1. 9/15/2013 1
The Gravity Field
Physics on the Road
Lesson 11

2. 9/15/2013 2
LI…
 Explain the difference between
mass and weight
 Know that the weight of an object
is proportional to it’s mass
 Describe a gravitational field

3. 9/15/2013 3
Newton’s 3 Ideas
 Mass and weight
 The gravity field
 Forces change velocities

4. 9/15/2013 4
Mass and weight: on
Earth, on the Moon
 The force needed to push the sack is related to it’s
mass and resistance to this is called inertia.
 The force needed to lift the sack is related to it’s
mass and called it’s weight.
 Would there be difference on the moon?

5. 9/15/2013 5
Mass and weight
 The pull of gravity on an
object is proportional to
it’s mass and is called it’s
weight.
weight = mass x gravity
 The force needed to
accelerate an object is
proportional to it’s mass
force = mass x acceleration
 The weight of an object
is a force and has units
of Newtons (N)
 Mass is the amount of
matter and has units of
kilograms (kg)
W = mg
F = ma
Gravity and acceleration are equivalent

6. 9/15/2013 6
The Gravity Field
 In physics we use the idea of invisible regions or fields filling
space to explain forces which act at a distance (non-
contact)
 Gravity (or weight) is a non-contact force and is produced by
a gravity field
 Newton’s famous idea was that masses produce their own
gravity fields. The bigger the mass the stronger the field.
planet
Planet surface

7. 9/15/2013 7
Gravity and
acceleration
 W = mg and F = ma are
equivalent formulas (because
weight is a force)
 So this means that for
objects in a gravity field the
field and the objects
acceleration are equivalent.
So gravity fields have two
units
1. Newtons/kilogram - Nkg-1
2. Metres/second/second - ms-2
F = mg and a = F / m
So for either mass…
acceleration = mg / m = g
ie. they fall with same
acceleration.
W w
gravity field

8. 9/15/2013 8
Forces change
velocities
 Newton explained that forces change velocities.
 Sometimes this means a change of speed, but
not always.
1. Use the ideas of vectors to explain that a change
of velocity can happen without a change of speed
2. Give an astronomical example of this and use a
diagram
3. Isaac Newtons ideas were slow to be accepted in
the 17th Century (particularly in France). Use the
differences in transport between then and now to
explain why.