mass and Weight Gravitation. The law of gravity – 1666, 3 laws of motion – 1686 . If no net force acts on it, then. First Law of Motion. A force is needed to start motion or change its velocity
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Chapter 2.7 2.15
The law of gravity – 1666, 3 laws of motion – 1686
A force is needed to start motion or change its velocity
A force is any influence that can change the speed or direction of motion of an object
an object remains at rest
an object in motion remains in motion
An object continues to be accelerated as long as a net force acts upon it
The reluctance of an object to change its state of rest or of uniform motion in a straight line is inertia
Mass is a property of matter that shows itself as inertia
The SI unit of mass is kilogram (kg)
1 liter (0.26 gallon) has a mass of 1 kg
1 liter = 10cm x 10cm x 10cm
More about mass and inertia
The more mass an object has, the less its acceleration for a given force.
F = net force
m = mass
a = acceleration
a = ---
The net force on an object equals to the product of the mass and the acceleration of the object.
The direction of the force is the same as that of the acceleration.
The second law of motion defines the unit of force.
F=ma, [m]=kg, [a]=m/s2 [F]=kg m/s2 = newton (N)
1 lb = 0.453 kg x 9.8 m/s2 = 4.45 N 1 N = 0.225 lb
The weight of an object is the force it is attracted by the earth’s gravitational pull.
Mass is a measure of the amount of matter included in an object.
When an object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.
Action force is equal to reaction force
Centripetal force is an inward force on an object moving in a curved path
Fc = ---- r is the radius of a circle
The centripetal force always points toward the center of curvature of the object’s path.
The Earth’s mass can be found from the law of gravity.
Weight of an object F = mg
G m M
F = ----------
Gravitational force on an object
G = 6.67 10-11 N m2/kg2
M = ------ = 6 1024 kg
R = 6378 km = 6.38 106 m
g = 9.8 m/s2
In order to launch a satellite, the gravitational force has to be overcome.
Centripetal force = gravitational force
------ = mg v2 = rg v = rg
Minimal orbital speed
Escape speed (ve): the speed required to leave the gravitational influence of a body.
For the Earth: v = 28,400 km/h, ve=40,000 km/h