–
Friction is due to surface bumps and also to
the stickiness of atoms on the surfaces of the
two materials.

The Force of Friction
CHECK YOUR UNDERSTANDING
The force of friction can occur
A.
with sliding objects.
B.
in water.
C.
in air.
D.
all of the above

The Force of Friction
CHECK YOUR ANSWER
The force of friction can occur
A.
with sliding objects.
B.
in water.
C.
in air.
D.
all of the above
Comment:
Friction can also occur for objects at rest. If you push
horizontally on your book and it doesn't move, then friction
between the book and the table is equal and opposite to
your push.

The Force of Friction
CHECK YOUR UNDERSTANDING
When Nellie pushes a crate across a factory floor at
constant speed, the force of friction between the crate and
the floor is
A.
less than Nellie's push.
B.
equal to Nellie's push.
C.
equal and opposite to Nellie's push.
D.
greater than Nellie's push.

The Force of Friction
CHECK YOUR ANSWER
When Nellie pushes a crate across a factory floor at
constant speed, the force of friction between the crate and
the floor is
A.
less than Nellie's push.
B.
equal to Nellie's push.
C.
equal and opposite to Nellie's push.
D.
greater than Nellie's push.

The Force of Friction
CHECK YOUR UNDERSTANDING
When Nellie pushes a crate across a factory floor at an
increasing speed
, the amount of friction between the crate
and the floor is
A.
less than Nellie's push.
B.
equal to Nellie's push.
C.
equal and opposite to Nellie's push.
D.
greater than Nellie's push.

The Force of Friction
CHECK YOUR ANSWER
When Nellie pushes a crate across a factory floor at an
increasing speed
, the amount of friction between the crate
and the floor is
A.
less than Nellie's push.
B.
equal to Nellie's push.
C.
equal and opposite to Nellie's push.
D.
greater than Nellie's push.
Explanation:
The increasing speed indicates a net force greater than
zero. The crate is not in equilibrium.

distance covered
travel time
Speed and Velocity
•
Speed
is the distance covered per amount of
travel time.
Equation for speed:
Speed =

•
Average speed
is the total distance traveled
divided by the time of travel.
Equation for average speed:
Average speed =
•
Instantaneous speed
is the speed at any
instant of time.
total distance covered
travel time
Speed and Velocity

Speed and Velocity
CHECK YOUR UNDERSTANDING
The average speed in driving 30 km in 1 hour is
the same as the average speed in driving
A.
30 km in 1/2 hour.
B.
30 km in 2 hours.
C.
60 km in 1/2 hour.
D.
60 km in 2 hours.

Speed and Velocity
CHECK YOUR ANSWER
The average speed in driving 30 km in 1 hour is
the same as the average speed in driving
A.
30 km in 1/2 hour.
B.
30 km in 2 hours.
C.
60 km in 1/2 hour.
D.
60 km in 2 hours.

change of velocity
time interval
Acceleration
•
Acceleration
is the rate at which velocity
changes with time. The change in velocity may
be in magnitude, in direction, or both.

#### You've reached the end of your free preview.

Want to read all 61 pages?

- Spring '14
- AnnM.Robinson