1. Instructor: Dr. Tatiana Erukhimova
Physics 218: Mechanics
Instructor: Dr. Tatiana Erukhimova
Lectures 13-15

2. Newton’s Laws
1st Law: A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration
2nd Law: The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force acting on the object.
3rd Law: For every action there is an equal, but opposite reaction

4. Reading quizzes (Chs 5,6)

5. Hockey Puck a)
Which of these three best represents a hockey puck in the real world? b) c)

6. Newton’s 1st Law
A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration

7. Aristotle: a natural state of an object is at rest; a force is necessary to keep an object in motion. It follows from common sense.
B.C.
Galileo: was able to identify a hidden force of friction behind common-sense experiments

8. Galileo: If no force is applied to a moving object, it will continue to move with constant speed in a straight line
Inertial reference frames
Galilean principle of relativity: Laws of physics (and everything in the Universe) look the same for all observers who move with a constant velocity with respect to each other.

9. Newton’s second law
The vector acceleration of an object is in the same direction as the vector force applied to the object and the magnitudes are related by a constant called the mass of the object.

10. A Recipe for Solving Problems
Sketch
Isolate the body (only external forces but not forces that one part of the object exert on another part)
2. Write down 2nd Newton’s law
Choose a coordinate system
Write 2nd Newton’s law in component form:
3. Solve for acceleration

11. Is it better to push or pull a sled?
You can pull or push a sled with the same force magnitude, FP, and angle Q, as shown in the figures.
Assuming the sled doesn’t leave the ground and has a constant coefficient of friction, m, which is better? FP FP

12. FFriction = mKineticN Kinetic Friction THIS IS NOT A VECTOR EQUATION!
For kinetic friction, it turns out that the larger the Normal Force the larger the friction. We can write
FFriction = mKineticN
Here m is a constant
Warning:
THIS IS NOT A VECTOR EQUATION!

13. FFriction  mStaticN Static Friction
This is more complicated
For static friction, the friction force can vary
FFriction  mStaticN
Example of the refrigerator:
If I don’t push, what is the static friction force?
What if I push a little?

14. Coefficient of friction: H
What is the normal force?
What is the velocity of the block when it reaches the bottom?

15. For every action there is an equal, but opposite, reaction
Newton’s 3rd Law
For every action there is an equal, but opposite, reaction

16. Skater Equal and opposite force Force provides an acceleration
Skater pushes on a wall
The wall pushes back
Equal and opposite force
The push from the wall is a force
Force provides an acceleration
She flies off with some non-zero speed

17. P m1 m1 m2 Free body diagram N2 N1 F12 P F21 m2 m1 m2g m1g F12=F21
No friction P m1 m1 m2
Free body diagram N2 N1
F12 P
F21 m2 m1
m2g
m1g
F12=F21

18. A small block, mass 2kg, rests on top of a larger block, mass 20 kg
A small block, mass 2kg, rests on top of a larger block, mass 20 kg. The coefficient of friction between the blocks is If the larger block is on a frictionless table, what is the largest horizontal force that can be applied to it without the small block slipping?

20. Force of tension
Massless, unstretchable string; frictionless pulley

21. A Problem With First Year Physics Strings and Pulleys
m1, m2 are given
m2>m1 m2
String is massless and unstretchable
Find accelerations of m1 and m2 (assume no friction)

22. V0
A block of mass m is given an initial velocity V0 up an inclined plane with angle of incline θ. Find acceleration of the block if
a)  = 0
b) non-zero 

23. A wedge with mass M rests on a frictionless horizontal tabletop
A wedge with mass M rests on a frictionless horizontal tabletop. A block with mass m is placed on the wedge and a horizontal force F is applied to the wedge. What must the magnitude of F be if the block is to remain at a constant height above the tabletop?

24. Block 1, of mass m1, is placed at rest on an inclined plane
Block 1, of mass m1, is placed at rest on an inclined plane. It is attached by a massless, unstretchable string to block 2, of mass m2. The pulley is massless and frictionless and just changes the direction of the tension in the string. The coefficient of friction between the plane and m1 is the constant . 1 2
Determine what range of values for mass m2 will keep the system at rest.
Find acceleration if mass 2 goes down.

25. Friction everywhere. 2 F 1
Find F necessary to drag the box 1 at constant speed.

26. The advantage of a pulley
What minimum force F is needed to lift the piano of mass M?

28. Quiz
a) A crate of mass m is on the flat bed of a pick up truck. The coefficient of friction between the crate and the truck is . The truck is traveling at the constant velocity of magnitude V1. Draw the free body diagram for the crate.
b) The truck starts to accelerate with an acceleration ac. Draw the free body diagram for the crate, if the crate does not slip.

29. The Elevator Problem

30. Have a great day! Reading: Chapter 7
Hw: Chapter 6 problems and exercises