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Homework Help: Need help on some conceptual physics problems

  1. Dec 14, 2008 #1
    Need help in understanding the physics behind these physical moments.

    Can someone please help me how in terms of physics the following things happen?

    1) Why does a rocket go up? (what I think is that force emmitted from the flames released from the rocket creates a force that heats the ground. Then based on newton's third law. an equal but opposite force he created on ground that acts on the rocket forcing the rocket to go up. That all I can think of.)

    2) In newtons's cradle what happens when a person lifts one marble and let it go? (I mean I understand that energy from the marble that is lifted gets transferred to all the other marbles inbetween cause the, marble at the other end to rise. But is this a good enough explanation?)

    3) Two hockey pucks are on ice. One is mass M and the other is mass m. M > m. Same force is applied to each. Same distance, A, is traveled. As m reaches A, is its speed larger or less than M. (I think as m reaches A it has a bigger velocity because since it has a lesser mass than M it can go faster because it has less I guess inertia preventing it from slowing down?)

    4) Two cars collide. What can you say about the forces each feels if one car is much heavier than the other. The speed of each car is different. (I think the heavier car feels a lesser force than the lighter car, can really think of a physical reason why though.)

    5) Why do astronauts experience weightlessness? Is there really no gravity in space?
    (I know that there is gravity in space but it is a lot less compared to the earth beause od distance. Since the force of gravity is so much less in space, people feel weightless.)

    I need to explain these concepts in atleast two sentences. What I have thought of so far is in the parenthesis. Can someone please help me out. I would greatly appreciate it...
  2. jcsd
  3. Dec 14, 2008 #2
    1) Newtons third law is a good start, but think about the expansion of the exploding gas also.

    2)Energy, Momentum both should be mentioned. Is it elastic? or Inelastic?

    3)Fdt=impulse so if you can use calculus integral of ma from time t initial to t final =impulse, both receive the same force so figure out which leaves faster, you can also use newtons 2nd law to logic through this is calculus is not allowed.

    4) That question seems vague. If M is the more massy car and m is less massy then when M >> m M feels barely any force m feels a large force, again thing impulses and conservation of momentum.

    5) Think centripetal acceleration, what could we call orbiting the Earth? Think about those planes that go up and give you the "weightless" experience, they aren't in space, think about why you are weightless on them, or why parachutists feel weightless.
  4. Jan 31, 2009 #3
    1.) Well the force pushes in the opposite direction of the rocket, which causes an equal and opposite reaction, causing the same amount of force to push the rocket in the opposite direction of the force. Newtons third law: For every action there is an equal and opposite reaction.

    2.) Newton's cradle is a demonstration of the conservation of momentum, and energy. The momentum from the falling marble is transfered throughout the other marbles and in an "ideal world" without energy being lost due to friction, heat, sound or deformation, this would be a never ending process.

    3.) Force = Mass x Acceleration. If the mass is smaller with the same applied force, the acceleration will be larger, therefore puck m would have a greater speed than puck M.

    4.) The forces are the same. The formula for force is F=ma. The heavier car has more mass, but less acceleration. The lighter car has less mass, but more acceleration. Newton's third law of motion: For every action there is an equal and opposite reaction.

    5.) Actual "weightlessness" does not exist. The sensation of being "weighless" is achieved by falling at the same rate as the environment around you. For example, Mthees08 mentioned the planes that go upward and give you the "weightless" feeling. The planes that do that actually travel up and the go into freefall for a certain amount of time. While the plane is in freefall, everything inside it is also. This provides the illusion of being weightless. Astronauts in space will always have weight, because gravity is always present. Now this weight might change due to the position of the astronaut in space, but it will still be present. You mentioned that gravity is less in space due to distance, this may be true in some cases. But, in other cases such as on Jupiter, the gravitational force is much larger, and you would weigh more on Jupiter than on Earth. Okay let me get to the question and stop rambling on though. ;) Astronauts feel "weightless" because they are traveling at the same speed as the vessel that they are in. If there was a difference in the speed of the rocket and the astronaut inside, then the astronaut would be pressed against the wall and probably crushed, depending on the difference of speeds.
    Last edited: Jan 31, 2009
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