WHAT WHY QUESTIONS

WHAT WHY QUESTIONS

SUMMARY/MAIN POINTS

1

Conversion Factors / Weight

Force

What is a 200 lb professor’s weight in Newtons? _______

Mass

What is a 30 kg dog’s mass in slugs? _______

Weight: force of gravity W = ______

200 lb on earth has a mass of ________

30 kg on earth has a weight of ________

2

Conversion Factors / Weight

Lazydog weighs 64 lb on the earth and gmoon is 5.31 ft/s2

what is the dog’s mass on the moon? __________ slugs __________ kg

what is the dog’s weight on the moon? ___________ lb ___________ N

3

The Original Alabama Bass Tracker

Alabama fan weighs 195 lbs. His “boat” and motor weigh 218 lbs and the motor provides a force of 143 lbs. The drag resistance of the water is 62 lbs.What is the acceleration of the boat?

What mass is used for kinetic diagram?

4

The Original Alabama Bass Tracker

Alabama fan weighs 195 lbs. His “boat” and motor weigh 218 lbs and the motor provides a force of 143 lbs. The drag resistance of the water is 62 lbs.

What are the forces on the fisherman?

WHAT WHY QUESTIONS

WHAT WHY QUESTIONS

SUMMARY/MAIN POINTS

1

A train consists of 3 locomotives and 100 coal cars.

Each locomotive weighs 432x103 lbs and provides a tractive effort of 183 x103 lbs.

Each coal car weighs 286x103 lbs.

Determine the acceleration of the train as it goes up a 1% incline.

Today’s Topics

• F=ma examples

• Non-constant forces

http://www.thedieselshop.us/Data%20ES44AC.HTML

What is the order of magnitude of the acceleration?

The train is going up an incline. What can you say about the normal force vs the weight?

2

3 Locomotives Total

Weight: 1296 kipsTractive Force: 549 kips

100 Coal cars Total

Weight: 28,600 kips

For the previous problem, find the force in the coupling between the engines and the cars.

What can you say about the coupling force vs the tractive force?

3

A 7600 kg helicopter is lifting a 1200 kg load straight up. The helicopter provides an upward lift force of 95000 N. Determine the tension in the cable connecting the load to the helicopter.

What is the tension in the cable compared to the weight of the load?

1200 kg

Ca

ble

7600 kg

4

Non-constant acceleration

A 25,000 lb airplane taking off from rest has a variable

thrust as shown below. Resistant forces are a constant

1000 lb. If the airplane takes off at t=45 seconds, what

is the take-off velocity?

0

2000

4000

6000

8000

10000

12000

0 15 30 45

Time (seconds)

Th

rust

(lb

s)

WHAT WHY QUESTIONS

WHAT WHY QUESTIONS

SUMMARY/MAIN POINTS

1

Table Saw BladeTable Saw BladeTable Saw BladeTable Saw Blade

The 10 inch diameter blade on a table saw spins at

3450 rpm. What is the speed of the teeth in mph?

What does NOT affect the speed of the teeth?

Circular Motion Relationships

1 cycle = (2π )rad

f =1

T

ω = 2π f

ω =2π

T

v = rω

atan = rα

ω =dφ

dt

α =dω

dt=

d2φ

dt2

2

A 687 N person is riding a 15 m radius Ferris Wheel that is moving at 3 rpm

(0.314 rad/s). The seat bottom stays horizontal throughout the ride. Find

the vertical force on the person at each position?

Ferris WheelFerris WheelFerris WheelFerris Wheel

=

A

B

CD

30°

=

=

=

3

A 687 N person is riding a 15 m radius Ferris Wheel. The seat bottom stays horizontal

throughout the ride. How fast would the wheel have to spin to feel weightless at A?

Ferris WheelFerris WheelFerris WheelFerris Wheel

=

A

W

N

WHAT WHY QUESTIONS

WHAT WHY QUESTIONS

SUMMARY/MAIN POINTS

1

Test Your Knowledge

W=12 lb

Q

μs = 0.5, μk = 0.4

+xP

Q Maximum friction force

0

8 lb

P Q Friction force

5 lb 0

-5 lb 0

7 lb 0

7 lb 8 lb

2

Finding μs: Block on an Inclined Plane

θ

3

Pushing box across a floor

Find the minimum force required to start

the 50 lb box moving if the angle of the

push is 20° above horizontal.

μs=0.8 μk=0.6

What can we say about the normal force vs. the weight

4

Pushing box across a floor

If we push with a force of 65 lb, what is

the acceleration of the box?

μs=0.8 μk=0.6

WHAT WHY QUESTIONS

WHAT WHY QUESTIONS

SUMMARY/MAIN POINTS

1

Horizontal CurveHorizontal CurveHorizontal CurveHorizontal Curve

A car goes around a 285 ft radius curve. The road is flat

and μs = 0.90. Determine the maximum speed the car

can go without sliding.

FBD = KD

Top View

Front View

2

Horizontal CurveHorizontal CurveHorizontal CurveHorizontal Curve

A car goes around a 285 ft radius curve. The road is flat

and μs = 0.90. The car is increasing speed at the rate of

8 ft/s2. Determine the maximum speed the car can go

without sliding.

FBD = KD

Top View

Front View

3

Banked CurveBanked CurveBanked CurveBanked Curve

The turns at Bristol Motor Speedway are banked

at 36° and have a radius of 285 ft.

Determine the speed of a race car so that there

is no friction force perpendicular to the car.

Direction of centripetal acceleration?FBD = KD

Solving for normal force: � ���

��� �

Substitute: ��

��� �sin � �

��

�

Cancel m: g�tan � � ���

�

Solution: � � ���������

4

Banked CurveBanked CurveBanked CurveBanked Curve

The turns at Bristol Motor Speedway are banked at

36° and have a radius of 285 ft. The coefficient of

static friction is 0.90.

Determine the speed at which it will start sliding up

the track. (Assume constant speed)

FBD = KD

Solving for normal force:

� ���

��� � ���� ! ���

Substitute: ��

��� � ���� ! ����sin � + $%cos (�)) =

��

�

Cancel m: �(� !(�)(����� (�))

��� � ���� ! (�)=

��

