Chapter 4 Force and Motion

1. Chapter 4 Force and Motion

2. Force and Motion • A force is a push or a pull exerted on an object. • Can cause objects to speed up, slow down, or change direction • Pg 89 “A force exerted on an object causes the object’s velocity to change; that is a forces causes an acceleration (or deceleration)”

3. Forces and Motion • Everything around the object that exerts forces on it is called the external world.

4. Contact and Field Forces • Field Forces are exerted without contact • Gravity • Magnetism • Contact forces are exerted with contact. • Push, pull, drag, touch • An agent is the thing that is exerting the force.

5. Free Body Diagrams • Help you analyze how forces affect motion • Create a Pictorial Model • Go to Page 89 • Read “ To make a physical representation…..” paragraph

6. Drawing Free-body Diagrams • Represent the object with a dot • Represent each force with a arrow that points in the direction that the force is applied. Make the length of each arrow proportional to the size of the force. • Draw the force arrows pointing away from the particle. Use the symbol F with a subscript label to identify both the agent and the object on which the force is exerted

7. Combining Forces • Net Force: the vector sum of all the forces on an object. • Pg 92 Figure 4-5

8. Newton’s 2nd Law F = ma The sum of the forces acting on the object is equal to the mass of the object times the acceleration of an object. F = force a = acceleration m = mass • Force = Newtons or (kg * (m/s2)) • accel = m/s2 • mass = kg

9. Newton’s 1st Law • An object at rest will remain at rest, and an object in motion will continue moving in a straight line with a constant speed until the net force of the object isn’t zero. • AKA: Law of Inertia Inertia: an object’s ability to resist change • Equilibrium • When net force = 0 • Object is at rest or moving at a constant velocity

10. Different Forces we will use Pg 94 table 4-2 Do Pg 93 7 and 8 What is the magnitude of a force on a 3 kg that is acceleration a 2 m/s2 A force of 15 N is applied to a chair that weighs 10 kg. What is the acceleration of the chair? What is the acceleration if there is an opposing force of 10 N too?

11. Free Falling Object • No touching anything • Air resistance is neglected • Gravity is the only force • Ball’s acceleration is g, 9.8 m/s2 • F = ma • Think about how g is difference in other places.

12. Scales (bathroom) • Based on springs • The scale exerts an upward force on your b/c you are in contract with it. You are not accelerating so spring force must equal your weight (another type of force due to gravity) • Scales measure weight

13. Mass vs. weight • Weight is dependent on gravity • Mass isn’t • Example: Your mass is the same on every planet because the amount of matter inside you stays the same. • Weight, however, depends on the magnitude of the gravity • 15-18

14. Apparent Weight • Ex. Standing on a scale in an elevator • How would you feel when you start moving up? Would you scale increase in force or decrease? • How would you feel when you start moving down? Or if you slow down? What would the scale say?

15. Apparent Weight • The force an object experiences as a result of all the forces acting on it, giving the object an acceleration • What if the cable breaks? You and the scale would accelerate with a = -g. You would be weightless • Weightlessness does not mean that an object’s weight is actually zero. But no contact forces so no apparent weight. • 19-20

16. Drag Force and Terminal Velocity • Air exerts a balanced force on all sides- no net force • Drag Force is the force exerted by a fluid on the object moving through fluid. • As the speed of the object increases, so does the magnitude of the drag force. • Size and shape of the object also affects the drag force • It is also affected by the properties of the fluid (ie. Viscosity and temperature)

17. Terminal Velocity • Pg 101 Read first paragraph • The constant velocity that is reached when the drag force equals the force of gravity is called terminal velocity. • How can we apply this idea to life situations? • Competitive skiers, sky divers, bikers • 21-27

18. Interaction Forces • Forces come in pairs • Interaction pair is two forces that are in opposite directions and have equal magnitudes. • Forces exist together or not at all. • Ex. Roller Bladers

19. Newton’s 3rd Law • All forces come in pairs. F A on B = -F B on A The force of A on B is equal in magnitude and opposite in direction of the force on B on A • Example: Holding a book • Pg 104 28-31

20. Forces of Ropes and Strings • Tension is the force exerted by a string or rope • Strings/ropes are considered massless • Ex bucket tried to string in equilibrium. • The tension in the rope is equal to the weight of the bucket (like a scale) • Example: Tug-of-War • Pg 106 32-33

21. Normal Force • Any time two objects are in contact, they each exert a force on each other. • The normal force is the perpendicular contact force exerted by a surface on another object. • Always equal to weight of the object