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Ch4 Laws of Motion
4.2,3,4 Newton’s law
Isaac Newton(1642-1727)
He studied various subject in his life:
Calculus
law of gravitation, law of motion
Optics
Alchemy
Theology
“If object 1 and object 2 interact, the force F12 Example
exerted by object 1 on object 2 is equal in
magnitude but opposite in direction to the force
2
F21 exerted by object 2 on object1”
examples
1.Force:
One pushes a chair
Reaction force: The chair pushes the person.
2.Force:
Earth pulls a person
Reaction force: A person pulls Earth.
Newton’s First law
3.Normal force is not the reaction force of
“An object moves with a velocity that is constant weight of an object.
in magnitude and direction unless a non-zero
Action and reaction forces acts on different
net force acts on it”
object.
Galileo discovered the first law of motion and
Newton restated it in his law of motion.
Galileo’s thought experiment
SI unit
N = kg m/s2
B
A
1
n2
n1
F
T
T
w1
w2
m

3.2 free-body diagram.
Diagram of forces acting on one object.
3.3 types of forces
Newton’s Second law
Reaction forces does not appear since it acts on There are four fundamental forces in the
“The acceleration a of an object is directly
a different object.
nature, but we will discuss the fundamental
proportional to the net force acting on it and
Drawing a free body diagram.
forces later time.
inversely proportional to its mass”
1. Isolate an object.
Some forces have names since they appear so

In many problems, objects are connected. You often and we will discuss about these forces.

F  ma
need to draw a separate diagram for each
Weight (w)
Force and mass are defined by this equation. object.
The gravitational force acting on an object.
Force is a vector so we can compute x-,y- and z- 2. Find the forces acting on the object.
w = mg
direction independently.
Tension(T)
inertia: tendency of an object to continue in its 3. Draw force vectors and label them according Pulling force exerted by each end of string, rope
original state of motion.
to the step 2.
or cable.
Mass: measure of inertia. This is one definition
Normal force(n)
of mass.
Force exerted by a surface. Its direction is
always perpendicular to the surface and
Newton’s Third law
opposite to the force acting on the surface.
Friction(f)
Force resists a motion on a surface or through a
viscous medium.
The direction of friction is opposite the actual
motion or impending motion.
When an object is at rest or moving at a
constant velocity, f = - F. Thus, friction has the
same magnitude as the force exerted and
opposite direction in this case.
Static friction
When an object is stationary,
f  s n
In this case f is equal magnitude and opposite
direction of the force exerted. μs is the
coefficient of static friction.
When an object is in motion.
f  k n
Where μk is the coefficient of kinetic friction.
As you can see in the equations, friction is
proportional to the normal force exerted by the
surface on the object.
The coefficient of the friction is nearly
independent of the area of contact between the
surfaces.
A traffic light weight 1.00 x 102 N hangs from a vertical A woman weight a fish with a spring scale attached to
cable tied to two other cables that are fastened to
the ceiling of an elevator, as shown in figures.
While the elevator is at rest, she measured a
a support. The upper cables make angles of 37.0°
weight of 40.0 N.
and 53.0° with the horizontal. Find the tension in
each of the three cables.
(a) What weight does the scale read if the elevator
accelerates upward at 2.00 m/s2?
(b) What weight does the scale read if the elevator
accelerates downward at 2.00 m/s2?
(c) If the elevator cable breaks, what does the scale
read?
Suppose a block with a mass of 2.50 kg is resting on a The objects of mass m1 and m2, with m2 >m1, are
connected by a light, inextensible cord and hung
ramp. If the coefficient of static friction between
over a frictionless pulley, as in Figure. Both cord
the block and ramp is 0.350, what maximum
and pulley have negligible mass. Find the
angle can the ramp make with the horizontal
magnitude of the acceleration of the system and
before the block starts to slip down
the tension in the cord.