Question

Question: You're driving down a country road at 25 m/s, when a deer suddenly starts crossing the road. As soon as you react, you hit the brakes to decelerate the car at a rate of 4.20 m/s². If the deer is 77 m ahead of you, do you hit the deer with your car?

Activity: Develop a method for calculating your average reaction time using only a ruler or meter stick, NO STOPWATCH (you will need a partner to help you). Use your reaction time (the time it would take you to hit the brakes once you saw the deer) to solve the physics question listed above, showing your calculations in the Data Analysis section. Write up an individual lab report using the correct format, shown on the next page.

Please type the answer, no handwritten answers on paper, please.

Answer 1

Q) Answer: Distance = 74.40 m.

Explanation:

Initial velocity of the car, u = 25 m/s

since our target is to stop the car, the final velocity of the car, v = 0 m/s

The acceleration of the car, a = -4.20 m/s (-ve since its deceleration )

where s is the distance the car travels.

If you hit the brake at the moment you saw the deer, you won't hit the deer, ().

This does not include the reaction distance.

Reaction distance = Velocity * Reaction time

Therefore, the total distance the car travels is

As given in question, calculate your Reation time and pluf into this equation to get total distance.

For not hitting the deer, total distance should be less than 77m

Therefore, for not hitting the deer, your reaction time should be less than 0.103 sec.

Activity to calculate your reaction time:

1. Put your thumb and index finger slightly open at the bottom of the ruler, with the ruler between their fingers, held about 1 cm apart. Place your forearm on the lab bench with the fingers overhanging the edge. This is to prevent you from moving your arm downwards, chasing after the falling ruler
2. Ask your friend to suspend the ruler between your thumb and finger, held about 1 cm apart and ask him/her to drop the ruler without warning while the you attempts to catch it.
3. Record the distance the ruler fell.
4. Repeat the same 10 times.
5. Calculate your reaction time for each run and then calculate your average reaction time.

Theory:

If an object is released from rest it can be proven that the distance it fall is given by the equation:

where, y is the length of ruler dropped below your hand

t is the time of fall of the meter stick (time from when the meter stick is release to when it is caught). This is your reaction time.

take g = 9.8 m/s²

Calculate t and use this reaction time t in the problem to find the distance the car travels.

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