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Physics Test review

  1. Feb 20, 2013 #1
    I have a physics test in a week and am having trouble coming up with the proper equations to solve the problems. Does any body want to offer hint or guidelines to solving these? I have been working on these but I dont think I am approaching the questions appropriately.

    1) A 920-kg sports car collides into the rear of a 2300-kg SUV stopped at a red light. The
    bumpers lock, the brakes are locked, and the two cars skid forward 2.8 m before stopping.
    The police officer, knowing that the coefficient of kinetic friction between tires and road is
    0.80, calculates the speed of the sports car at impact. What was that speed?

    2) A toy car (m = 100 g) is launched horizontally along a track by a spring loaded mechanism
    (k1 = 1000 N/m). The mechanism is “armed” by compressing the spring by and then placing
    the car in front of it. After being launched, the car rolls down an inclined ramp until it
    reaches another level section 0.6 meters below the first. On this section of track it encounters
    another spring (k2 = 2000 N/m). The car compresses this second spring by 6 cm before
    momentarily coming to rest. Assume the whole system is frictionless.
    a) How much was the launching spring compressed by before firing?
    b) If the car had instead been launched up an inclined section of track, to what height would
    it have reached before coming back down?

    3) A 1500-kg car traveling at 90.0 km/h east collides with a 3000-kg car traveling at 60.0 km/h south. The two cars stick together after the collision. What is the velocity (speed and
    direction) of the cars just after the collision (before friction has had time to act)?

    4)A certain rope swing is 4 meters long and will break if it has to support more than 1666 N of
    force. What is the maximum height that an 85 kg person should launch themselves from so
    that the rope doesn’t break as they swing on it? (Hint: The most force will be applied to the
    rope when the swinger is at the bottom of their swing).

    5)Given that the acceleration of gravity at the surface of Mars is 0.38 of what it is on Earth, and that Mars’ radius is 3400 km, determine the mass of Mars.
  2. jcsd
  3. Feb 20, 2013 #2
    1) All of the motional energy of the collided cars can be considered to have been absorbed by work done by friction on their tires. Use the distance traveled and the coefficient of friction to calculate the work done (hint: there's a piece missing here). Then note that all of that energy came from the kinetic energy of the sports car. That allows you to calculate the velocity.

    2) Again, conservation of energy: the energy stored in the second spring was delivered by the toy car. The energy of the toy car came from the release of the first spring, and then the drop down the incline. Two of these you know, so calculate the third.

    3) This one's conservation of momentum. The net momentum of the smashed-together cars is equal to the vector sum of the two cars individually.

    4) A combination of conservation of energy and elementary circular motion. At the bottom of the swing the tension in the rope must pull hard enough to both overcome the swinger's weight and generate the centripetal force necessary to keep the swinger moving in a circle. The centripetal force is related to the velocity at the bottom of the swing; use conservation of energy to calculate that velocity by assuming that all of the swinger's kinetic energy came from their gravitational potential energy at the height they began swinging.

    5) Take a good, long look at Newton's Law of Gravity.

    If you're looking for a general approach, I demonstrated that there really isn't one.
  4. Feb 20, 2013 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    1) What is the k.e. of the sport car just before the collision?
    What is the force of kinetic friction of the system of cars?
    What is the energy dissipated by that force as the two vehicles coast to a stop?
    Combine that data.
  5. Feb 20, 2013 #4


    User Avatar

    Staff: Mentor

    Welcome to the PF.

    You must show your attempt at a solution, before we can be of any tutorial help. This post would normally be deleted, but since you got help, I will leave it posted.

    Please re-read the Rules link at the top of the page, and then in future threads here, fill out the sections on the Relevant Equations and show your Attempt at a Solution.
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