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# Changing speed and velocity

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### Changing speed and velocity

1. 1. Changing speed and velocity:
2. 2. How do you describe that a car or object changes its speed or velocity? • When it slows down • When it speeds up In what situations or places where cars happen to change the speed? • When it is travelling in a high way its speed increases. • When it travels in a traffic road the speed decreases • When it turns a curve path the car also slows down.
3. 3. What do you call this rate of change in velocity of the object? • It is called acceleration. • In formula we can write acceleration as: • Acceleration= change in velocity • elapsed time • In symbol: • a = Δv or a = vf - vi Δt tf - ti
4. 4. Vf is final velocity , vi is initial velocity tf is final time , ti is initial time What is the MKS unit of acceleration? The MKS unit of acceleration is meter per second squared, in symbol m/s². Other unit is the km/h²
5. 5. Application: • Solve the given problems: • 1. A car on a straight level road is stopped for a red light. On the green light, it uniformly accelerates to a velocity of 18m/s in 3 minutes. What is its acceleration? • A jet plane flying at 100m/s is uniformly accelerated to a cruising speed of 150m/s in 20 minutes. What is its acceleration during this time?
6. 6. Reinforcement: • Which of the following is a graph of acceleration? • A. B. C. v v v t t t Answer: All of them show a graph of acceleration. Why?
7. 7. • 2. How do you describe the slope of graph A? graph B? Graph C ? Answer: Graph A has a positive slope Graph B has zero slope Graph C has negative slope 3. What does a positive slope mean? A zero slope? A negative slope?
8. 8. • Answer: • Positive slope means it has a constant acceleration, the velocity changes but at constant rate.Constant acceleration is also called uniformly accelerated motion. • Zero slope means no acceleration but the car is still moving with constant velocity. • Negative slope means the acceleration is constant at decreasing velocity, when the velocity decreases its called deceleration.
9. 9. Analyze the following graphs of acceleration • A. B. C. v v v t t Which graph shows constant acceleration? Answer: All of them Which graph shows highest or greatest acceleration? Answer: graph A, why? t