1. Newton’s Laws of Motion (Applications)
Lecture 11
Newton’s Laws of Motion (Applications)
General Physics 1, Lec By/T.A. Eleyan

2. General Physics 1, Lec 11 By/T.A. Eleyan
Definition of equilibrium
If the net for force exerted on an object is zero, the acceleration of the object is zero and its velocity remains constant. That is, if the net force acting on the object is zero, the object either remains at rest or continues to move with constant velocity. When the velocity of an object is constant (including when the object is at rest), the object is said to be in equilibrium.
The first condition of equilibrium
General Physics 1, Lec By/T.A. Eleyan

3. General Physics 1, Lec 11 By/T.A. Eleyan
Example: A traffic light weighing 122 N hangs from a cable tied to two other cables fastened to a support, as in Figure The upper cables make angles of 37.0 ° and 53.0 ° with the horizontal. Find the tension in the three cables.
Solution:
Note that the traffic light in equilibrium. Then
General Physics 1, Lec By/T.A. Eleyan

4. General Physics 1, Lec 11 By/T.A. Eleyan
since
From the condition of equilibrium
By solve the eqs.
Example: Suppose the two angles are equal. What would be the relationship between T1 and T2?
General Physics 1, Lec By/T.A. Eleyan

5. General Physics 1, Lec 11 By/T.A. Eleyan
Example: A person weighs a fish of mass m on a spring scale attached to the ceiling of an elevator, as illustrated in Figure. Show that if the elevator accelerates either upward or downward, the spring scale gives a reading that is different from ward, the weight of the fish.
Solution:
General Physics 1, Lec By/T.A. Eleyan

6. General Physics 1, Lec 11 By/T.A. Eleyan
We chose upward as the positive y direction
(1) If the elevator is either at rest or moving at constant velocity, the fish does not accelerate, and so, Or
(Remember that the scalar mg is the weight of the fish.)
(2) If the elevator moves with an acceleration a relative to an observer standing outside the elevator in an inertial frame , Newton’s second law applied to the fish gives the net force on the fish:
(*)
General Physics 1, Lec By/T.A. Eleyan

7. General Physics 1, Lec 11 By/T.A. Eleyan
Thus, we conclude from (*) that the scale reading T is greater than the weight mg if a is upward, so that ay is
Positive, and that the reading is less than mg if a is downward, so that, ay is negative.
For example:
if the weight of the fish is 40.0 N and a upward, so that ward, ay = 2.00 m/ s2, the scale reading from (*) is
(2) If a is downward so that ay = m/s2, then (*) gives us,
General Physics 1, Lec By/T.A. Eleyan

8. General Physics 1, Lec 11 By/T.A. Eleyan
Normal Force
General Physics 1, Lec By/T.A. Eleyan

9. General Physics 1, Lec 11 By/T.A. Eleyan
Example: A car of mass m is on an icy driveway inclined at an angle as in Figure.
Find the acceleration of the car, assuming that the, driveway is frictionless.
Solution:
General Physics 1, Lec By/T.A. Eleyan

10. General Physics 1, Lec 11 By/T.A. Eleyan
Now we apply Newton Newton’s second law in component form, noting that ay = 0:
(1)
(2)
From (1)
From (2)
Example: solve the example if the mass 1000 kg and the angle 30.
General Physics 1, Lec By/T.A. Eleyan

11. General Physics 1, Lec 11 By/T.A. Eleyan
Example: When two objects of unequal mass are hung vertically over a frictionless pulley of negligible mass, as in Figure, the arrangement is called an Atwood machine. The device is sometimes used in the laboratory to measure the free-fall acceleration. Determine the magnitude of the acceleration of the two objects and the tension in the lightweight cord.
Solution:
When Newton’s second law is applied to object m1, we
obtain
General Physics 1, Lec By/T.A. Eleyan

12. General Physics 1, Lec 11 By/T.A. Eleyan
Similarly , for object m2 we , find
When (2) is added to (1), T cancels and we have
Then we have
Example: find the acceleration and the tension of Atwood's machine in which m1=2kg, m2=4kg.
General Physics 1, Lec By/T.A. Eleyan

13. General Physics 1, Lec 11 By/T.A. Eleyan
example: A ball of mass m1 and a block of mass m2 are attached by a lightweight cord that passes over a frictionless pulley of negligible mass, as in Figure. The block lies on frictionless incline of angle . Find the magnitude of the acceleration of the two objects and the tension in the cord.
Solution:
Equations of motion for m1
(1)
(2)
General Physics 1, Lec By/T.A. Eleyan

14. General Physics 1, Lec 11 By/T.A. Eleyan
Equations of motion for m2
(3)
(4)
Now, solve (2) , (3) simultaneously . The acceleration
When this is substituted into (2) we find the normal force
Example: if m1=10kg, m2=5kg and =45 find a and T.
General Physics 1, Lec By/T.A. Eleyan

15. General Physics 1, Lec 11 By/T.A. Eleyan
Example:  A 3.0 kg mass hangs at one end of a rope that is attached to a support on a railroad car. When the car accelerates to the right, the rope makes an angle of 4.0° with the vertical.  Find the acceleration of the car.
Solution:
In the vertical direction:                      
General Physics 1, Lec By/T.A. Eleyan

16. General Physics 1, Lec 11 By/T.A. Eleyan
In the horizontal direction: 
                                                     
Problem: A train is given an acceleration of 5 m/s2. 
What is the tension between the two cars with a mass of 2000 kg.
General Physics 1, Lec By/T.A. Eleyan

17. General Physics 1, Lec 11 By/T.A. Eleyan
Forces of Friction
In most cases we need to include friction.  For many surfaces the force of sliding friction is proportional to the normal force. That is, as the normal force increases, the force of friction increases linearly.  When this is true, we write  
with the proportionality constant called , the coefficient of kinetic friction.  This is the friction when the object is moving.
    
General Physics 1, Lec By/T.A. Eleyan

18. General Physics 1, Lec 11 By/T.A. Eleyan
If I push a table and it doesn't move, there must also be a force pushing back.  That force comes from friction, as well.  It is the friction when the object is not moving, or static, called the coefficient of static friction.  It is always greater than or equal to the coefficient of kinetic friction.  It is often harder to start something sliding than to keep it sliding, due to the difference in these coefficients.
General Physics 1, Lec By/T.A. Eleyan

19. General Physics 1, Lec 11 By/T.A. Eleyan
The Static force of friction ( fs ) is the force of friction between two objects when there is no motion.
The Kinetic force of friction ( fk ) is the force of friction between two objects when there is motion.
General Physics 1, Lec By/T.A. Eleyan

20. General Physics 1, Lec 11 By/T.A. Eleyan
Example:  An 18.0 kg box is released on a 37.0° incline and accelerates down the incline at 0.27 m/s2.  Find the coefficient of friction and the frictional force.  (Draw diagram)
Solution:
Forces in y-direction are:
General Physics 1, Lec By/T.A. Eleyan

21. General Physics 1, Lec 11 By/T.A. Eleyan
Forces in x-direction are:
Now, we can relate these to the coefficient of kinetic friction,
General Physics 1, Lec By/T.A. Eleyan