Why constant (or straight line) motion? Remember, if an object turns at a constant speed it is accelerating.

Transcription

1 Newton s 1st Law

2 Newton s 1st Law of Motion - An object in constant motion will continue in constant motion or an object at rest will stay at rest unless acted upon by an unbalanced force. Unbalanced force means draw the FBD and look for a net force. If there is a net force the object will change it s state of motion. This is also called the Law of Inertia. Why constant (or straight line) motion? Remember, if an object turns at a constant speed it is accelerating. Mass is a measure of inertia that essentially tells you the amount of matter that an object contains. In physics class, we will measure it in kilograms [kg]. It is the same everywhere! Weight is the force of gravity that pulls an object to the earth. It depends on the location. For instance an object weighs less on the moon but has the same mass as it does on earth. The moon just pulls on it less. In Physics class, we will measure it in Newtons [N].

3 DEFINITION Mass inertia, or resistance to acceleration Weight is the FORCE of the planet on an object SYMBOL m W or F g or F EonO UNITS CONSTANT OR VARIABLE? SCALAR OR VECTOR? Measured in kilograms (kg) Always stays the same Scalar quantity Measured in Newtons (N) Varies with the gravitational pull Vector quantity

4 A force is any push or pull on an object. (Hint: Do Inertia or g push or pull? NEITHER! Inertia (mass) and g (acceleration due to gravity) are NOT FORCES.) A force is an interaction between two objects. Forces are either: contact forces, objects are TOUCHING (Normal, Tension, friction) long-range forces, force acts through field (Gravity or Force of the Earth)

5 Units of Force Newton (SI system) 1 N = 1 kg m /s 2 1 N is the force required to accelerate a 1kg mass at a rate of 1 m/s 2 Pound (British system) 1 lb = 1 slug ft /s 2

6 You need to identify several types of forces and be able to draw them on a FBD: They are all vectors (magnitude + direction)! Weight Gravitational Pull - always perpendicular to the surface of the earth. Pulls down to earth. This is the only non-contact force we will use in this unit. It will usually point straight down toward the center of the earth. symbol F EonO = m g W = m g

7 Contact Force Normal Force always perpendicular to the surface of what is pushing it. Pushes away. If there is more than one contact force there may be more than one normal force. It may point in any direction as long as it is perpendicular to the surface. A common one is an upward support force from a table, floor or road. symbol

8 In the simplest case.the normal force is equal to the weight of an object for objects resting on horizontal surfaces ONLY when no other forces act vertically. n = F EonO = W =mg F N W = mg

9 Tension always along a rope or string. Pulls along the rope. If there is more than one pull there may be more than one tension force. It may point in any direction. symbol

10 Force Exerted by a Spring F SonO = kδl k is the spring constant for a given spring Unit for k is N/m ΔL is the change in length of the spring symbol F SonO

11 Friction - opposes (opposite) motion or attempted motion. Pushes parallel to surface. symbol Sometimes we will get more specific and specify the type of friction. There are two types of sliding friction. Static friction Kinetic Friction force before sliding occurs force after sliding occurs Static friction is the sticky force that keeps an object from sliding across a surface. Static Friction is much greater than Kinetic Friction. Has a maximum value that must be met in order for the object to start sliding.

12 Rolling Friction You are familiar with this from the bowling ball demonstrations that you have seen this year

13 Air/ Wind resistance This is complicated because it changes as velocity and surface area change. Flying squirrels can adjust air resistance to reduce the speed of its fall Terminal velocity is reached for sky divers when air resistance equals weight

14 Drawing Free Body Diagrams (FBD) The purpose is to simplify a situation so that Newton s Second Law may be applied. In each case the rock is acted on by one or more forces

15 Drawing Free Body Diagrams (FBD) Try a few more

16

17 Equilibrium or Not? Zero Net Force or Not? F = 0N or F 0N

18 Sum of the Forces is Zero Bowling ball resting on the desk

19 Another Example F = 0N Static Equilibrium, equilibrium at rest

20 Think Equilibrium An object at rest has F net = 0 Newtons ΣF x = 0 N and ΣF y = 0 N

21 Suppose you see a motion diagram like this Describe the motion What is the change in the velocity of the moving object?

22 Suppose the previous motion diagram was drawn based on the motion of a bowling ball moving at a constant velocity.

23 Equilibrium in motion. dynamic equilibrium Also known as motion with a constant velocity. F still = 0N

24 Can you determine from this force diagram whether the bowling ball is resting on the desk or moving at a constant speed in a straight line? N = F D on B W = F E on B

25 Remember the girl on the skateboard? Could this force diagram show the forces acting on the girl if she is traveling at a constant velocity? Why? Or Why not?

26 Equilibrium revisited An object MOVING AT A CONSTANT SPEED IN A STRAIGHT LINE also has F net = 0 Newtons ΣF x = 0 N and ΣF y = 0 N

27 If there is zero net force on a body in motion, it cannot accelerate, and therefore must move at constant velocity, which means it cannot turn, it cannot speed up, it cannot slow down

28 Shopping cart Egg drop 7 demos Seat Belt Football to face Water balloon

29 N1L Find a good N1L video to share with the class. Egg in glass huaa

30 Video Clip : Focus on the crash test dummies in the back of the station wagon. How is this similar to a magician pulling a tablecloth out from beneath a stack of dishes?

31 Video Clip : West German Highway Used Live Volunteers to test Seat belts mph into a tree.

32 Video Clip : Rear end collision filmed in the car doing the hitting.

33 Video Clip : Same collision from a different angle.

34 Video Clip : than Wear your seat belt! A short clip of a rollover crash with crash test dummies. SUV s tend to do this more readily cars because of their high center of gravity

35 Video Clip : 180 MPH crash. She walks away because she was wearing her seatbelt.

36 Demo: Pulling table cloth out from dishes

37 Demo: Try and pull the dollar out without toppling the bottles Draw a FBD of the top bottle as the dollar is pulled

38 A person is trying to lift a 100N block (purple). What force does he need to apply to the rope to hold the block in place? Does the pulley configuration matter?

39 Demo: Lifting a 1 kg mass using the pulley configuration from the previous slide. What tension is in the rope in each case?

40 Demo: Pie Pan & Egg (Whiplash Demo) Describe the motion of the egg as the broom strikes the pie pan Broom will strike edge of pie pan and then be brought to rest by the edge of the table

41 Demo: Strike the wood block quickly with a hammer Describe the motion of the block if it is struck quickly as shown Describe the motion of the can as the block is struck Try drawing a FBD for the block as it is struck Try drawing a FBD for the can as it is struck

42 Demo:

43 Demo: Putting hammer head on hammer