# Acceleration Acceleration the rate at which velocity changes

• Slides: 19

Acceleration • Acceleration – the rate at which velocity changes • Can be an: • Increase in speed • Decrease in speed • Change in direction

Types of acceleration • Increasing speed • Example: Car speeds up at green light • Decreasing speed • Example: Car slows down at stop light • Changing Direction • Example: Car takes turn (can be at constant speed) screeeeech

Question • How can a car be accelerating if its speed is a constant 65 km/h? • If it is changing directions it is accelerating

Calculating Acceleration • If an object is moving in a straight line ¡ Units of acceleration: l m/s 2 Acceleration is simply expressed as velocity per seconds. And Velocity as meter per seconds. So the SI notation is m*s*s.

Calculating Acceleration Final Initial 0 s 0 m/s 1 s 4 m/s 2 s 8 m/s 3 s 12 m/s 4 s 16 m/s

Graphing Acceleration • Can use 2 kinds of graphs • Speed vs. time • Distance vs. time

Graphing Acceleration: Speed vs. Time Graphs 1)Speed is increasing with time = positive acceleration 2)Line is straight = acceleration is constant

Graphing Acceleration: Speed vs. Time Graphs Rise = 4 m/s Run = 2 s 3) In Speed vs. Time graphs: Acceleration = Rise/Run 4) What is the acceleration of the object between time 2 seconds and 4 seconds? = 4 m/s ÷ 2 s = 2 m/s 2

• You can use a graph to calculate acceleration. • The slope of a speed-time graph is acceleration. • The graphs below show 3 scenarios. • Figure 1 shows a constant velocity and zero acceleration. • Figure 2 shows a direct relationship with increasing velocity and positive acceleration. • Lastly Figure 3 shows a inverse relationship with decreasing velocity and negative acceleration.

Positive acceleration Negative acceleration

Graphing Acceleration: Distance vs. Time Graphs 1)On Distance vs. Time graphs a curved line means the object is accelerating. 2)Curved line also means your speed is increasing. Remember slope = speed.

Question: 35 Distance (m) 30 25 20 15 10 5 0 0 1 2 3 4 5 Time (s) The black and red lines represent a objects that are accelerating. Black is going a greater distance each second, so it must be speeding up. Red is going less each second, so 1)be Which must slowingline downrepresents an object that is accelerating? Remember: in distance vs. time graphs: curved line = accelerating, flat line = constant speed

Question Run = 3 s Rise = -6 m/s Above is a graph showing the speed of a car over time. 1) How is the speed of the car changing (speeding up, Slowing down, or staying the same)? 2) What is this car’s acceleration? 1) The car is slowing down 2)Acceleration = rise/run = -6 m/s ÷ 3 s = -2 m/s 2

Question: Hard one Above is a graph showing the speed of a car over time. 1)What would a distance vs. time graph for this look like?

Question: Hard one Is there acceleration? YES! What shape will the distance/time graph need to be? Curved!

Question: Hard one ¡What is the speed doing? ¡What is the distance doing? ¡What Going down! Going UP! does that mean for the GAIN in distance over time? Distance over time goes up slowly!

Distance (m) Question: Hard one 45 40 35 30 25 20 15 10 5 0 0 1 2 3 4 5 6 Time (s) Distance over time goes up slowly in a curved line!

Exit Question A car does not change its speed, but it is accelerating. How is this possible? (use your acceleration notes) • The car is changing direction! (A change in direction is a change in velocity)