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Gravity
100 kg
5 kg
30 N
1. A chihuahua with a mass of 5 kg exerts a
force of 30 N on a Happy Face with a mass of
100 kg. If the friction is 15 N, how fast did the
chihuahua accelerate the Happy Face?
2. If the chihuahua pushes too far and they
both fall off the edge of the table, which one
accelerates faster?
Gravity
Objectives:
• Describe gravitational force.
• Distinguish between mass and weight.
• Explain why objects that are thrown will follow
a curved path.
• Compare circular motion with motion in a
straight line.
Gravity
Gravity- an attractive force between two
objects that depends on the masses of the
objects and the distance between them
Gravitational force increases as:
 Masses increase
 Objects move closer together
Gravity
Force of gravity increases as mass increases!
Gravity
Force of gravity increases as objects get closer together!
Gravity
Gravitational Acceleration (g):
Gravity causes objects to accelerate toward the center of Earth.
When all forces, except gravity, acting on a falling object can be
ignored, the object is said to be in free fall. All objects near Earth’s
surface would fall with the same acceleration.
The acceleration of an object in free fall close to the Earth’s surface.
g = 9.8 m/s2
• Weight- the gravitational force exerted on an object
• Mass- measures the amount of matter
– Weight and mass are related:
— Weight (N) = mass (kg) x g (acceleration of gravity, m/s2)
— How much would you weigh on the Moon?
• Weight is different, but mass stays the same.
Calculating Weight
• WEIGHT ON EARTH
W = 70 kg × 9.8 m/s2 = 686 N
• WEIGHT ON THE MOON
W = 70 kg × 1.6 m/s2 = 112 N
Weight Calculator for the Solar System
Why do all objects have the same
free-fall acceleration?
 Newton’s second law shows that acceleration
depends on both the force on an object and its
mass.
 A heavier object experiences a greater gravitational
force than a lighter object. However, a heavier
object is also harder to accelerate because it has
more mass.
 The extra mass of the heavy object exactly
compensates for the additional gravitational force.
Newton’s Law of Universal Gravitation
or Did an apple really fall on his head?
Projectile MotionAn object with horizontal velocity is pulled down by
gravity.
Gravity
Centripetal ForceThe net force exerted toward
the center of a curved path.
– Car going around a curve
– People on an amusement park ride
– Earth’s gravity pulls the Moon into
circular orbit
Inertia
Centripetal
Force
Gravity
Centripetal Force“Centripetal” is not the name
of a force – it actually
describes a force.
• Centripetal force may be
due to :
gravity
— friction
— a physical structure
—
Inertia
Centripetal
Force