Physics

1. Physics • https://www.youtube.com/watch?v=2vYI2NcVsXY • https://www.youtube.com/watch?v=HuZZpJJF71U

2. In Motion https://www.youtube.com/watch?v=rZo8-ihCA9E Physics for Science 20F

3. Distance vs. Displacement • difference between a scalar and a vector. • A Scalar is a value - has only a number & a unit ex: 25km, 150lbs • A Vector - value that has a number, unit and a direction ex: 30 cm left, 40km north (N)

4. Distance is how far you’ve traveled in total ex: 25 km • Displacement is how far you are from your starting point. This needs a direction ex: 25 km from home or 40km north of the city • So, distance is a scalar value whereas displacement is a vector.

5. Displacement with respect to time… • Displacement - always calculated by final position minus initial position.

6. Positive/negative displacement • A line going up to the right is a positive • A line going down to the right is a negative. Draw it. • A flat line represents no displacement (like you’re standing still). Draw it. • We need to translate a graph to a given situation (or vice versa).

7. Velocity • Velocity is a vector that relates average displacement to average time. • Speed is the total distance divided by the total time for a trip. • For both, the formula is: v = ∆d ∆t

8. If you’re finding it from the graph, you are actually finding the slope of the section of line. • If you’re given numbers, simply plug them into the formula. • An easy way to manipulate the formula is to use the magic triangle: d v t

9. The units for the velocity or speed are just the units for distance slash the units for time. ex:Eiko skates to school, a total distance of 4.5 km. The journey takes her 0.62 hours. What is her average speed? v = d/t v = 4.5 km/0.62 h v = 7.3 km/h

10. Write down Newton’s 3 laws.

11. Try these… • Graphing Distance vs Time Worksheet • Graphing Speed vs Time Worksheet

12. Acceleration • Acceleration - change in velocity per unit of time. • Calculated as the slope of a straight line velocity-time graph • Formula: a = Δv t • Remember that Δv = final velocity – initial velocity!

13. ex: What’s the average acceleration of a rollercoaster that starts with an initial speed of 5 m/s but whose speed 4 seconds later is 25 m/s? a = vf – vi t = 25 – 5 4 = 20 4 = 5 m/s2 **The units for acceleration are usually m/s2 but watch out for time units that aren’t the same (change km/h to m/s before doing the questions!)**

14. Try these… • Acceleration Worksheets

15. Newton’s Three Laws • Newton came up with three laws that are very important for our study of science • His first law was that an object in motion will stay in motion (or an object at rest will stay at rest) until acted upon by a force. • Also called… Law of Inertia ex: A ball will keep rolling but only stops because the force of friction slows it and eventually stops it.

16. Newton’s second law states that a force ( a push or a pull) that equals an object’s mass times it’s acceleration. • This can be shown by f = ma • The mass must be in kg and acceleration in m/s2. • The units for force are Newtons (N) ex: How much force will a 2 kg mass exert if it accelerates at 4 m/s2? F = ma = (2kg)(4 m/s2) = 8N F m a

17. Newton’s third law is that for every action there is an equal and opposite reaction • This is important in car collisions that we will look at later. • Newton’s three laws video • https://www.youtube.com/watch?v=mn34mnnDnKU • Bill Nye • https://www.youtube.com/watch?v=iHeMqK3XZbc

18. Momentum • Which causes a bigger dent in your car? A cyclist travelling at 50km/h or a tank travelling at 50 km/h? • Which would hurt you more? A bullet travelling at 1 m/s or one travelling at 30m/s? • Momentum depends on 2 factors, mass and velocity and therefore has the formula: p = mv

19. Mass must be in kg and the velocity is in m/s. • Therefore, the units for momentum is kg m/s • The momentum of an object at rest is always zero • There is a Law of Conservation of Momentumwhich means that the momentum before a collision equals the momentum after the collision

20. p = mv = (500)(16.67) = 8333.33 kg m/s ex: How much momentum will a 500 kg truck travelling at 16.67m/s have? • If you have to change grams to kg, divide by 1000! p m v

21. Impulse • Impulse - the impact of a force on a moving object over an interval of time. • Impulse changes the momentum of an object. It can make it larger or smaller. • It is a vector so it has direction • It can be represented by the formula: I = Ft • It will have the units Ns (Newton seconds) I F t

22. Impulse can be increased in 2 ways: • Increase the force being applied to change the object’s motion • Increase the time that the force is being applied for to change the object’s motion • Which of the two above would be gentler?

23. Friction • Friction is a resistive force exerted by a surface on a moving object • It is a force that opposes motion • Generally, a rough surface will have greater friction and a smooth surface will have lesser friction. • Friction has a great effect on the braking distances of cars

24. There are 3 factors that effect the braking distance: 1) the velocity of the vehicle 2) friction (of the surface AND the brakes) 3) reaction time of the driver • The mathematical formula for calculating braking distance on various surfaces is: d = kv2 where d is distance, k is a constant (table), and v is velocity in m/s

25. There are three situations that you may have to calculate the braking distance: 1) Rubber tires on dry concrete (k = 0.06) 2) Rubber tires on wet concrete (k = 0.1) 3) Rubber tires on ice (k = 0.25) ex: What is the braking distance on dry concrete for a car travelling at 100km/h? 100km/h ÷ 3.6 = 27.8 m/s d = kv2 = (0.06)(27.8)2 = 46.3 m