04 : Forces & Newton’s Laws of Motion

1. 04 : Forces & Newton’s Laws of Motion A force is a push or pullexerted on an object. A force causes change in velocity. Unit: 1 kg m/s2 = 1 newton (N) One newton is the unit of force that accelerates exactly 1 kg of mass at a rate of 1 m/s2 .

2. Mass vs. Weight Ms. Garcia • Mass is the amount of matter an object contains. • Scalar quantity. • SI unit: kilogram (kg) • Use a Balance • Weight is a force caused by the Earth’s gravity. • Vector quantity. • SI unit: Newton (N) 1 N = kg·m/s2 • English unit: pound (lb) • Use a Spring Scale

3. link Contact vs. Field Forces Contact force: force that arises from the physical contact of two objects. Ex: car crash Field force: force that can exist between objects, even in the absence of physical contact between the objects. Ex: The electric field around the balloon exerts an attractive force on the pieces of paper.

4. Balanced vs. Unbalanced Forces • The net force (Fnet)is the sum of the forces acting on a body. Balanced forcesdo notcause a change in motion. They are opposite in direction but equal in size.  The forces canceleach other out. The object is in equilibrium.  There is no net force. Unbalanced forcesalways cause a change in motion. They are opposite but not equal.  There is a net force. Balanced Forces

5. Recognize ForcesLink object

6. “Free-body diagram” FN A free-body diagram is used to analyze all the force vectors acting on a body & how each force affects the motion; used to find the net force. 1) The forces acting on the boy cancel each other since they are equal; balanced. 2) Many forces are involved in pulling a wagon. Forces shown are for the wagon; unbalanced with a net force. Fg “Net Force” = Zero FN Fapplied Ffriction  Fg

7. Balanced or Unbalanced Forces and why? A free falling object is falling under the sole influence of gravitydownwards at a rate of 9.8 m/s2 . Free-falling objects do not encounter air resistance. Fg Gravity (Fg) always goes down! • But as time passes, the air drag increases with her speed until it equals her weight. At this point, she has reached terminal velocity(constant velocity). • Thus, balanced forces, no change in motion. Fdrag Fg

8. Practice: Free-body Diagrams FN • Draw a force diagram of a tire suspended motionless from a tree limb by a rope. • An egg is free-falling from a nest in a tree. Neglect air resistance (no drag). Diagram the forces acting on the egg as it falls. • Diagram the forces on the racehorse as it accelerates towards the finish line.  Fg Fg FN Fground  Ffriction Fg

9. GP: Baseball in Air • A baseball is hit into the air at an angle of 45° above the horizontal. Which free-body diagram shows all the forces acting on the ball at the moment it reaches its maximum height? B D A C

10. Net Force • The net force (Fnet)is the sum of the forces acting on a body. • Horizontal ΣFx = Fright - Fleft (assume right is + ; left is -) • Vertical ΣFy = Fup - Fdown (assume up is +; down is -) • Four forces act on a hot-air balloon, shown below. Find the magnitude and direction of the resultant force on the balloon. Horizontal Net Force, Fx: -1520 N + 950 N = -1425 N West Vertical Net Force, Fy: 5120 N + -4050 N = 1070 N North Resultant = = 529 N NW 5120 N 950 N 1520 N 4050 N

11. Newton’s First Law of Motion(Law of Inertia) Link • “Every object will remain at rest, or will continue to move with constant speed, unless the object is acted on by a force”. • Inertia is resistance to change in direction & speed. • The amount of inertia an object has depends on its mass. (the measure of inertia)

12. Lab: Newton’s 1st Law: • Why do we wear seatbelts? • Why can you pull a cloth out from under dishes without moving the dishes? • Why will the coin drop into the glass when a force accelerates the card?

