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UNIT 3 Dynamics 26 May 2018 Physics 11.

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1 UNIT 3 Dynamics 26 May 2018 Physics 11

2 (gravitation, friction,
Unit 3 Dynamics (Ch 5,8,9) 1. Forces (gravitation, friction, Normal, applied) 4. Momentum 2. Hooke’s Law 3. Newton’s Law 26 May 2018 Physics 11

3 4.2 Gravitational Force (Fg)
4.0 Dynamics Dynamics: The study of the forces that cause motion 4.1 Force (F) Force: -a push or a pull -may cause the movement of objects -Variable symbol: (F) -Units: Newtons (N) 4.2 Gravitational Force (Fg) -Def: Attraction of any 2 objects of mass (pull towards the center of the earth.) -We use a spring balance to measure the force of gravity (aka: weight) in Newtons (N) Test of the Notes 26 May 2018 Physics 11

4 26 May 2018 Physics 11

5 Weight = force of gravity (N) Mass = amount of matter (kg)
NB: Weight ≠ Mass Weight = force of gravity (N) Mass = amount of matter (kg) An object weighs LESS on the moon but mass is the same. Gravitational Field Strength Earth: Moon: 26 May 2018 Physics 11

6 Force of Gravity on an object:
Eg. On mercury a 57 kg man has a weight of 201 N. Find the gravitational field strength on mercury. 26 May 2018 Physics 11

7 A force is a vector quantity as it has direction and magnitude
4.3 Free Body Diagrams A force is a vector quantity as it has direction and magnitude We use Free body diagrams to show all the forces acting on an object All forces emanate from a center of mass EG) Draw a free body diagram of a sailboat being pushed along by the wind. 26 May 2018 Physics 11

8 ALWAYS act to oppose motion NEVER causes an object to move!!!
4.4 Friction Forces ALWAYS act to oppose motion NEVER causes an object to move!!! There are several types of friction: Sliding: 2 surfaces rubbing Static: preventing motion (VIDEO: #03-02) Rolling (tire of cars) Air / Viscous 26 May 2018 Physics 11

9 Sliding Friction depends on: Type of surface (smoothness=μ)
Force pushing the surfaces together (Force Normal= FN ) Sliding Friction does NOT depends on: Speed Surface area (VIDEO: #03-03) 26 May 2018 Physics 11

10 4.5 Co-efficient of Friction (μ)
A measure of the force of friction on a sliding object Always a value between 0 and 1 VIDEO #03-05 Note: μ has NO units!! 26 May 2018 Physics 11

11 EG: A 250.0 kg motorcycle skids along an icy road with an average friction force of 40.0 N. Find μ.
26 May 2018 Physics 11

12 You can use a force to stretch or compress some materials
4.6 Hooke’s Law You can use a force to stretch or compress some materials Most materials will respond with an opposing ELASTIC force that is directly proportional to the change in the length of the material 26 May 2018 Physics 11

13 The force is directly proportional to the change in the length of the material, and will produce a straight line when graphed 26 May 2018 Physics 11

14 Thus the equation for Hooke’s Law is:
NB: k (spring constant) will be the slope of a F vs x graph. 26 May 2018 Physics 11

15 EG: A spring stretches from 10cm to 16cm when a 150 g mass is hung from it. What is the spring constant of the spring? 10cm 16cm 26 May 2018 Physics 11

16 If the graph of F vs X is not linear (a curve) then the material does not follow Hooke’s Law
Eg: sometimes the material does not spring all the way back to it’s original shape. (over stretched!!! You have exceeded the elastic limit for that material.) 26 May 2018 Physics 11

17 EG: Why do cars have seatbelts?
4.7 Newtons First Law “An object at rest will stay at rest, and an object moving will continue to move at a constant speed unless unbalanced forces act on that object” forces are balanced for objects at rest or objects moving with a constant velocity The tendency of an object to resist a change in it’s state of motion (moving or not) is described as: INERTIA EG: Why do cars have seatbelts? 26 May 2018 Physics 11

