Adrian Wilson is a strong safety (6’3″ high, 230 pounds) with the Arizona Cardinals. In this YouTube video (http://www.youtube.com/watch?v=7vL19q8yL54) he is seen to jump a 66 inch high gate with only a short start. The reason he accelerates into the air is
A.
He is a professional football player.
B.
His feet push against the floor with a force larger than his 230 lbs weight.
C.
The floor pushes against his feet with a force larger than his 230 lbs weight.
D.
His feet pushes against the floor with a force exactly equal to his 230 lbs weight.
10 points
Question 2
The next several questions involve watching an amazing acrobatic video which is taken from the show “La Nouba” which is presented by Cirque du Soleilat Disney World in Florida.
Download the video from Week 3 – Assignments.
It is well worth watching the whole thing, but for this question please look at the single sequence as the old acrobat falls off the building to the trampoline at 2:06 to 2:10 (two minutes, two seconds to two minutes, six seconds) into the video. What you see is the old acrobat leaping into the air backwards off the building, initially slowly rotating with his arms and legs extended. He quickly pulls his arms and legs into a tuck and begins to rapidly somersault 4 or 5 times before landing on his back on the trampoline. Disregarding for the moment this very complicated motion, what is true about the motion of the center of mass of the acrobat during this fall after he leaves contact with the building?
His center of mass initially accelerates upwards, but decreases as he reaches his maximum height, then rapidly increases as he falls to the trampoline.
His center of mass falls diagonally off the building to the trampoline, his acceleration is in the same direction.
His acceleration is always downwards. His center of mass follows a parabolic trajectory, just like a ball thrown off the top of the building.
His center of mass follows little circles as he falls. His acceleration is always toward the center of the circles to keep him rotating.
10 points
Question 3
Considering again the sequence from 2:06 to 2:10: After leaving the building, the old acrobat initially slowly rotates with his arms and legs extended. He quickly pulls his arms and legs into a tuck and begins to rapidly somersault 4 or 5 times before landing on his back on the trampoline. During this period when his rotation suddenly speeds up, what is true?
He is exerting a large torque with his muscles in the direction of his rotation which accelerates him.
His angular momentum dramatically increases as he exetrs a positive torque.
His angular momentum is a constant. By going into a tuck position he increases his moment of inertia and so his rotational speed increases.
His angular momentum is a constant. By going into a tuck position he decreases his moment of inertia and so his rotational speed increases.
10 points
Question 4
Now watch the sequence 7:50 to 7:56 which shows an acrobat taking a long run down the trampoline, bouncing extremely high into the air several times, but then coming to a full stop on the trampoline. Consider two cases: (A) where the woman falls down from a great height onto the trampoline and rebounds back up to a great height, and (B) where the woman falls down from a great height, and comes to a full stop. For which case is the change in the woman’s momentum the largest? For which case is the Impulse the largest?
The change in the momentum is largest for (A) and the impulse is the largest for (B).
The change in the momentum is largest for (B) and the impulse is the largest for (A).
The change in the momentum and the impulse is the largest for (A).
The change in the momentum and the impulse is the largest for (B).
10 points
Question 5
Watch this extreme slow motion video of a tennis ball hitting a hard floor: http://www.youtube.com/watch?v=0KfD-v23isA&feature=related
From Newton’s third law, the force of the floor on the ball and the force of the ball on the floor are equal in magnitude and in opposite directions. Between the floor and the ball, which received most of the collision energy?
The ball received most of the collision energy and the force of the floor on the ball is in the upward direction.
The ball received most of the collision energy and the force of the floor on teh ball is in the doward direction.
The floor received most of the collision energy and the force of the ball on the floor is downwards.
The floor received most of the collision energy and the force of the ball on the floor is upwards.
10 points
Question 6
Watch this extreme slow motion video of a tennis ball hitting a tennis racket: http://youtu.be/5qjRWLGYncU.
From Newton’s 3rd law the force of the ball on the racket is equal to the force of the racket on the ball. Based on your observations where does the collision energy go?
Only to the ball.
Only to the racket.
Approximately equally distributed between the racket and the ball.
Mostly to the tennis player.
10 points
Question 7
7-man Bungee jumping from the new river gorge bridge in WVa: http://www.youtube.com/watch?v=-cVMUZgZIzg
It is 267 meters (876 ft) from the center of the bridge to the New River below. In this video you first see a basket of 7 men falling from the bridge which is attached to a large bungee cord. In the video the basket very nearly reaches the river, before springing back up. Eventually the basket reaches equilibrium nearly approximately halfway down to the river. If only 4 men fell in the basket, where would the equilibrium position be?
The basket would stabilize at exactly the same place which is determined by the length of the cord.
The basket would stabilize closer to the bridge.
The basket would stabilize closer to the river.
The basket would reach the river.
10 points
Question 8
In 1974 Philippe Petit walked for 40 minutes on a wire between the two World Trade Center towers. The 450 pound cable was stretched 140 ft between the two towers, 1368 ft in the air. You can see pictures of his walk here:
A description of the 6 years of planning is here: http://en.wikipedia.org/wiki/Philippe_Petit
When Philippe Petit was in the middle of the wire the wire would depress by 1 ft. If that day someone twice his weight were to accomplish the same feet the cable would have depressed by
1 ft
2 ft
3 ft
4 ft
10 points
Question 9
When Philippe Petit was in the middle of the wire between the World Trade Center Towers the wire depressed 1 ft. If during his 40 minutes of tricks the wire began to oscillate up and down from this equilibrium position by 6 inches, where is his kinetic energy the greatest?
6 inches above his equilibrium position.
at the equilibrium position.
6 inches below the equilibrium position.
His kinetic energy is a constant at every position.
10 points
Question 10
After travelling all night, you are driving down a narrow country lane. You briefly fall asleep at the wheel, but wake up as the car moves off the shoulder of the road. You wake up but rapidly lose control of the car and it zooms off the road. You are heading towards (1) a stack of bales of hay, or (2) a brick wall. You have seconds to choose, and luckily you choose to hit the hay stack and come to a stop.
Compare in detail the cases of hitting the brick wall versus hitting the hay stack. In which scenario do you transfer more momentum? In which scenario is your impulse larger? In which scenario is the force on the car largest? In which scenario do you experience the larges acceleration? Explain why using physical principles you have learned.