1. Activity 81 Major Concepts • If more than one force acts on an object , the forces will reinforce or cancel one another, depending on their direction and magnitude. • Unbalanced forces result in a nonzero net force that will cause the object to accelerate (a change in speed or direction).

2. Activity 81 Analysis • Balanced Forces: • Unbalanced Forces: 1. Describe an example and draw a force diagram of a situation with:

3. 2. Imagine that a parked car is hit from the left with 30,000 N of force. At the exact same time it is hit from the right with 40,000 N of force. • Draw a Force Diagram: • Draw Net Force Diagram: 40,000N 30,000N 10,000N

4. 3. What is the Missing Force? 10 N 20N 10 N

5. 4. Could the block in Part A have been moving? Explain • Yes • Balanced forces – means no acceleration • No acceleration – no change in speed nor direction

6. 5. For each situation, explain why there is or is not a zero net force acting on the car. • A car parked on a level parking space. • A traffic light turns green, and a car starts to move. • A car drives steadily at 25 mph. • A car is slowing down from 30 mph to 10 mph. • A car goes around a corner at 10 mph.

7. Read page E-41

8. Activity 82 Title: Braking Distance Problem: How can friction lower the risk of getting into an accident? Hypothesis: If _____________, then ____________________________.

9. In the next two activities you will examine some of the factors involved in slowing down a car to prevent a collision from occurring.

10. Background Information: Braking Distance • Braking distance is the distance a vehicle travels in the time between the moment the brakes are applied and the moment when the vehicle comes to a stop. • How do brakes stop a vehicle? • In this activity, the friction between the wheels and the track is increased by placing a cloth on the track rather than trying to fit the cart with brakes.

11. When you cover the track with the cloth, will the cart’s braking distance become longer or shorter? • The roughness of the cloth’s surface will bring the cart to a stop in a shorter distance. • In this activity, you will investigate what happens to a cart’s braking distance when it hits the cloth “brakes” at different speeds.

12. Read the procedure on pages E-42 to E-43

13. What do you know? • How does notch height affect speed? • What variables in this experiment are controlled? -braking surface -mass -vehicle shape • What is the tested variable? -notch height • What is the responding variable? -braking distance

14. After you finish: • Do analysis questions 1-4 • Read pg. E-45 and set up your notebook for Activity 83 -Activity # -Title -Problem -Hypothesis

15. Activity 82 Analysis Questions

16. 1. In this model of a stopping car: • What does the cloth represent? • brakes • List some strengths and weaknesses of this model.

17. 2. What effect does speed have on the braking distance? Explain, using evidence from this activity. • Initial speed increased  braking distance increased • What about the amount of changes in speed, how much effect does that have on braking distance?

18. 3. Use evidence from this activity to support the statement… A car moving at 55 mph needs less distance to stop than an identical car moving at 45 mph. -Not supported by our evidence -Faster Speed, more Braking Distance

19. 4. Reflection: Why do you think speeding is a factor in a high percentage of car accidents? • This activity showed, speed, stopping distance What affect will this have? • If it does hit something, a speeding car will be traveling faster…… • Activity 76 showed that collisions at higher speeds result in………