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Bellringer/11.2 Notes
Recall that weight is defined as a measure of the gravitational force exerted on an
object. Use knowledge you have about gravity to answer the questions in the following
situations:
1. Elvis is a student whose mass is 70 kg. On Earth’s surface, Elvis weighs about 690 N.
Suppose Elvis could stand on the surface of the following bodies in the solar
systems. Match Elvis’ weight with the letter of the appropriate body. (Note that
Earth has a mass of 6.0 x 1024 kg.)
Planet
a. Jupiter (m = 1.9 x 1027 kg)
b. Venus (m = 4.9 x 1024 kg)
c. Neptune (m = 1.0 x 1026 kg)
d. Mercury (m = 3.3 x 1023 kg)
e. Earth’s moon (m = 7.4 x 1022 kg)
2.
Elvis’ weight
780 N
113 N
260 N
1800 N
620 N
Suppose Elvis is in orbit around Venus at a distance twice as far from the planet’s
center as the surface of Venus is. Would you expect his weight to be greater than,
less than, or equal to his weight on the surface of the planet?
Section 2
Gravity
I. Law of Universal Gravity
A. The gravitational force exerted by
objects is related to:
1. Distance between the objects
2. Size of their masses
B. Gravitational force increase as mass
increases
Since the moon is smaller than Earth, the
gravitational force placed on objects on the
moon is much less.
C. The distance between objects will
significantly decrease the gravitational forces
between two objects.
II. Free Fall and Weight
A. In the absence of air resistance, all objects
fall at the same rate:
B. On Earth, free-fall acceleration due to
gravity (g) = 9.8m/s2.
1. Force of gravity on an object = weight.
Example:
My weight in Newtons
72.5 kg x 9.8 m/s2 = 710 N
C. Mass and weight are not equal.
On the moon, gravity is about 1.6 m/s2.
1. Even on Earth, the gravitational
force can change
On Earth at sea level g = 9.8 m/s2
Weight = 72.5 kg x 9.8 m/s2 = 710 N
(159.6 pounds)
On Earth atop a high mountain g = 9.78 m/s2
Weight = 72.5 kg x 9.78 m/s2 = 709.5 N
(159.5 pounds)
D. Terminal Velocity – Air resistance = Gravity
Unbalanced
+ Acceleration
Unbalanced
+ Acceleration
Unbalanced
+ Acceleration
Balanced
0 Acceleration
E. Free fall only occurs when gravity is the
only force acting on an object (in space).
III. Projectile Motion and Gravity
A. Projectile Motion – curved path an object
takes due to:
1. Horizontal motion
2. Vertical motion
Path of a ball:
Horizontal motion
(force applied by person)
Vertical
motion
(gravity)
Path of ball