1. Fundamentals of Physics School of Physical Science and Technology
Mechanics
(Bilingual Teaching)
张昆实
School of Physical Science and Technology
Yangtze University

2. Chapter 5 Force and Motion
5-1 What Causes an Acceleration?
5-2 Newton's First Law
5-3 Force
5-4 Mass
5-5 Newton's Second Law
5-6 Some Particular Forces
5-7 Newton's Third Law
5-8 Applying Newton's Laws
5-9 Ininertial Reference System and
Inertial Forces

3. 5-1 What Causes an Acceleration?
★If the velocity of a particle changes in either maglitude or direction (acceleration) an interaction
between the body and its surroundings.
★ An interaction that can cause an acceleration of a body is called a force.
★ Newtonian mechanics: The study of the relationship between a force and the acceleration it causes.
★ Quantum mechanics
★ Special theory of relativity
Modifications
are necessary

4. 5-2 Newton's First Law
★Observations: Send a puck sliding over a extremely slippery surface, over which the puck would hardly slow.
★Conclusion: a body will keep moving with constant velocity if no force acts on it.
★ Newton’s First Law:
If no force acts on a body, then the body’s velocity cannot change; that is, the body cannot accelerate.
at rest remain at rest;
in motion move with constant
velocity.

5. 5-3 Force is measured by acceleration it produces.
★ Define the unit of force in terms of the acceleration that a force gives to a standard reference body (a mass of 1 kg).
★A force (vector)
is measured by
acceleration it
produces.
magnitude; direction
★Principle of superposition for forces
When two or more forces act on a body, a
net force or resultant force can be found. The
net force has the same effect on the body as
all the individual forces together.
mass
acceleration
force
1kg
1 m/s 1N
2 m/s 2N
a m/s aN

6. 5-3 Force ★ Newton’s First Law (restate):
If no net force acts on a body ( ), then the body’s velocity cannot change; that is, the body cannot accelerate.
★ Inertial Reference Frames
An inertial reference frame is one in which
Newton’s laws hold.
Example: the ground, any reference frame
moving with constant velocity
with respect to the ground.
★ Noninertial frame: a accelerating frame;
a rotational frame. F
net =

7. 5-4 Mass ★ What is mass ? the less massive baseball
receives a greater acceleration
the more massive bowling ball
receives a smaller acceleration
★Conjecture: The ratio of the masses of two bodies is equal to the inverse of the ratio of their accelerations when the same force is applied to both. a F m a x F m x

8. 5-4 Mass
★ What is mass ? a F m a x F m x F = 1N F = 8N

9. 5-4 Mass
★ Mass is an intrinsic characteristic of a body--- a characteristic that automatically comes with the existance of the body.
★ The mass of a body is the characteristic that relates a force on the body to the resulting acceleration.

10. 5-5 Newton's Second Law
★ Newton's Second Law:
The net force on a body is equal to the product of the body’s mass and the acceleration of the body.
(Newton's second law)
★ Caution: m is the mass of a body, is the vector sum of all the forces act on that body.
★ Equivalent equations:
(5-1)
(5-2)

11. 5-5 Newton's Second Law If (a) at rest stays at rest
★ The acceleration component along a given axis is caused only by the force component along that same axis, and not by force component along any other axis. If
(a) at rest stays at rest
(b) In motion move at constant
velocity
the forces and the body: in equilibrium
(5-2)

12. 5-5 Newton's Second Law ★ The SI unit of force: Newton (N)
★ the free-body diagram
To solve problems with Newton’s second law,
we often draw a free-body diagram in which
only one body, represented by a dot, is
considered. The external forces on the body
are drawn. A coordinate system is usually
included.
★ external forces: any force on the bodies inside the system (a collection of two or more bodies) from bodies outside the system.
(5-3)

13. 5-5 Newton's Second Law ★ Example of a free-body diagram
★ internal forces: forces between two bodies inside the system (a collection of two or more bodies).
★ Example of a free-body diagram
Sample Problem 5-3 (P79)

14. 5-6 Some Particular Forces
★ The Gravitational Force on a body is a pull by another body. In most situations
the other body is Earth or some other
astronomical body.
For Earth, the force is directed down toward the ground, which is assumed to be
a inertial frame.
★ A body(mass m) is in free fall with the free-fall acceleration of magnitude g. the only force acting on the body is the gravitational force (neglecting the effects of the air).
★ choose a vertical y axis along the body’s
path, with the positive direction upward.

