1. Forces and Newton’s Laws of Motion
Chapter 4
Forces and Newton’s Laws of Motion

2. 4 Forces and Newton’s Laws of Motion
Slide 4-2

3. Slide 4-3

4. Slide 4-4

5. Slide 4-5

6. Slide 4-6

7. Reading Quiz A “net force” is
the sum of the magnitudes of all the forces acting on an object.
the difference between two forces that are acting on an object.
the vector sum of all the forces acting on an object.
the force with the largest magnitude acting on an object.
Answer: C
Slide 4-7

8. Answer
A “net force” is
the sum of the magnitudes of all the forces acting on an object.
the difference between two forces that are acting on an object.
the vector sum of all the forces acting on an object.
the force with the largest magnitude acting on an object.
Answer: C
Slide 4-8

9. Reading Quiz
Which of the following is NOT one of the steps used to identify the forces acting on an object?
Name and label each force the object exerts on the environment.
Name and label each contact force acting on the object.
Draw a picture of the situation.
Identify “the object” and “the system.”
Name and label each long-range force acting on the object.
Answer: A
Slide 4-9

10. Answer
Which of the following is NOT on of the steps used to identify the forces acting on an object?
Name and label each force the object exerts on the environment.
Name and label each contact force acting on the object.
Draw a picture of the situation.
Identify “the object” and “the system.”
Name and label each long-range force acting on the object.
Answer: A
Slide 4-10

11. Reading Quiz Which of these is not a force discussed in this chapter?
The tension force.
The normal force.
The orthogonal force.
The thrust force.
Answer: C
Slide 4-11

12. Answer Which of these is not a force discussed in this chapter?
The tension force.
The normal force.
The orthogonal force.
The thrust force.
Answer: C
Slide 4-12

13. Reading Quiz An action/reaction pair of forces
point in the same direction.
act on the same object.
are always long-range forces.
act on two different objects.
Answer: D
Slide 4-13

14. Answer An action/reaction pair of forces point in the same direction.
act on the same object.
are always long-range forces.
act on two different objects.
Answer: D
Slide 4-14

15. What causes motion?
Ice Boats travel at up to 60 mph as they fly across the frozen lake!
With our knowledge so far, we could describe the boats’ motion with…
Pictures
Graphs
Equations
All these are KINEMATICS
Kinematics is the how something moves but does not tell us why.

16. Why does motion occur?
What is the “natural state” of an object if left to itself?
A moving object is not in its natural state, and thus requires an explanation:
WHY is this object moving?
What keeps it going and prevents it from being in its natural state?

17. Why does motion occur?
The questions that deal with “why” require us to turn our attention to a different branch of physics: dynamics
Why do the boats accelerate briskly before reaching their top speed?
What is the cause of motion!?
Dynamics joins with kinematics to form mechanics, the general science of motion.
THIS chapter is all about studying dynamics qualitatively
(we will work quantitatively in the next 4 chapters)
Voyager space probe
Launched in 1977

18. What Causes Motion?
In the absence of any forces acting on it, an object will continue moving forever. Motion needs no “cause.”
Slide 4-15

19. The idea of “Forces”
Galileo’s ideas were completely counter to those of the ancient Greeks.
There is no longer a need to describe why the sled continues to slide across the ice; that motion is its “natural state”.
What needs explanation, from this viewpoint, is why objects don’t continue in uniform motion.
Why does the sled eventually slow down to a stop?

20. Newton’s First Law
“An object at rest stays at rest and an object in motion stays in motion…”
…with the same speed and in the same direction unless acted upon by an unbalanced force.”

21. Seat Belts: An Application of Newton’s First Law
Slide 4-16

22. Ok… So?
Newton’s first law tells us that an object in motion subject to no forces will continue to move in a straight line forever.
But this law does not explain in any detail exactly what a force is.

23. What Is a Force? A force... ... is a push or a pull.
... acts on an object.
... requires an agent.
... is a contact force or a long-range force.
... is a vector.
Slide 4-17

24. Forces – 2 types Contact Force Long-range forces
A Force that acts on an object by touching it at a point of contact.
Examples:
Long-range forces
Forces that act on an object without physical contact.

25. Force – Agent and object
Every force has an agent:
something which acts or pushes or pulls
a specific cause
If a force is being exerted on an object* then you must be able to identify a specific cause or agent!
*the thing being studied
Example 

26. Homework
Pg. 125 Questions: #1, 2, 3, 4, 5