13. Newton’s Second Law of Motion Newton’s 2nd Law says: A body accelerates when acted upon by a non-zero net force. • Fnet = ma • F = force (N) • 1 N = 1 kgm/s2 • m = mass (kg) • a = acceleration (m/s2) Weight is a gravitational force. where acceleration is 9.8 m/s2 . • Fg= ma

14. F = ma: Acceleration vs. Force • The acceleration of an object is directlyproportional to the net force acting on the object. • (More Force  More Acc.) • Force causes acceleration. • Direct Relationship

15. F = ma: Acceleration vs. Mass • The acceleration of an object is inversely proportional to the mass of the object. • (More mass  Less acceleration) • Mass resists acceleration. • Indirect Relationship

16. Ex. 1: Force • An airplane with a mass of 5,000 kg needs to accelerate at 5 m/s2to take off before it reaches the end of the runway. How much force is needed from the engine? Givens: m = 5,000 kg a = 5 m/s2 F = ? • Fnet = ma F = ma F = 5000 kg x 5 m/s2 F = 25,000 kgm/s2 or 30,000 N

17. Ex.2: Acceleration • A car rolls down a ramp and you measure a force of 2.0 N pulling the car down. The car has a mass of 0.5 kg. Calculate the acceleration of the car. Givens: F = 2.0 N  2.0kgm/s2 m = 0.5 kg a = ? F = ma 2.0 = 0.5 x a 2.0 / 0.5 = a a = 4 m/s2

18. Ex. 3 Involving Friction • A strong man is pulling a truck with a mass of 4500 kg with an applied force of 54100 N. There is a friction force of 15100 N. What is the strong man’s acceleration? • Givens: • m = 4500 kg • F app = 54100 N • F fric = 15100 N • a = ? • Fnet = ma • (54100  15100) = (4500) a • 39000 / 4500 = a • a = 8.7 m/s2 Fapplied Ffriction 4500 kg

19. Ex. 4: Involving finding acceleration In performance testing, a 1250-kg car accelerates from 0 to 27.78 m/s in 8.0 seconds. What is the average net force pushing the car during the test? Givens: m = 1250 kg vi= 0 m/s vf= 27.78 m/s t = 8.0 s F =? 1) Find the acceleration in m/s2: a = (vf – vi) / t a = (27.78 – 0) / (8) a = 3.47 m/s2 2) Find the force: F= ma F = 1250 kg x 3.47 m/s2 F = 4340 N or 4300 N

20. Gravitational Acceleration • Gravitational Acceleration (g) is the acceleration experienced by an object when gravity is the onlyforce acting on it and is directed toward the center of the Earth. • g = 9.8 m/s2 g = 32.2 ft/s2 • If no forces other than gravity are acting on two objects of different mass, both objects will hit the ground at the same time because gravitational acceleration is constant. • As the mass of the object increases, the gravitational force increases but the gravitational acceleration remains the same.

21. Why do you feel “heavier” when you go up in an elevator? • Because the floor of the elevator is pushing upwardwith a greater force than when you are standing still or moving at a constant speed.

22. Ex. 4.3 Weight Measured During Acceleration • A 637 Nperson is placed on a scale in an elevator. The elevator accelerates upward at a rate of 2.50 m/s2. What weight (force) does the scale read during the acceleration? • Givens: • Fg = 637 N • a = 2.50 m/s2 • Fnet = Fscale − Fg • Fscale= Fnet + Fg Find mass of girl: Fg = ma 637 = m x 9.8 637/9.8 = m 65.0 kg = m (girl) 2) Find net force on girl: Fnet= ma Fnet = (65.0)(2.50) Fnet = 163 N 3) Fscale= Fnet + Fg Fscale = 163 + 637 Fscale = 800. N

23. Lab: Friction

24. Newton’s Third Law of Motion • For every action force, there is a reaction force equal in strength and opposite in direction. • Ex. Tire on the road. • Action: Tire pushes on road. • Reaction: Friction from road pushes back. • Ex. When you throw a ball. • Action: your hand pushes the ball • Reaction: ball pushes back on your hand.

25. Lab: Balloon Races • Tie a string across the room. • Blow up a balloon and clip with paperclip. • Tape the balloon to a straw that is on the string. • Let it go and time it from point A (0 m) to point B (10 m). • Obtain the mass of the balloon. • Calculate the Force.

26. Review Forces(click on picture)