Transcript Sliding friction

Do Now – get out Velocity HW for
Checking then solve the following:
• The average velocity of an object can be found by dividing the
distance an object travels by the amount of time it took to travel
that distance. Sometimes positions in races are determined by the
highest average speeds obtained by the cars as they qualify in
individual heats.
• Suppose a race-car driver need to average 100 mpt in a
qualifying heat in order to gain the top spot. The heat is 100
miles. At exactly the halfway mark, her crew radios her that she
has only averaged 50 mph. If the car is capable of going 150
mph, can she still make the top spot? Defend your answer.
Friction
Sliding
Rolling
Fluid
Sliding friction
When two solid surfaces
slide over each other the
sliding friction acts between
the surfaces. When you push
a chair across the floor,
sliding friction opposes your
motion.
Sliding friction
The amount of sliding friction present depends on
two factors: the weight of the object that is
moving and the types of surfaces that the object
slides across. There is more friction when a stack
of cartons is pushed than when just one carton is
pushed. But there is less friction opposing the
motion if the cartons are pushed across a smooth
floor rather than across a carpeted one.
Rolling friction
tends to oppose motion less than sliding
friction. So wheels are often placed under
objects to make it easier to use them.
Fluid friction
All liquids and gases are fluids. Air,
water, and oil are fluids. When an
object moves through a fluid, fluid
friction opposes the motion.
Air resistance is an example of fluid
friction. Substances called lubricants
change sliding friction to fluid friction.
Speed
Speed = distance
time
Speed is the
distance traveled
by a moving
object per unit of
time.
You can calculate the speed of a
moving object by dividing the
distance the object travels by the
time it takes to travel that distance.
Speed
Speed that does not change is
called constant speed. But the
speed of a moving object is not
always constant. Dividing the
total distance by the total time
gives the average speed.
Total Distance = Average Speed
Total Time
Velocity
• Velocity is speed in a given direction
Velocity Homework
1. 75km N in 1.5 hrs. = 75km/1.5 = 50km/hr
2. 3000mi x 5hrs = 15,000mph
3.
Work HW
1. W = F x d. The force on the mountain
climber is 900 N. (What force is this?) and
he climbs 100 m. So the work done here is
900N x 100m = 900N-m or 9000J.
2. The force required to push the lawn mower
is 200N and 4000J or work is done. Since
W = F x d, then d = W/F. The distance the
lawn mower is moved is 4000J/200N or 20
meters.
Power HW
3. The Power of 40,000J of work in 20 sec is
40,000J = 2000W
20s
4. P = W/t = F x d/t so the power of the diver is
750 N x 10 m = 5000W
1.5s