Download Chapter 13: The Nature of Forces I. Forces A. Any _push__ or

Chapter 13: The Nature of Forces
I. Forces
A. Any _push__ or _pull__acting on an object.
B. Forces can make an object:
1. Stop
2. Start moving
3. Change speed, direction, or shape
C. _Balanced___forces mean there is no change in the __motion___
__Unbalanced__forces change the motion of an object.
D. Examples of forces are:
1. Friction
2. Gravity
E. We measure force with __spring scales___
F. The unit of force is the __Newton___
G. Like velocity and acceleration, a force is described by its __strength__ and the __direction___
In which it acts.
II. Friction is a _force__that opposes __motion_____.
It can slow or even __stop__ an object. It is always in the _opposite___direction of the motion.
A. Two factors that affect friction are
1. the types of __surfaces__involved.
2. how hard the surfaces are _pushed together___
B. There are 4 types of friction
1. Sliding Friction
This occurs when 2 solid surfaces slide over each other. It makes moving objects slow down or stop. Without
this, a person on a sled wouldn’t stop until they hit a wall.
2. Static Friction
This acts between objects that are not moving. You need to overcome this friction to move something.
Example: You need to push a couch across a room. If you don’t push hard enough to overcome static friction, it
won’t move.
3. Fluid Friction
This occurs when a solid object moves through a fluid. It is easier to overcome than sliding friction. This is why
sidewalks become slippery when they are wet.
4. Rolling Friction
This occurs when an object rolls across a surface. It is much easier to overcome than sliding friction. This is why
it is easier to push a bike when the wheels can turn, but more difficult if you apply the brakes.
B. Examples of when friction is bad: Things wear out, it reduces speed, it makes it harder to move things.
C. Examples of when friction is good or helpful. Slowing down or stopping, walking, driving on ice
III. Gravity
A. A force of __attraction__between 2 objects.
It is a force that __pulls___objects toward each other.
B. 2 factors that affect the gravitational attraction between objects are:
1. The mass of the objects
2. The distance between the objects
C. The more mass an object has, the __greater_its gravitational force.
D. It is responsible for __accelerating_an object toward the earth. All objects accelerate at
the same rate if there is no air resistance. (9.8 m/s2) A marble, a rock, and a boulder would all hit the
ground at the same time if there were no air resistance.
E. __Weight___is a measure of the force of gravity on an object.
W = m g. (Weight = mass x gravity)
The lower the __gravity_, the lower the __weight__. You weigh _less__ on the moon
than on Earth.
F. __Air_ resistance
Any falling object meets air resistance. A leaf and a rock to not fall at the same rate. The shape of the object
causes different amounts of air resistance. At the earth’s surface, with no air resistance, the acceleration due to
gravity of a falling object is 9.8 m/s/s. (For an object that starts at rest and falls, after one second it would be
going 9.8 m/s. After 2 seconds, it would be going 19.6 m/s. After 3 seconds, 29.4 m/s. if there were no air
resistance.)