1. Chapter 12 Forces and Motion
Name: _______________________________ Class: ____ Newton’s 1st & 2nd Laws of Motion
Chapter 12
Forces and Motion

2. What are we going to learn?
Newton’s Laws of Motion
(Imagine a toy car with an unanchored load – what will change its motion?)

3. What is a Scientific Law?
A statement of what will happen in a given set of circumstances.
FYI: Newton developed the Law of Universal Gravitation in addition to the Laws of Motion

5. Newton’s First Law of Motion
An object at rest will remain at rest unless acted upon by a nonzero net force
An object moving at a constant velocity will continue moving at a constant velocity unless acted upon by a nonzero net force

7. Inertia
The tendency of an object (moving or not) to resist a change in its motion
Newton’s 1st Law is also known as the Law of Inertia
Ex: You move forward in your seat when your car stops suddenly
Relate inertia to safety bars on a roller coaster
_______________________________________

8. Inertia Relate inertia to safety bars on a roller coaster
They keep you in your seat when changes in the coaster’s motion moves you out of your seat

9. Mass and Inertia
The greater the mass of an object, the greater its inertia...and the greater the force required to change its motion

10. Use this information to explain why you feel tossed around whenever a roller coaster goes over a hill or through a loop...

11. Use this information to explain why you feel tossed around whenever a roller coaster goes over a hill or through a loop
Because of inertia, you resist changes in motion. When the coaster accelerates over a hill, inertia keeps you moving at a constant velocity. You feel tossed around.

12. Example
What keeps a ball rolling down a hill?

13. Example
What keeps a ball rolling down a hill?
Gravity and Inertia

14. What force is acting on the ball, resisting motion?

15. What force is acting on the ball, resisting motion?
Friction between the ball and the ground

16. If you put a ball on the floor and roll it across the floor so it hits the wall, why did the ball stop moving in the direction it was rolled?

17. If you put a ball on the floor and roll it across the floor so it hits the wall, why did the ball stop moving in the direction it was rolled?
It rolled into the wall, so it could not keep going in the direction it was rolled

18. Now I get it!
Now I know that Newton’s first law of motion states that _____________________________

19. Now I get it!
Now I know that Newton’s first law of motion states that objects only change motion when a nonzero net force acts on them

21. Newton’s Second Law of Motion
An object’s acceleration depends on its mass and on the net force acting on it
Ex: Is a full or empty shopping cart easier to push?
Ex: Who can cause a greater acceleration on the cart, a small child or grown adult?

22. Newton’s Second Law of Motion
Ex: Is a full or empty shopping cart easier to push? Empty cart – less mass
Ex: Who can cause a greater acceleration on the cart, a small child or grown adult? Grown adult – more force

23. Formula
The acceleration of an object is equal to the net force acting on it divided by the object’s mass
Acceleration = Net force
Mass or
Net force = Mass x Acceleration

24. Mass x Acceleration = Net force x Mass
which gives
Mass x Acceleration = Net force
when flipped gives
Net force = Mass x Acceleration

25. Changes in Force and Mass
If you increase the force on a cart without changing its mass, the acceleration will increase
If something falls out (decrease mass), it will accelerate faster if you keep pushing just as hard (same force)

26. If you want to increase the acceleration of a bicycle, what must you increase?

27. If you want to increase the acceleration of a bicycle, what must you increase?
The force you apply to the pedals

28. Why are racing bicycles made of lightweight metals like titanium?

29. Why are racing bicycles made of lightweight metals like titanium?
The less mass a bicycle has, the greater the acceleration the rider can produce

30. Suppose 4 dogs pull a sled carrying 2 people
Suppose 4 dogs pull a sled carrying 2 people. Change the dog/person arrangement to change the sled’s acceleration in the 2 questions.

31. Suppose 4 dogs pull a sled carrying 2 people
Suppose 4 dogs pull a sled carrying 2 people. Change the dog/person arrangement to change the sled’s acceleration in the 2 questions.

32. Which part of the formula would you change if you added dogs to the team?

33. Which part of the formula would you change if you added dogs to the team?
Net force

34. How does changing the top number of a fraction affect the value of the answer?

35. How does changing the top number of a fraction affect the value of the answer?
It increases the value

36. Write an “if-then” sentence to show the effect of adding more dogs

37. Write an “if-then” sentence to show the effect of adding more dogs
If you add dogs to the team, then you increase the acceleration

38. What else could be done to increase acceleration?

39. What else could be done to increase acceleration?
By removing some people from the sled or a combination of adding dogs and removing people

41. Remember:
Mass units are kg
Acceleration units are m/s2
Force units are N (kg · m/s2)

45. Force = Mass x Acceleration

47. 2 kg x 2 m/s2 = 4N

49. It would be cut in half to become 1 m/s2

50. I get it! Now I know that Newton’s second law of motion describes the relationship ________
____________________________________

51. I get it! Now I know that Newton’s second law of motion describes the relationship among force, mass and acceleration