Newton’s Laws and Forces 1

1. Newton’s Laws and Forces 1 Honors Physics

2. Biblical Reference Then he pushed with all his might, and down came the temple on the rulers and all the people in it. Judges 16:30

3. Facts About Force: Unit:Newton (N) = kg x m/s2 • Named after Isaac Newton Definition: a push or a pull • Can exist during physical contact (Tension, Friction, Applied Force, Normal Force, Spring Force, etc.) • Can exist with NO physical contact, called Field Forces (Gravity, Electric, Magnetic, etc.) Vector: has magnitude and direction

4. What are Forces? • A push or pull on an object is a force. • A force is a vector. • It has both size and direction. • Arrows are used to show the size and direction of the force. • A force can change an object’s speed and the direction in which the object is moving.

5. Types of Forces • A contact forceis a push or a pull applied by one object to another object that is touching it. • Contact forces are also called mechanical forces. • A force that pushes or pulls an object without touching it is a noncontact force.

6. Contact Forces • Friction • Resistance to motion • Normal Force • Support force • Tension • Stretching • Applied Force • Hitting, Pushing, etc. • Spring Force • Force exerted by a stretched or compressed object

7. Friction • Friction is a contact force that resists the sliding motion between two objects that are touching. • The force of friction acts in the opposite direction of the motion. • Rougher surfaces produce greater friction than smooth surfaces.

8. Noncontact Forces • Magnetic • Opposite Poles Attract • Same Poles Repel • Electric • Opposite Charges Attract • Like Charges Attract • Gravity • The pull of the Earth downward

9. Gravity • Gravityis a noncontact attractive force that exists between all objects that have mass. • Gravitational force depends on the mass of the objects and the distance between them. • Gravity comes from the Latin word, Gravitare, which means to unite or join together.

10. Gravity • Weight is a measure of the force of gravity acting on an object. • As two objects get farther apart, the gravitational force between the objects decreases.

11. Gravity • As the mass of one or both objects increases, the gravitational force between them increases. • Doubling the mass of one of the objects doubles the force of attraction.

12. Combining Forces • When several forces act on an object, the forces combine to act as a single force. • The sum of the forces acting on an object is called the Net Force.

13. Combining Forces • When different forces act on an object in the same direction, you can find the net force by adding the forces together.

14. Combining Forces • When forces act in opposite directions, you must include the direction of the force when you add them. Which team will win? - 400 + 300 - 100 - 120 -180 +110 + 90 +100

15. Balanced Forces • When the net force on an object is 0 N, the forces acting on it are balanced forces. • If the forces acting on an object are balanced, the object’s motion does not change.

16. Unbalanced Forces • When the net force acting on an object is not 0, the forces acting on the object are unbalanced forces. • Unbalanced forces cause objects to change their motion, or accelerate.

17. Sir Isaac Newton • Forces are measured in a unit called a Newton (N), named after English scientist Sir Isaac Newton, who studied the motion of objects. • Newton summarized his findings in three laws of motion.

18. Newton’s First Law – The Law of Inertia Inertia – a quantity of matter, also called Mass • Italian for “Lazy”. • Unit for Mass = Kilogram (kg) • Resistance to acceleration • More massive objects have greater resistance to change in motion Weight(Force due to Gravity) is how your mass is affected by gravity.

19. NOTE:Massand Weightare NOT the same thing. Massnever changes when an object moves to a different planet. Question: What is the weight of an 85.3-kg person on earth? On Mars (g=3.2 m/s/s)?

20. Newton’s First Law of Motion • Objects in motion stay in motion, and objects at rest stay at rest, unless acted upon by an outside force. • Newton’s First Law is sometimes called The Law of Inertia. • Inertia is the tendency to resist a change in motion. Example

21. Newton’s First Law of Motion There are two conditions and one constraint: Condition #1– The object can move but at a constant speed Condition #2– The object is at rest Constraint– The forces must be balanced - the sum of all the forces is zero. There is NO ACCELERATION, and the object must be at EQILIBRIUM ( all the forces cancel out).

22. Newton’s First Law • When the forces acting on an object are balanced, the object is either at rest or moving with a constant velocity. • Unbalanced forces can cause an object to accelerate or decelerate. • Unbalanced forces can also cause an object to change direction. Demonstration

23. Examples of Newton’s First Law • A beverage in a car that either starts or stops suddenly • The liquid wants to stay at rest or keep moving • Unrestrained people fly forward when a vehicle comes to a stop. • When you hit the bottom of a ketchup bottle, the ketchup slides to the front of the bottle. • A skateboarder flies forward when he hits a curb.

24. Newton’s Second Law of Motion • Newton’s Second Law of Motionstates that the acceleration of an object is equal to the net force exerted on the object divided by the object’s mass. • Acceleration comes from the Latin word, “celer”, which means “swift”.

25. Acceleration and Force • Both acceleration and force are vectors. • They have both a magnitude and a direction. • The direction of acceleration is the same as the direction of the net force. • Newton’s Second Law predicts the combination of force and mass needed to achieve a certain acceleration. Demonstration

26. Newton’s 2nd Law (F=ma) Newton’s 2nd Law is based on direct and inverse relationships. As twoof the variables change, oneof them must remain constant. If force is constant, acceleration and mass change.

27. Newton’s 2nd Law (F=ma) If we add a second dog pulling with 100N just like the first dog, we could pull the sled with twice the acceleration, if the mass of the sled was constant. F(net)=ma 2F=m(2a) 3F=m(3a)

28. Forces Newton’s Second Law Newton’s First Law

29. Example: Solving for Acceleration Determine the accelerations that result when a 12-N force is applied to a 3-kg object. Given: F = 12N, m = 3 kg Find: a

30. Example: Solving for Mass A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. What is the mass of theencyclopedia? Given: F = 15N, a = 5 m/s2Find: m

31. Example: Solving for Force Suppose that a 10-kg sled is accelerating at a rate of 2 m/s2. What is the net force on the sled? Given: m = 10 kg, a = 2 m/s2Find: F

32. Newton’s Third Law of Motion • Newton’s Third Law of Motionsays that when one object exerts a force on a second object, the second object exerts a force of the same size, but in the opposite direction, on the first object. • When two objects apply forces on each other, one of the forces is called the action force, and the other is called the reaction force. Demonstration

33. Newton’s Third Law • When two objects exert forces on each other, the two forces are a force pair. • Force pairs are not the same as balanced forces. • Balanced forces combine or cancel each other out because they act on the same object. Each force in a force pair acts on a different object.

34. Examples of Newton’s Third Law Two ice skaters – one pushes the other and they both move in opposite directions. Fish use fins to propel themselves through water. Thrusters on satellites expel mass to move in the opposite direction. A rifle recoils when fired.

35. Example: A 2.75 kg box sits on a frictionless table. A rope exerts a force of 20.0 N at an angle of 35 above the horizontal. There is another horizontal force of 12.0 N in the opposite direction of the rope. What is the net force in the x-direction? What is the acceleration?

37. Example: When a 5.0 N force is exerted on a dog’s leash at an angle of 65 above the horizon, the dog accelerates 0.061 m/s2 in the x-direction. What is the dog’s mass? 5.0 N 65 0.061 m/s2

38. 5.0 N 65

39. Example: A baseball is hit by a bat with a force of 325 N at a direction of 105 above the horizon. If the mass of the baseball is 0.15 kg, what is the acceleration in the x-direction? 325 N 105

40. 325 N 105 75