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Classy Jack

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1. Your friend is convinced that she's found a situation where the total energy in the universe is not

being conserved. To demonstrate, she takes a magnet out of her pocket and uses it to pick up a nail.

Analyze this situation when the nail is accelerating upward (i.e. using energy concepts). How would

your friend argue that energy isn't conserved? Try and give an argument to your friend proving that

the total energy is conserved.

2. You are called to investigate a traffic collision (involving two very expensive cars) where the facts are

in dispute. Driver A claims that Driver B rammed into him from behind while he was stopped at a

red light. The damage is very light, but the repairs will be expensive. You investigate the scene and find no skid marks on the road. Driver A explains this by saying that his car (car A) weighs a bit more

than the other car (car B) which is why you don't see any skid marks. Use momentum and energy

concepts to reason about what actually happened in the collision.

3. You hook a fire hose directly up to a hydrant that sits beneath a water tower at UC Davis. If the

water in the tank is at a maximum height of 23 m, with what velocity does the water exit the hose on

the ground (assuming no loss to thermal energy systems)? Using this velocity, calculate the maximum

height to which the water will rise if you point the hose straight up. How would this height change if

you pointed the hose at an angle?

4. A merry-go-round is fixed to the ground at its pivot point (it is free to rotate about this point).

A child runs with a speed v and leaps onto the merry-go-round (she holds on tight), setting it in

motion. Consider the system of merry-go-round and child together and analyze the Energy, Momentum,

and Angular Momentum of this system before and after the child leaps onto the merry-go-round.

(Which are conserved? Which change?) If any of the above quantities change, account for this change

physically.

5. A large adult giraffe has a neck that is about 2 m long. These animals can move their heads from being

vertically upward, to nearly vertically downward.

(a) Calculate the change in blood pressure in the giraffe's head as it lowers its head from being

vertically upward to the ground. Express this pressure in units of torr = mmHg which are the

usual units for blood pressure.

(b) How does this pressure relate to normal blood pressure ranges for healthy animals (the same as

humans, 70 mmHg-140 mmHg). Comment on the plausibility of your results from (a).

6. At the scene of a highway collision, skid marks stretch out for 90m away from the collision site.

Assuming the deceleration of the vehicle was at least 6.00 m/ s 2 ; estimate the speed of the car before

the incident.

7. A book flies past your dorm room window (which is 1 m tall), taking 0.25 s to pass. Assuming it

was dropped, from what height (above your window) was the book dropped?

8. You fire three shots from your gun (let's label the shots A, B, and C). One straight up (A), one straight

down (B) and one horizontally (C). Assuming that you shoot from the hip (i.e. your hand is in the

same place each time, right by your holster) and you ignore air resistance,

(a) (30%) which of the three bullets (A, B, or C) will have the greatest speed as it reaches the ground

and why?

(b) 10% Now include air resistance, how does your answer change and why?

9. If the speed of a car is increased by 150% (i.e. v' = 2.5v), by what factor will its minimum braking

distance be increased, assuming all else is the same? Ignore the driver's reaction time.

10. You are driving in a large truck when a small car crashes into it. Is is possible that all kinetic

energy is lost immediately after the collision? If not, then explain why it is not possible to lose all

kinetic energy in this collision. If so, describe how it is possible.

11. A child on a sled (total mass = 20 kg) slides (from rest) down a hill, through a snow-bank, and into a

net.

(a) If the hill is 4 m high, calculate the speed of the child before she hits the snow-bank.

(b) If the child stretches the net 0.6 m and it acts like a spring with stiffness constant k = 4000 N/ m, how much mechanical energy was lost in the snow-bank?

(c) If the child drank one can of Sprite soda (155 kcal = 657 kJ), how many times could the

child run back up the hill (carring the sled) if she could somehow channel this energy with 100%

efficiency?

12. A light bowling ball (mass = m) is sitting at rest. A much heavier ball (mass = 20m) is moving

with a speed v and collides headson with the first ball. Assume an elastic collision happens and find

the speeds of each ball after the collison.

13. We claim that momentum and angular momentum are conserved. Yet most moving or rotating

objects eventually slow down and stop. Explain.

14. I have a wrench that is 30 cm long and 3 cm wide, the maximum amount of force I can exert on

the end of the wrench is 400 N. Suppose I cannot turn a certain bolt that is stuck. Two friends offer

to loan me their wrenches to do the job. One is 25 cm long and 5 cm wide, the other is 35 cm long but

only 2.1 cm wide. Which wrench should I use to do the job and how much extra torque will it provide

me?

15. (a) Is it possible to have a system where the net force is zero, but the net torque is not zero? If not,

explain why not. If yes, then give an example.

(b) Is it possible to have a system where the net torque is zero, but the net force is not zero? If not,

explain why not. If yes, then give an example.

(c) Is it possible to have a system where neither the net torque or the net force is zero? If not, explain

why not. If yes, then give an example.