Newton’s 1st and 2nd
laws of Motion
4 Forces of the Universe
• Nuclear : Strong forces holding the
nucleus of an atom together (gluon)
• Weak : Forces as a result of some
atoms that decay the nucleus (
radioactive )
• Electromagnetic : a force based on
electricity, magnetism and light
properties
• Gravitational : The weakest force
caused by the 2 factors, MASS and
DISTANCE
Mechanical Forces
• Inertia: Newton’s First Law, net or unbalanced
force is needed to change the state of motion
• Friction: the contact force that acts to oppose
sliding motion between surfaces
• Normal : the contact force exerted perpendicular
to the surface contact
• Tension : the pull exerted by a cable when
attached to a body
• Compression : the push exerted to the object
• Weight : force of attraction due to distance from
the center of the earth and mass of the object
• Units are Measured in NEWTONS ( N )
Newton's First Law of
Motion ( inertia )
• An object at rest tends to stay at
rest and an object in motion tends
to stay in motion with the same
speed and in the same direction
unless acted upon by an
unbalanced force.
Stationary and Moving objects
Balanced and Unbalanced
Forces
• The book is said to be at equilibrium.
There is no unbalanced force acting
upon the book and thus the book
maintains its state of motion
Balanced and Unbalanced
Forces
• The book is not at equilibrium and
subsequently accelerates
Newton’s Second law of motion
• The acceleration of a body is directly
proportional to the net force on it and inversely
proportional to the mass. ( F = ma )
• Unit of Force = kg-m or N (Newton)
s2
• Calculation:
• mass of an apple is 100g or .1 kg
• F = ma = (.1kg) (10 m/s2) = 1 kg-m/s2 = 1 N
Sample Force Problem #1
• Using Newton’s Second law of Motion ( the
acceleration of a body is directly proportional to the
net force on it and inversely proportional to the
mass, F = ma).
• What net force is
required to accelerate
a 1500 kg car
at 3.00 m/s2?
Solution Problem #1
• F=ma Given:
• F = 1500 kg x 3.00 m/s2 m = 1500 Kg
• F = 4500 kg-m/s2 a = 3.00 m/s2
or
• F = 4500 N Find:
F=?
Static Balance (free body)
Problem #2
• Forces of 10 N and 15N are arranged
according to diagram. What is the net force
on this object. What is the rate of
acceleration of the 12 Kg object?
Solution #2
• Given:
• Force 10 N and 15 N in opposite directions
• Mass = 12 Kg
• Find :
• Net Force
• Acceleration
Net Force = F1 - F2 = 15N – 10N = 5 N
F = ma
5N = 12 Kg (a)
5N =a
12 Kg
0.417 m/s2 = a
Sample Question #3
• The figure shows a block that is being
pulled along the floor. According to the
figure, what is the acceleration of the
block?
Solution Problem #3
• F=ma Given:
• 40N = 20 kg (a) m = 20 Kg
• 2 m/s2 = a Ffriction= 10 N
Fpull = 50 N
Find:
a =?
Sample problem #4
A 50-kg child on a
skateboard experiences
a 75-N force as shown.
What is the expected
acceleration of the child?
Solution Problem #4
• F=ma Given:
• 75 N = 50 kg (a) m = 50 Kg
• 1.5 m/s2 = a Fpush= 75 N
Find:
a =?
Uniform Circular
Motion
• Circular motion at a constant
speed HAS an ACCELERATION!
• Keeping the magnitude of the velocity
constant, but changing the direction will
create a change in acceleration
• Using vector addition, a centripetal = v2
r
Sample Problem #3
• During a hammer throw, an athlete rotates a
7.00 kg hammer tied to the end of a 1.3 m
chain. The hammer moves at the rate of 3
m/s in a circle.
• What is the centripetal acceleration of the
hammer
• What is the tension in the chain?
• If the chain breaks at the point shown which
direction will the hammer fly?
Solution Problem #3
• m = 7.00 kg v c = 3.0 m/s r = 1.3 m
• a c = v2 / r
• a c = (30 m/s)2 / 1.3 m
• a c = 900 m2/s2 / 1.3 m
• a c = 692.3 m/s2
• Fc=mac
• F c = 1.3 kg ( 692.3 m/s2)
• F c = 900 N @ the tangent (direction 2)
Summary
• Forces are invisible measurements that
explain the states of motion
• Newton derived laws of motion that
explained the way things move.
• Forces can be balanced (equilibrium) or
unbalanced (acceleration).
• Uniform circular motion is accelerating
because of the constant change in direction