1. PHYSICS 231 INTRODUCTORY PHYSICS I
Lecture 5

2. Main points of last lecture
Range (yi=yf):
Relative velocity:
Newton’s Laws:
If SF = 0, velocity doesn’t change.
vbe = vbr + vre

3. Newton’s Third Law Force on “1” due to “2”
Single isolated force cannot exist
For every action there is an equal and opposite reaction
Action and Reaction Forces act on different objects

4. Free-body Diagram Draw arrows for all forces acting ON object
If the object is in “equilibrium” (at rest or constant v)
Otherwise, find acceleration by
Ignore rotational motion for now. Treat object as a particle.

5. Example 4.1 Two blocks sit on a frictionless table. The masses are
M1=2 kg and M2=3 Kg. A horizontal force F=5 N
is applied to Block 1.
What is the acceleration of the blocks?
What is the force of block 1 on block 2? F M2 M1
1. a = 1 m/s2
2. F21= 3 N

6. Mechanical Forces Gravity: w=mg (down) Normal forces
Strings, ropes and Pulleys
Friction
Springs (later)

7. Rules for Ropes and Pulleys
Force from rope points AWAY from object
(Rope can only pull)
Magnitude of the force is Tension
Tension is same everywhere in the rope
Tension does not change when going over pulley
Approximations: Neglect mass of rope and pulley,
neglect friction in pulley

8. Example 4.2
I pull a 5 kg mass up with a rope, so that it accelerates 2 m/s2. What is the tension in the rope?
T = 59 N

9. Example 4.3 - Atwood Machine
a) Find acceleration
b) Find T, the tension in the string
c) Find force ceiling must exert on pulley
7 kg
5 kg
a) a = g/6 = m/s2 b) T = 57.2 N c) Fpulley=2T = N

10. Example 4.4a T1 is _____ T2 cos(10o)=0.985 sin(10o)=0.173
Which statements are correct? Assume the objects are in static equilibrium.
T1 is _____ T2
A) Less than
B) Equal to
C) Greater than
cos(10o)= sin(10o)=0.173

11. Example 4.4b T2 is ______ T3 cos(10o)=0.985 sin(10o)=0.173
Which statements are correct? Assume the objects are static.
T2 is ______ T3
A) Less than
B) Equal to
C) Greater than
cos(10o)= sin(10o)=0.173

12. Example 4.4c T3 is ______ Mg cos(10o)=0.985 sin(10o)=0.173
Which statements are correct? Assume the objects are static.
T3 is ______ Mg
A) Less than
B) Equal to
C) Greater than
cos(10o)= sin(10o)=0.173

13. Example 4.4d T1+T2 is ______ Mg cos(10o)=0.985 sin(10o)=0.173
Which statements are correct? Assume the objects are static.
T1+T2 is ______ Mg
A) Less than
B) Equal to
C) Greater than
cos(10o)= sin(10o)=0.173

14. Example 4.4
Given that Mlight = 25 kg, find all three tensions
T3 = N, T1 = N, T2 = N

15. Cable Pull Demo

16. Inclined Planes
Choose x along the incline and y perpendicular to incline
Replace force of gravity with its components
30.0

17. Example 4.5 Find the acceleration and the tension
a = 4.43 m/s2, T= 53.7 N

18. Example 4.6 M
Find M such that the box slides at constant v
M=15.6 kg

19. Frictional Forces
RESISTIVE force between object and neighbors or the medium
Examples:
Sliding a box
Air resistance
Rolling resistance

20. Sliding Friction Parallel to surface, opposing direction of motion
~ independent of the area of contact
Depends on the surfaces in contact
Object at rest: Static friction
Object in motion: Kinetic friction

21. f F Static Friction, ƒs Just enough force to keep object at rest.
ms is coefficient of static friction
N is the normal force f F

22. f F Kinetic Friction, ƒk mk is coefficient of kinetic friction
Friction force opposes
direction of motion
N is the normal force F

23. Coefficients of Friction

24. Example 4.7
The man pushes/pulls with a force of 200 N. The child and sled combo has a mass of 30 kg and the coefficient of kinetic friction is For each case: What is the frictional force opposing his efforts?
What is the acceleration of the child?
f=59 N, a=3.80 m/s / f=29.1 N, a=4.8 m/s2

25. Example 4.8 a) ms = 0.5 b) a=1.96 m/s2 c) 39.25 N
Given m1 = 10 kg and m2 = 5 kg:
a) What value of ms would stop the block from sliding?
b) If the box is sliding and mk = 0.2, what is the acceleration?
c) What is the tension of the rope?
a) ms = b) a=1.96 m/s2 c) N

26. Example 4.9
What is the minimum ms required to prevent a sled from slipping down a hill of slope 30 degrees?
ms = 0.577

27. Other kinds of friction
Air resistance, F ~ Area  v2
Rolling resistance, F ~ v
Terminal velocity:

28. Coffee Filter Demo

29. Example 4.9
An elevator falls with acceleration a = 8.0 m/s2. If a 200-lb person stood on a bathroom scale during the fall, what would the scale read?
36.9 lbs

30. Accelerating Reference Frames
Equivalent to “Fictitious” gravitational force

31. Fictitious Force: Derivation
Eq. of motion in fixed frame
F-maf looks like force in new frame, maf acts like fake gravitational force!

32. Example 4.10
You are calibrating an accelerometer so that you can measure the steady horizontal acceleration of a car by measuring the angle a ball swings backwards.
If M = 2.5 kg and the acceleration, a = 3.0 m/s2: a) At what angle does the ball swing backwards?
b) What is the tension in the string? q
q = 17 deg T= 25.6 N

33. Example 4.11a
A fisherman catches a 20 lb trout (mass=9.072 kg), and takes the trout in an elevator to the 78th floor to impress his girl friend, who is the CEO of a large accounting firm. The fish is hanging on a scale, which reads 20 lb.s while the fisherman is stationary. Later, he returns via the elevator to the ground floor with the fish still hanging from the scale.
In the instant just after the elevator begins to move upward, the reading on the scale will be ______________ 20 lbs.
Greater than
Less than
Equal to

34. Example 4.11b
A fisherman catches a 20 lb trout (mass=9.072 kg), and takes the trout in an elevator to the 78th floor to impress his girl friend, who is the CEO of a large accounting firm. The fish is hanging on a scale, which reads 20 lb.s while the fisherman is stationary. Later, he returns via the elevator to the ground floor with the fish still hanging from the scale.
On the way back down, while descending at constant velocity, the reading on the scale will be ________________ 20 lbs.
Greater than
Less than
Equal to

35. Example 4.11c
A fisherman catches a 20 lb trout (mass=9.072 kg), and takes the trout in an elevator to the 78th floor to impress his girl friend, who is the CEO of a large accounting firm. The fish is hanging on a scale, which reads 20 lb.s while the fisherman is stationary. Later, he returns via the elevator to the ground floor with the fish still hanging from the scale.
In the instant just before the elevator comes to a stop on the 78th floor, the mass of the fish will be ______________ kg.
Greater than
Less than
Equal to