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General Physics 101 PHYS. Dr. Zyad Ahmed Tawfik. Email : zmohammed@inaya.edu.sa. Website : zyadinaya.wordpress.com. Lecture No. 4 Review and resolve quizzes on Newton's laws. Facts about FORCE. Force unit is the NEWTON (N). Its definition a push or a pull.
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General Physics101 PHYS Dr. Zyad Ahmed Tawfik Email : zmohammed@inaya.edu.sa Website : zyadinaya.wordpress.com
Lecture No. 4 Review and resolve quizzes on Newton's laws
Facts about FORCE • 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.
Adding Forces Forces are vectors (They have both magnitude and direction) and so add as follows:
1-Adding Forces In one dimension • 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 +6 N 0 N +3 N -3 N
1- The angle between them is 90°. . Where cos 90=0 When the forces F1 and F2 are the action on the object and the angle between them 90°. The magnitude of the resultant force
2- with angle When the forces F1 and F2 are the action on the object with θ degree. The magnitude of the resultant force
Newton’s First Law 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.
Newton’s First Law (The Law of Mass) NOTE:MASS and WEIGHT are NOT the same thing. MASS never changes When an object moves to a different planet. Unit for MASS = kilogram. Weight(W) or Force due to Gravity is how our MASS (m) is effected by gravity (g).
1- with out angle FN Fp Ff mg 1. Friction Force Fn 2. Normal Force Ff In this case the friction force is opposite the pull So, Ff = - Fpull and Fn=mg
2- with angle . Fx = F cos θ & Fy = F sin θ So the Ff and Fn in this case FN F Fy Ff Fx mg In this case F analysis in x and y direction
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.
Note If the surface is smooth, the friction force, Ff= 0
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
Q 1 • 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
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
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? Q 7 • 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?
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: • The force of friction • the acceleration of the man
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. • What is the mass of the suit case ? • What is the weight of suit case ?
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
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 ? • 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 • 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
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: The force of friction the acceleration of the boy
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?
