Newton s First & Second Law

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Dwayne McDowell
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1 ewton s irst & Second Law Physics 1

2 Unit is the EWTO() Is by definition a push or a pull Can exist during physical contact(tension, riction, Applied orce) Can exist with O physical contact, called IELD ORCES ( gravitational, electric, etc)

3 IERTIA a quantity of matter, also called MASS. Unit for MASS kilogram. Weight or orce due to Gravity is how your MASS is effected by ygravity. W mg OTE: MASS and WEIGHT are OT the same thing. MASS never changes When an object moves to a different planet. What is the weight of an 85.3-kg person on earth? On Mars3. m/s/s)? W W mg W (85.3)(9.8) (85.3)(3.) 7. MARS )

4 An object in motion remains in motion in a straight line and at a constant speed OR an object at rest remains at rest, ULESS acted upon by an EXTERAL (unbalanced) orce. There are TWO conditions here and one constraint. Condition #1 The object CA move but must be at a COSTAT SPEED Condition # The object is at REST Constraint As long as the forces are BALACED!!!!! And if all the forces are balanced the SUM of all the forces is ZERO. The bottom line: There is O ACCELERATIO in this case AD the object must be at EQILIBRIUM ( All the forces cancel out). acc 0 0

5 A pictorial i representation ti of forces complete with labels. f W 1,g 1 or m 1 g T T m g Weight(mg) Always drawn from the center, straight down orce ormal( ) A surface force always drawn perpendicular to a surface. Tension(T or T ) force in ropes and always drawn AWAY from object. riction(f)- Always drawn opposing the motion.

6 f mg

7 Since the net 0, a system moving at a constant speed or at rest MUST be at EQUILIBRIUM. TIPS for solving problems Draw a BD Resolve anything into COMPOETS Write equations of equilibrium Solve for unknowns

8 A 10-kg box is being pulled across the table to the right at a constant speed with a force of 50. a) Calculate the orce of riction b) Calculate the orce ormal 50 a f a mg ( 10)(9.8) ) 98 n f mg

9 Suppose the same box is now pulled at an angle of 30 degrees above the horizontal. a) Calculate the orce of riction b) Calculate the orce ormal ax cosθ 50cos30 a f ax a 30 f mg ax ay mg! + ay mg mg 73 ay (10)(9.8) 50sin 30

10 If an object is OT at rest or moving at a constant speed, that means the ORCES are UBALACED. One force(s) in a certain direction over power the others. THE OBJECT WILL THE ACCELERATE.

11 The acceleration of an object is directly proportional to the ET ORCE and inversely proportional to the mass. 1 ET aα ET aα m ET a ET m ma Tips: Draw an BD Resolve vectors into components Write equations of motion by adding and subtracting vectors to find the ET ORCE. Always write larger force smaller force. Solve for any unknowns

12 A 10-kg box is being pulled across the table to the right by a rope with an applied force of 50. Calculate the acceleration of the box if a 1 frictional force acts upon it. In which direction, et ma a is this object accelerating? f a f ma The X direction! mg a So.S.L. is worked out using the forces in the x direction only a 3.8 m / s

13 A mass, m kg, is resting on a frictionless horizontal table is connected to a cable that passes over a pulley and then is fastened to a hanging mass, m 11.0 kg as shown below. ind the acceleration of each mass and the tension in the cable. et ma T m g T ma T m1a T m g m1 a m a m 1 g m g m a + m a m g a( m + m ) m g m g (11)(9.8) a 7.7 m / s m + m

14 et ma T (3)( 3)(7.7) ) 3. 1 m g T m a T m1a ET t et ma a Rise Slope Run m

15 Where does the calculus fit in? r ma r dv m dt d x m dt There could be situations where you are given a displacement function or velocity function. The derivative will need to be taken once or twice in order to get the acceleration. Here is an example. You are standing on a bathroom scale in an elevator in a tall building. Your mass is 7-kg. The elevator starts from rest and travels upward with a speed that varies with time according to: v ( t) 3t + 0.0t When t 4.0s, what is the reading on the bathroom scale (a.k.a. orce ormal)? dv d(3t + 0.0t ) a t dt dt a(4) (4) 4.6 m/s/s a mg ma ma + (7)(9.8) + (7)(4.6) net ma mg

Unit 5 Forces I- Newtonʼ s First & Second Law

Unit 5 Forces I- Newtonʼ s First & Second Law Unit 5 orces I- Newtonʼ s irst & Second Law Unit is the NEWTON(N) Is by definition a push or a pull Does force need a Physical contact? Can exist during physical contact(tension, riction, Applied orce)