15. 5-6 Some Particular Forces
★ The Gravitational Force
Newton’s second law
the vector form:
★ Weight (scalar)
The weight W of a body is the magnitude of the net force required to provent the body from falling freely.
upward force=grivatational force
balanced keep the ball at rest
the weight of the ball is the
magnitude of the upward force 2N
(5-8)
(5-9)
Upward force 2N

16. 5-6 Some Particular Forces
In general: a body has relative
to the ground (inertial frame), two
forces acting on it are balanced.
in vertical direction:
(weight, with ground as inertial frame)
Substituting for : (weight)
The weight of a body is equal to the magnitude
of the gravitational force on the body.
(5-10)
(5-11)
(5-12)

17. 5-6 Some Particular Forces
Spring scale
Measuring the weight of a body:
Equal-arm balance
When the device is in balance,
The gravitational force on the
Body (L) is equal to the gravi-
tational force on the reference
bodies (R).
The body stretches a spring, moving a pointer along a scale (in mass or force units)

18. 5-6 Some Particular Forces
★The weight of a body must be measured when the body is not accelerating vertically relative to the ground.
● apparent weight :
elevator (lift)
cab
P89 Sample Problem 5-8
apparent
weight
apparent
weight

19. 5-6 Some Particular Forces
★The weight of a body is not the mass of the body.
Mass
Weight
Measurement of inertial
Mass is constant
(v<<C speed of light)
Measured in kg
The magnitude of the gravitational force
varies in different places
latitude altitude
Measured in N

20. 5-6 Some Particular Forces
★The Normal Force y
Normal force
When a body presses against
a surface, the surface ( even
a seeminly rigid surface )
deforms and pushes on the
body with a normal force
that is perpendicular to the
surface.
Exp. A block rests on a tabletop
(5-13)
(5-14)

21. 5-6 Some Particular Forces
★ Friction
(more in Chapter 6)
Direction of
attempted
slide
● A frictional force is the force on a body when the body slides or attempts to slide along a surface.
● The force is always parallel
to the surface
● The force is directed so as to oppose the motion of the body.
Exp. A frictional force opposes the attempted
slide of a body over a surface.
★ Friction negligible frictionless

22. 5-6 Some Particular Forces
★ Tension: the cord is bing pulled taut
The cord is in the state of tension
The tension is the maglitude T of the force on the body.
● P83 Checkpoint 5 Sample Problem 5-4

23. 5-7 Newton's Third Law
Newton’s Third Law :
When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction.
● A pair of action-reaction
forces
a third law force pair
Book B
Crate C B C
● Scalar form:
Book B leans against crate C
● Vector form:
●Exp. Cantaloupe-table-earth three bodies

24. 5-8 Applying Newton's Laws
★ When you read the sample problems,
pay attention to:
Problem solving procedures;
How to draw a free-body diagram
with appropriate axes;
How to use Newton's Laws to solve
problems.
★ Sample problems: 漆安慎P79 例3

25. ★ 5-9 Noninertial Reference System and Inertial Forces
1. Lineal accelerating reference frame
A car is moving with
acceleration from rest.
A steel ball is put on a
frictionless surfice in the
car
No net force on the ball,
The ball accelerats with
relative to the car !
Newton’s laws don’t hold !
Suppose: a
acting on it, then: hold !
No net force acts on the ball,
The ball rests related
to the ground.
Newton’s laws hold
observer on ground
observer in the car

26. ★ 5-9 Noninertial Reference System and Inertial Forces
1. Lineal accelerating reference frame
A steel ball is connected
to a spring
As the car moves with a
acceleration from rest,
the spring is stretched *
A elastic force acts on the ball, But the ball rests
relative to the car !
Newton’s laws don’t hold !
Suppose: a
acting on it, then: hold !
A elastic force acts
on the ball, The ball
Moves with ,
Newton’s laws hold
observer on ground
observer in the car

27. ★ 5-9 Noninertial Reference System and Inertial Forces
In a lineal accelerating reference frame the Inertial Force acting on a body is equal to the product of and ( the acceleration of the noninertial reference system ), in opposite direction.
interacting force
Inertial Force
1.There is a third law force pair; Can be observed in both inertial and noninertial reference frame.
1. There is no reaction
force !
2. Can be observed only
in noninertial reference
frame !

28. ★ 5-9 Noninertial Reference System and Inertial Forces
Dynamical equation in a lineal
accelerating reference frame

29. ★ 5-9 Noninertial Reference System and Inertial Forces
2. Rotating noninertial reference frame *
A circular plate is rotating
about axis with a angular
speed , a ball is fixed by
a rope with its other end
fixed at the vertical axis.
observer on ground
observer on the plate
A centripetal force
acts on the ball,
The ball is in uniform
circular motion. Newton’s laws hold
A centripetal force acts on the ball, the ball rests on the plate!
Newton’s laws don’t hold !
Suppose: a
acting on it, then: hold !

30. ★ 5-9 Noninertial Reference System and Inertial Forces
In a rotating noninertial reference frame
the Inertial Force acting on a body in the radial
direction is
It is called the inertial centrifugal force
(离心惯性力)
If a body rests on a rotating noninertial frame ,
then

31. ★ 5-9 Noninertial Reference System and Inertial Forces
Coriolis force (科里奥利力):
If a body has motion relative to a rotating
noninertial reference frame, the body may
experience the Coriolis force
Coriolis acceleration
Coriolis force
The examples of Coriolis force

32. ★ 5-9 Noninertial Reference System and Inertial Forces
Coriolis force (科里奥利力):
Coriolis force
Hurricane (typhoon) in north hemisphere