Physics Help Forum cheack my answaers thanks !

 Jan 4th 2013, 03:56 AM #1 Junior Member   Join Date: Dec 2012 Posts: 7 cheack my answaers thanks ! (1) A person can survive a feet first impact at a speed of about 1ms on concrete 15ms on soil and 34 on water. Why the different values for different surfaces? The more the velocity the more the force of impact. The harder the surface the less the velocity, therefore the force would be less. (2) Automobiles in the past times were manufactured to be as rigid as possible, whereas modern autos are designed to crumple upon impact why ? Modern auto’s are done like this to increase the time of impact. Which decreases the force of impact. (3)Bronco dives from a hovering helicopter and finds his momentum increasing, does this violate the conservation of momentum? Explain. Yes as an external force is acting on him hence his momentum is increasing his momentum . Momentum is only conserved when there is no external force acting on the system. (4)if a mack truck and a Mini Cooper have a head on collision which vehicle will experience the greater force of impact ? the greater impulse ? the greater change in momentum ? the greater deaccelaeration ? The one with the lower mass will experience the greater deacceleration . momentum is conserved . (not sure ) (5) Superman meets an asteroid in outer space and hurls it at 800 ms as fast as bullet. The asteroid is a thousand times more massiv than super man. In the comic superman is seen at rest after the throw. Taking physics into account what would be his recoil velocity ? 800 m/s (6) two people who weigh th same climb a flight of stairs. The first person climbs the stairs in 30s, and the second person climbs them in 40s. Which person does more wor? Which uses more power ? Work= force x distance The person with the 40 sec time uses less power, therefore has done less work. The person with 30 sec has more power more work. (7) Belly- flop Bernie dives from atop a tall flagpole into a swimming pool below. His potential energy at the top is 10000J ( relative to the surface of the pool). What is his kinetic energy when his potential energy reduces to 1000 j? 9000 J (8)in the hydraulic machine, it is observed that when the small piston is pushed down 10 cm, the large piston is raised 1cm. If the small piston is pushed down with a force of 100 N, what is the most weight that the large piston can support ? Work = force x distance 100x 0.10= 10 J ( not sure ) can some1 cheack my answears and give the right answears for the wrong answear ? thanks !
Jan 4th 2013, 07:15 AM   #2
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 Originally Posted by Tyson (1) A person can survive a feet first impact at a speed of about 1ms on concrete 15ms on soil and 34 on water. Why the different values for different surfaces? The more the velocity the more the force of impact. The harder the surface the less the velocity, therefore the force would be less.
Not right. Think about the cushioning distance of hitting concrete versus dirt versus water. When you hit concrete you decelerate to zero velocity almost instantaneously, whereas when you hit water you body penetrates the water some number of meters before coming to a complete stop. What does this imply about the force you feel?

 Originally Posted by Tyson (2) Automobiles in the past times were manufactured to be as rigid as possible, whereas modern autos are designed to crumple upon impact why ? Modern auto’s are done like this to increase the time of impact. Which decreases the force of impact.
Good.

 Originally Posted by Tyson (3)Bronco dives from a hovering helicopter and finds his momentum increasing, does this violate the conservation of momentum? Explain. Yes as an external force is acting on him hence his momentum is increasing his momentum . Momentum is only conserved when there is no external force acting on the system.
Correct, except rather than saying "yes" I think you mean "No - conservation of momentun is not violated."

 Originally Posted by Tyson (4)if a mack truck and a Mini Cooper have a head on collision which vehicle will experience the greater force of impact ? the greater impulse ? the greater change in momentum ? the greater deaccelaeration ? The one with the lower mass will experience the greater deacceleration . momentum is conserved . (not sure )
Force of impact is the same - Newton's law about equal and opposite forces.
For impulse: consider that impulse = F delta T.
Change in momentum: Consider F delta t = m delta v.
Greater deceleration: you have that correct.

 Originally Posted by Tyson (5) Superman meets an asteroid in outer space and hurls it at 800 ms as fast as bullet. The asteroid is a thousand times more massiv than super man. In the comic superman is seen at rest after the throw. Taking physics into account what would be his recoil velocity ? 800 m/s
No. Consider conservation of momentum, so the product of m delta v must be the same for both the asteroid and Superman.

 Originally Posted by Tyson (6) two people who weigh th same climb a flight of stairs. The first person climbs the stairs in 30s, and the second person climbs them in 40s. Which person does more wor? Which uses more power ? Work= force x distance The person with the 40 sec time uses less power, therefore has done less work. The person with 30 sec has more power more work.
No. Both do the same amount of work - consider Work = force times distance, and in this case the force is the force of gravity (ie their weight) and the distance is the vertical height of the stairs. Both of these are equal for both people, so both do the same amount of work. Power is work per unnit time, or stated another way: force times velocity. We know that both are experiencing the same force, but their velocities are different, so the power output is different.

 Originally Posted by Tyson (7) Belly- flop Bernie dives from atop a tall flagpole into a swimming pool below. His potential energy at the top is 10000J ( relative to the surface of the pool). What is his kinetic energy when his potential energy reduces to 1000 j? 9000 J
Good.

 Originally Posted by Tyson (8)in the hydraulic machine, it is observed that when the small piston is pushed down 10 cm, the large piston is raised 1cm. If the small piston is pushed down with a force of 100 N, what is the most weight that the large piston can support ? Work = force x distance 100x 0.10= 10 J ( not sure )
Not quite. If work is the same for both, then F1 x D1 = F2 x D2. Yuo have values for F1 and the ratio of D1/D2, so solve for F2.

 Jan 4th 2013, 02:39 PM #3 Junior Member   Join Date: Dec 2012 Posts: 7 hey thanks man !

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