Download lecture-no-4-Quiz-law-of-newton

Dr. Zyad Ahmed Tawfik
Email : [email protected]
Website : zyadinaya.wordpress.com
Lecture No. 4
Review and resolve quizzes
on Newton's laws
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• Force unit is the NEWTON (N).
• Its definition a push or a pull.
 What change the stateof object is called
“force”.
 means that we can control the magnitude
of the applied force and also its direction,
so force is a vector quantity, just like
velocity and acceleration.
Forces are vectors (They have both
magnitude and direction) and so add
as follows:
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 In one dimension, note direction using a + or – sign
then add like scalar quantities (regular numbers
with no direction associated with them)
F=F1+F2
 Examples:
+
+3 N
+
+3 N
=
-3 N
=
+3 N
0N
+6 N
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 .When the forces F1 and F2 are the action on
the object and the angle between them 90°.
The magnitude of the resultant force
F 
F12  F22
Where cos 90=0
 When the forces F1 and F2 are the action on
the object with θ degree. The magnitude of
the resultant force
F  F12  F22  2 F1 F2 COS 
Newton’s First Law
An object at rest tends to stay at rest and an
object in motion tends to stay in motion unless
acted upon by an unbalanced force.
Unit for MASS = kilogram.
Weight (W) or Force due to Gravity is how our
MASS (m) is effected by gravity (g).
W  mg
NOTE: MASS and WEIGHT are NOT the
same thing. MASS never changes When an
object moves to a different planet.
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1. Friction Force Fn
2. Normal Force Ff
In this case the friction force is opposite the pull
FN
Ff
mg
Fp
So, Ff = - Fpull
and Fn=mg
In this case F analysis in x and y direction
.
Fx = F cos θ &
Fy = F sin θ
So the Ff and Fn in this case
Fx  F cos 
FN F
Ff
mg
Fy
Fx
F f  Fx
FN  mg!
FN  Fy  mg
FN  mg  Fy
Newton’s Second Law
Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a measurement of how quickly an
object is changing speed.
Net Force
The net force is the vector sum of all the forces
acting on a body.
Fnet  F  F1  F2  F3  
F  ma
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Note
If the surface is
smooth, the friction
force, Ff= 0
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Newton’s Third Law
Newton’s Third Law
For every action there is an equal and opposite
reaction.
Coefficients of friction
 IiCoefficients
of friction is the ratio
between friction force and normal force.
 Symbol is the Greek letter mu (μ)
μ= Ff / FN
 The coefficient of friction has no units.
Or
Friction Force = Coefficient of friction  Normal Force
Ffriction = Fnormal
Quizzes
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Q1
 Calculate the force required to accelerate a 15Kg block
along the floor at 3.0 m/s2.
Q2
 The forces F1=10 N and F2=5N are the action on the
block of mass 3 kg. Find the resultant force and
acceleration of the block ?
Q3
 An object of mass m=3Kg is subject to a force F=9N.
Find :
a) Wight of the object
b) the acceleration of the
object
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Q4
 The forces F1=2 N and F2=4N are the action on
the object with 60°. Find the magnitude of the
resultant force ?
Q5
 An object of mass m=5Kg is pulled by a force F on
a smooth horizontal floor. If the magnitude of
the force F= 16N and its direct 30°above the
horizontal. Find :
a) the normal force N.
b) the acceleration of the object
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Q6
 A man is pulling a bag of 20 Kg mass on a horizontal
floor. The pulling force is 40 N inclined at 30° above
the horizontal and the coefficient of friction between
the bag and the floor is 0.1.
a) What is the force of friction?
b) What is the acceleration of the suite case?
Q7
 A man of 60 Kg sits on a chair while his feet is resting
on the ground. The ground exerts a force of 350 N on
the feet.
Find the force exerted by the chair on him?
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Q8
 A young man of 40kg mass is skidding down an
inclined rough surface . The surface is inclined at 45°
above horizontal as shown in the figure, the
coefficient of friction between his skies and the
surface is 0.1 .
Find:
a) The force of friction
b) the acceleration of the man
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Q9
 A child of 30kg mass is running with speed 5m/s
on a rough horizontal floor skids a distance 3 m
till stopped . Find the force of friction
Q10
 A man is pulling a suitcase on a horizontal rough
floor. The pulling force is 40N action horizontally
and the coefficient of friction is 0.2.
a) What is the mass of the suit case ?
b) What is the weight of suit case ?
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Q 11
 A lady is pulling a suitcase of 20 Kg mass on a
rough horizontal floor. The pulling force
T=100 N and is inclined at 30° above the
horizontal. If the coefficient of friction is 0.3.
a)Find the components Tx and Ty of the
pulling force.
b) Find the force of friction acting on the
suitcase.
c) Find the acceleration of the suitcase
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Home work
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1. The forces F1=20 N and F2=15N are the action
on block of mass 3 kg. Find the resultant force
and acceleration of the block ?
2. A girl of 50kg mass is running with speed 7m/s
on a rough horizontal floor skids a distance 4
m till stopped . Find the force of friction
3. An box of mass m=10Kg is pulled by a force F on
a smooth horizontal floor. If the magnitude of
the force F= 20N and its direct 45° above the
horizontal. Find :
a) the normal force N.
b) the acceleration of the object
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5. A boy of 40kg mass is skidding down an inclined
rough surface . The surface is inclined at 30° above
horizontal as shown in the figure, the coefficient of
friction between his skies and the surface is 0.2 .
Find:
a) The force of friction
b) the acceleration of the boy
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6. A woman of 80 Kg sits on a chair while his feet is
resting on the ground. The ground exerts a force of
400 N on the feet.
Find the force exerted by the chair on him?
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Thank You
for your
Attention