18 Inertia is dependant on an object’s mass only
26 May 2018 Physics 11

19 4.8 Newton’s Second Law Newton’s first law is about balanced forces. What if the forces on an object are NOT balanced? Newtons second law deals with this… “The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.” What then is a net force? 26 May 2018 Physics 11

20 Newtons 2nd Law: EG # 1: A 25.0 kg toy rocket produces a thrust of N against a N friction force. Find the initial acceleration of the rocket upward at takeoff. 26 May 2018 Physics 11

21 26 May 2018 Physics 11

22 A boy pushes a 110.0 kg box with a force of 900.0 N.
EG # 2: A boy pushes a kg box with a force of N. a) What acceleration would result if μ=0.50, and the box is being pushed with a 50.0 N wind from behind the boy b) If the box started from rest, how far will it get in 2.0 mins? FN F a=900N Ff F W=50.0N Fg 26 May 2018 Physics 11

23 a) b) 26 May 2018 Physics 11

24 4.9 Newton’s Third Law “For every action force, there is an equal and opposite reaction force.” EG #1: Explain (using Newtons 2nd and 3rd laws) what happens when a man on a cart, pushes off from a wall A push on the wall will create an equal reaction force acting on the man… The reaction force on the man is greater than the opposing friction, thus a net force is created, and the man accelerates creating motion! 26 May 2018 Physics 11

25 Reaction forces are responsible for normal forces
The force of gravity is balanced with the reaction force (Fn) and thus the objects do not accelerate vertically 26 May 2018 Physics 11

26 4.10 Momentum Momentum describes how difficult it will be to stop a moving object Momentum is dependent on an object’s: mass and velocity Momentum is a vector quantity and thus has a direction equal to that of a mass’ velocity (thus an object CAN have negatively directed momentum) 26 May 2018 Physics 11

27 4.11 Impulse An object’s momentum can be changed by a force
This change in momentum (Δρ) is called IMPULSE EG: A golf ball on a tee has a momentum of zero (ρ=0 ) EG: On impact, the force of the club on the ball creates motion, and thus it changes the ball’s momentum. The change in an object’s momentum will depend on the magnitude of the force and the application time of that force 26 May 2018 Physics 11

28 Impulse can also be calculated using the following equation:
EG: The contact time between a golf ball and a golf club on impact is 0.15 s. What force was applied to a 40.0g golf ball on impact with the club if the ball’s speed immediately after the impact was m/s ? 26 May 2018 Physics 11

29 4.12 Conservation of Momentum
Momentum cannot be created or destroyed in an isolated system (no external forces), it can only be transferred from one object to another… the momentum before and after a collision between two objects must be equal EG: A 1.0 kg ball is rolling east at 2.0 m/s when it strikes a stationary 0.50 kg ball. After the collision, the 1.0 kg ball rebounds with a velocity of 1.0 m/s. What is the speed and direction of the 0.50 kg ball? 26 May 2018 Physics 11

30 A B A B 26 May 2018 Physics 11

31 (5)(0) + (0.015)(0) = (5)(V) + (0.015)(500) V =-1.5 m/s
Practice #1: A 5.0 kg rifle fires a 15.0 g bullet at a velocity of m/s. Find the recoil speed of the rifle. p(before) = p(after) (5)(0) + (0.015)(0) = (5)(V) + (0.015)(500) V =-1.5 m/s Practice #2: A 40.0 kg girl on skates going 4.0 m/s overtakes a 60.0 kg boy from behind going 2.0 m/s and grabs onto him. The couple coast off together. What is their speed? p(before) = p(after) (40.0)(4.0) + (60.0)(2.0) = (40.0)(V) + (60.0)(V) V = 2.8 m/s 26 May 2018 Physics 11

32 26 May 2018 Physics 11

33 26 May 2018 Physics 11

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