Transcript Lecture 21 (Oct. 30) - McMaster Physics and Astronomy

Friction
• Newton’s 2nd Law with Friction
• Air drag
• Examples
Outline
QQ38: Inequality
Example: A car is moving at a constant speed of
24m/s. The driver hits the brakes which causes the
car to slide. Given that μk=0.8 for rubber on concrete:
a) what is the acceleration of the car ?
b) how far will it move before stopping ?
Demo
Example: Two blocks are pulled with a force of 28N over a
rough surface. Block 1 has a mass of 1kg and a coefficient
of friction of 0.3 with the surface while Block 2 a mass of
2kg and a coefficient of friction of 0.2 with the surface.
What is the acceleration of the system ?
QQ38: Inequality
Quick Quiz:
A horizontal force of 150 N is applied to a 50 kg
box on a level floor. The coefficient of static
friction between the box and the floor is 0.5. The
magnitude of the frictional force on the box is:
(A)
(B)
(C)
(D)
(E)
150 N
250 N
500 N
3750 N
15000 N
QQ38: F prop N
Quick Quiz:
The pilot of the helicopter that you are in has fallen
asleep, and the helicopter is now in freefall. Can you
run to the cockpit and save the day?
a) Yes
b) No
c) Depends of μk of the floor
d) Depends of μs of the floor
QQ38: Inequality
Quick Quiz:
A force of 150 N is applied at an angle of 300 to the
horizontal to a 20 kg box on a level floor. How does
this force affect the normal force:
(A) No change
(B) Decrease the normal force
(C) Increase the normal force
QQ38: Inequality
Terminal Speed
A falling object in the presence of air resistance will
reach some maximum velocity beyond which it cannot
accelerate.
This is due to the fact that air friction (drag)
increases as your velocity increases. Once this drag
force is equal to the force of gravity, there is not net
force and acceleration is zero:
D
D
Fg
Fg > D
a>0
Fg
Fg = D
a=0
QQ38: Inequality
Terminal Speed
The air drag is equal to D=¼ Av2, where A is the
surface area of the object.
Hence, the maximum speed will be when D=Fg:
¼ Av2 = mg
vterm =
4mg
A
Note: more massive objects have a larger vterm,
as do objects with a smaller surface Area
QQ38: Inequality
Example
a) Estimate the terminal velocity of a 60kg person
falling down in a horizontal position.
b) Estimate the terminal velocity of a 60kg person
falling down in a vertical position.
Do for next class:
• Read: Sections 5.6
•Suggested problems: 5.50, 5.51, 5.53
• Do as many more of the problems from the end of the
chapter as it takes you to realise that they are all the
same. If this takes you more than two full problems,
make sure that you are following the procedure that I
followed for the examples in class.