1. Vern J. Ostdiek Donald J. Bord Chapter 2 Newton’s Laws

2. Friction Friction is a resistive force to relative motion between two bodies or substances in contact.  Between your tire and the road  Between your car and the air Friction always opposes movement.

3. Friction, cont’d Static friction is the resistance you experience when you try to start sliding an object. It is static because there is no relative motion between the surfaces. Kinetic friction is the resistance you experience while sliding an object. It is kinetic since there is relative motion.

4. Newton’s first law of motion Newton’s First Law of Motion states: An object will remain at rest or in motion with constant velocity unless acted on by a net external force. An external force is a force applied to the object from some other object.  force from an impact, gravity, air resistance, etc.

5. Newton’s first law of motion, cont’d The net force is the vector sum of all external forces.  If you and a friend push on opposite ends of a truck with the same force, the net force is zero — the forces cancel.  If you push in the same direction, there is a non-zero net force.

6. Newton’s first law of motion, cont’d If the two forces are in different directions, the net force is an “in-between” direction.

7. Mass Mass is a measure of an object’s resistance to acceleration, i.e., changes in the object’s motion. It also indicates the amount of matter in an object.  SI units: kilogram (kg) or gram (g)

8. Mass, cont’d An object of little mass requires little force to accelerate it. A massive object requires a much larger force to give it the same acceleration.

9. Mass, cont’d The weight of an object depends on which planet you measure the weight. The mass of the object is independent of the planet.

10. Newton’s 2 nd law of motion Newton’s 2 nd Law of Motion states that an object is accelerated when a net external force acts on it. The net force equals the object’s mass times its acceleration:

11. Newton’s 2 nd law of motion, cont’d Think about why you might add more dogs to a sled…

12. Newton’s 2 nd law of motion, cont’d The object will accelerate according to

13. Newton’s 2 nd law of motion, cont’d The SI unit of force, the newton, is defined according to Newton’s 2 nd law:

14. Newton’s 2 nd law of motion, cont’d A falling object accelerates due to the Earth’s gravity at So the force the object feels from Earth, i.e., its weight, is:

15. Example Example 2.2 An automobile manufacturer decides to build a car that can accelerate from 0 to 27 m/s in 10 s. The car’s mass is to be about 1,000 kilograms. What is the force required?

16. ANSWER: The problem gives us: The force is: Example Example 2.2

17. DISCUSSION: What produces this force?  The friction between the car’s tires and the road  If the road is too slick, the tires cannot get enough “grip” and you “peel-out.” Example Example 2.2

18. Types of motion free-fall Free-fall is a type of motion in which the only force acting on the object is the object’s weight.  There is no friction, no air resistance, etc. Gravity always acts toward the Earth’s center.

19. Types of motion free-fall, cont’d A ball thrown upward will slow-down because gravity is “pulling” down on the ball.

20. Types of motion free-fall, cont’d  Eventually it will stop.  It will then fall back to the Earth due to gravity. The time it takes to go up is the same amount of time for it to fall.

21. Types of motion falling with air resistance What happens when you “really” drop an ball? Think about the forces acting on the ball.  Gravity is accelerating it downward.  The air offers resistance, trying to prevent the ball’s descent.

22. Types of motion falling with air resistance, cont’d The ball’s weight pulls it down. The air has to be forced out of the ball’s way — it exerts an upward trying to slow the ball down.

23. Types of motion falling with air resistance, cont’d As the ball’s speed increases, the air resistance force increases. Eventually the ball no longer accelerates. At this point, you’ve reached the terminal speed.

24. Newton’s third law of motion Newton’s 3 rd Law of Motion states that forces always occur in pairs:  When one object exerts a force on a second object, the second exerts an equal and opposite force on the first.

25. Newton’s third law of motion, cont’d If object A exerts a force on object B, then object B exerts an equal force in the opposite direction on A:

26. Newton’s third law of motion, cont’d Think about pushing off against a wall.  You push against the wall.  The wall pushes back.  If the wall is weak, it might fall down.  If not, you move away.

27. Newton’s third law of motion, cont’d Consider an airplane’s wing.  Due to the angle of attack, the air impacts the bottom of the wing.  The wing pushes the air out of the way.  The air pushes back and provides some lift.

28. The law of universal gravitation Newton’s Law of Universal Gravitation states that every object exerts a gravitational force on every other object.  The force increases as either object’s mass increases.  The force decreases as the objects move farther apart.

29. The law of universal gravitation, cont’d Mathematically, we can write m 1 is the mass of one object m 2 is the mass of the other object d is the center-to-center distance separating the objects.

30. The law of universal gravitation, cont’d Note that Newton’s 3 rd law means:  the Earth pulls down on you with a force equal to your weight, and  you pull “up” on the Earth with you’re a force equal to your weight. You move more than the Earth because the Earth is much more massive than you.

31. The law of universal gravitation, cont’d If you could stand on a tower 4,000 miles high, you would weigh one-fourth your usual weight.  You are 2×’s farther from the Earth’s center.

32. The law of universal gravitation, cont’d Using the gravitational constant, we can write the law of gravity as