1. Chapter 3 Forces & Newton’s Laws

2. Action-at-a-Distance Forces
What is a Force?
4 Fundamental Forces
Gravity, Electromagnetism, Strong Nuclear, Weak Nuclear
Force –
Measured in Newtons
Changing Motion-
Contact Forces
Action-at-a-Distance Forces
Frictional Force
Gravitational Force
Tension Force
Electrical Force
Normal Force
Magnetic Force
Air Resistance Force
Applied Force
Spring Force

3. 3.1- Forces
Identifying Forces

4. 3.1- Forces Forces Net force – Rules for Adding Forces
Add forces in the same direction.
Subtract forces in opposite directions
Forces not in the same directions\ or in opposite directions cannot be directly added together.

5. 3.1- Forces
Calculate Net Force

6. 3.1- Forces
Balanced forces –
Unbalanced forces -

7. 3.1- Forces Drawing Force Diagrams Interpreting Force Diagrams
A book is at rest on a table top. Diagram the forces acting on the book.
A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.
Draw a Force Diagram to represent the item in Motion
Describe the motion of each box

8. Friction 3.1- Forces Friction- What causes friction? Static friction
Sliding friction
Rolling friction

9. Gravity 3.1- Forces Gravity- Law of Universal Gravitation
Force increases as mass increases or objects move closer

10. 3.1- Forces
Gravity
The Range of Gravity
The Gravitational Field

11. Gravity 3.1- Forces Weight –
Weight (N) = mass(kg) x gravitational strength (N/kg)
Fg =mg

12. Let’s Practice
If your mass is 70 kg on Earth, what is your weight? Remember: acceleration due to gravity is 9.8 m/s2.
A boy weighs 400 N on Earth. What is his mass?
An astronaut has a mass of 100 kg and a weight of 370 N on Mars. What is the gravitational strength of Mars?

13. Gravity 3.1- Forces Weight and Mass Weight on Earth
Finding other Planets

14. 3.2- Newton’s Laws of Motion
Newton’s 1st Law of Motion
Newton’s 1st Law –
Break it Down – Things want to keep doing what they are doing
Inertia –
Inertia and Mass
Mass corresponds to an object’s inertia
More massive objects have more inertia than less massive objects

15. 3.2- Newton’s Laws of Motion
Newton’s 2nd Law of Motion
Newton’s 2nd Law
Break It Down – F=ma
Acceleration is caused by net force
Acceleration is directly proportional to net force
Push an object, object accelerates
Push an object with twice as much force, then it accelerates twice as much
Direction of acceleration is always the direction of the net force

16. 3.2- Newton’s Laws of Motion
Newton’s 2nd Law of Motion
Mass and acceleration
More massive objects are harder to accelerate
Acceleration is inversely proportional to mass
Push a brick, brick accelerates
Push 2 bricks with same amount of force, accelerates ½ as much
3 bricks accelerates 1/3 as much

17. 3.2- Newton’s Laws of Motion

18. Let’s Practice!!!
With what force will a car hit a tree if the car has a mass of 3,000 kg and it is accelerating at a rate of 2 m/s2?
A 10 kg bowling ball would require what force to accelerate it at 4 m/s2?
If a helicopter’s mass is 4,500 kg and the net force on it is 18,000 N upward, what is it’s acceleration?

19. 3.2- Newton’s Laws of Motion
Newton’s 3rd Law of Motion
Newton’s 3rd Law
Break It Down - forces always occur in pairs

20. A Simple Rule Helps to Identify Action and Reaction
Action: Object A exerts a force on Object B Reaction: Object B exerts a force on Object A

21. Identifying Force Pairs
Identify the following Force Pairs
Action: Enclosed air particles push balloon wall outwards.
Reaction: ?
Action : Baseball pushes glove leftwards
Reaction: ?
Action: Shuttle pushes exhaust gases down
Reaction: ?

22. 3.2- Newton’s Laws of Motion
Action and reaction forces do not cancel each other out
Forces only cancel when they act on the same body
Action-Reaction forces work on different objects

23. 3.3- Using Newton’s Laws What happens in a crash?
Newton’s 1st Law and Safety Belts
Air Bags in Cars

24. 3.3- Using Newton’s Laws
Newton’s 2nd Law and Gravitational Acceleration
Air Resistance
Depends on size and shape
Depends on speed of object
More speed, more air
resistance
Terminal velocity -

25. 3.3- Using Newton’s Laws
Newton’s 2nd Law and Gravitational Acceleration
Free Fall –
Weightlessness
What does weightless mean?

26. 3.3- Using Newton’s Laws Centripetal Forces Centripetal force-
Force acting toward the center of circle

27. 3.3- Using Newton’s Laws Force and Momentum P = mv
Conservation of Momentum
Law of Conservation of momentum
Momentum conserved

28. 3.3- Using Newton’s Laws
Conservation of Momentum - inelastic

29. 3.3- Using Newton’s Laws
Conservation of Momentum - elastic

30. Let’s Practice!!!
Conservation of Momentum