Physics Chapter 4

1. Chapter 4
Dynamics and Newton’s
Laws of Motion

2. Isaac Newton
• 1642-1727
• Before age 30:
1) basic concepts and law of
mechanics
2) Law of universal gravity
3) Invented mathematical
method of calculus.
Published in 1687

3. Force and Mass
Force: push or pull
Force is a vector – it has magnitude and
direction

4. Force and Mass
Mass: measure of how
hard to change an
object’s velocity or
measure of Inertia
Mass : measure of the
quantity of matter in an
object.

5. Newton’s First Law of Motion
Stop pushing an object, does it stop moving?
Only if there is friction!
In the absence of any net external force, an
object will keep moving at constant velocity,
or remain at rest.
also known as the Law of Inertia.
No net force:
RestStay rest
MovingStaying moving with constant v

6. Ex: moving car, the use of seat belt… ice
hockey…
The tendency to stay the same or resistance to
change Inertia (laziness)
Mass is a measure of Inertia
Newton’s First Law of Motion
CYU:
1) throw a rock in space (no friction, no gravity), what will rock do?
2) A 2kg object moving with constant v, how much net force is needed?
3) Joe eats large quantity of food, what happens to inertia?

7. Newton’s Second Law of Motion
Two equal weights exert twice the force of one;
calibration of a spring:
No net force
Stay rest or moving
Net force
??

8. Newton’s Second Law of Motion
Acceleration is proportional to force:

9. Newton’s Second Law of Motion
Acceleration is inversely proportional to mass:

10. Newton’s Second Law of Motion
Combining the observations:
Or, total force or net force
m
F
a
∑=


amF

=∑
Unit: kg m/s2
= N

11. 5-3 Newton’s Second Law of Motion

12. Newton’s Second Law of Motion
Free-body diagrams:
A free-body diagram shows every force acting on an
object.

13. Newton’s Second Law of Motion
Example of a free-body diagram:

14. Vector Nature
yy
xx
maF
maF
=
=
∑
∑
x
y

15. Summary
Sketch the forces
• Isolate the object of interest
• Choose a convenient coordinate system
• Resolve the forces into components
• Apply Newton’s second law to each coordinate direction
yy
xx
maF
maF
=
=
∑
∑

16. Ex: pushing s stalled car. m=1850kg, two man pushing with
275N and 395N. Ground friction 560N, acceleration=?
Free body:
Reference frame:
Ex: Raft m=1300kg. Applied force 17 N due east. Wind
15N 67o
N of E. Ignore resistance, find acceleration.
ax=0.018, ay=0.011,a=0.021,θ=31.4

17. Find displacement at t=65s, if initial v is
0.15m/s due east.
X=48m, y=23m

18. Newton’s Third Law of Motion
If object 1 exerts a force F on object 2, then
object 2 exerts a force –F on object 1.
These forces are called action-reaction pairs.

19. Newton’s third law of motion
Whenever one body exerts a force on a second
body, the second body exerts an oppositely
directed force of equal magnitude on the
first body.
1) Pushing or hitting someone
2) Sitting on chair
3) A bug hitting windshields, which force is
greater? Bug hitting windshields or windshields
hitting the bug? Which one has greater a?

20. Newton’s Third Law of Motion
forces are the same, the accelerations are
different.

21. Combine 2nd
and 3rd
Law

22. Ex: In space, astronaut push the space station, what
happens? Ma =92kg, Ms=1100kg. P=32N on space station.
Find their acceleration.
2
Ex: A gun fires, gun would recoil, the acceleration of the
guns is greater, smaller and same?

23. Weight
weight of an object on the Earth’s surface is the
gravitational force exerted on it by the Earth.
F = ma =mg
Net force
Free fall
a=g
g=9.8 on surface of Earth
g=1/6 g on moon

24. Weight and mass
• Mass amount of matter, inertia
• Weight gravitational force
• Weight various in location.
Ex: if you weigh 600N on Earth, how much
do you weigh on moon?

25. Normal Forces
The normal force is the force exerted by a
surface on an object.

26. Normal Forces
The normal force is always perpendicular to the
surface.

27. Normal Forces
The normal force may be equal to, greater than,
or less than the weight.

28. Apparent weight:
Your perception of weight is based on the
contact forces
How do you weigh your self?
Measure normal or tension force

29. Apparent weight under acceleration
Person in elevator:
• Going up with v=constant
• Going up with a
• Going down with a
• Going down with g
Apparent weight ?
Free body diagram:

30. Photo 5-5
Astronaut candidates pose for a floating class picture during
weightlessness training aboard the “vomit comet.”
When free fall, apparent weight=0, weightless

31. Stephen Hawking in weightless
•http://www.msnbc.msn.com/id/17156385/
•The zero-gravity airplane flight produces the feeling
of weightlessness for about a half-minute at a time, by
following a parabolic up-and-down path at an altitude
of 30,000 feet. As the plane crests the top of the
parabola, the passengers and other objects in the
stripped-down, padded cabin essentially go into free-
fall — floating in the air as if they were in orbit.
•http://www.youtube.com/watch?v=JXQeQD0CJzY

32. Tension
pull on a string or rope, it becomes tautt 
tension along the string.

33. Ex: Jack and Jill lift up a 1.3kg water.
Jack 7.0 N at angle θ, Jill 11N 28o with vertical. Find θ so that
the bucket will accelerate straight up.
x y
F1
F2
W
Vector components:
=
=
∑
∑
y
x
F
F
48o
, 11.07m/s2

34. Ex: a 60kg block of ice acted on by two force with
magnitude of 13N and 11N as shown.
Find acceleration and normal force
F1 F2
60o
30o

35. Translational Equilibrium
translational equilibrium, net force on it is zero:
(6-5)

37. Small force big tension

38. Ex: θ=40o
, m=1.84kg. find Tension in each wire.
θFree body diagram:
94.6, 72.5N;

39. W=500N, find the tension in
each wire.

40. • Braces are used to apply forces to teeth to realign them. Shown in this figure are
the tensions applied by the wire to the protruding tooth. The total force applied to
the tooth by the wire, Fapp , points straight toward the back of the mouth.

41. W=500N, find the tensions.

42. Force
Approximate
Relative
Strengths
Range
Attraction/Rep
ulsion
Carrier
Particle
Gravitational −10 38 ∞ attractive only Graviton
Electromagne
tic
–10 2 ∞
attractive and
repulsive
Photon
Weak nuclear –10 13 < –10 18m
attractive and
repulsive
+W , –W , Z0
Strong
nuclear
1 < –10 15m
attractive and
repulsive
gluons
Extended Topic: The Four Basic Forces—
An Introduction