Physics

I was wondering if my answers are correct. Thanks in advance.

1. A boy pushes down on a car rolling horizontally down the road. The boy pushes down vertically with 10N of force as the car rolls 3.0m horizontally. What work is done?
my answer: 30 N•m (or would it be -30N•m?)

2. What is the speed of a 0.25kg ball if its kinetic energy is 15 J?
my answer: 11m/s

3. A ball is dropped off a high cliff. What quantity increases as the ball gets closer to the ground?
my answer: I'm not too sure. It's either kinetic energy or gravitational potential energy. I think kinetic energy

4. a 10.0 kg crate slides across the floor 13m against a force of friction of 7.5N. What is the thermal energy produced?
my answer: 98 J

5. In the process of being thrown, a 0.500 kg ball goes fromo rest to a speed of 19.9m/s over a distance of 0.75m. What is the force that was exerted on the ball?
my answer: 19 N

6. A 10.0kg crate initially at rest sitting on the floor with a static and kinetic coefficient of 0.20 is pushed with a force of 15N for 4.0s. What is the work done?
my answer: I'm not sure. I think it's 60 J

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  1. 1. A boy pushes down on a car rolling horizontally down the road. The boy pushes down vertically with 10N of force as the car rolls 3.0m horizontally. What work is done?
    my answer: 30 N•m (or would it be -30N•m?)

    NO, No, No !!
    Work is Force IN THE DIRECTION OF MOTION times the distance

    Answer is ZERO !!!

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  2. 2. What is the speed of a 0.25kg ball if its kinetic energy is 15 J?
    my answer: 11m/s
    (1/2)(.25) * v^2 = 15
    v = Yes, about 11 m/s

    3. A ball is dropped off a high cliff. What quantity increases as the ball gets closer to the ground?
    my answer: I'm not too sure. It's either kinetic energy or gravitational potential energy. I think kinetic energy

    Yes, potential energy goes down, kinetic energy goes up with speed (1/2) m v^2

    4. a 10.0 kg crate slides across the floor 13m against a force of friction of 7.5N. What is the thermal energy produced?
    my answer: 98 J

    Yes, 97.5 Joules

    5. In the process of being thrown, a 0.500 kg ball goes fromo rest to a speed of 19.9m/s over a distance of 0.75m. What is the force that was exerted on the ball?
    my answer: 19 N
    F = change in momentum / change in time
    = .5 (19.9-0)/t
    t = distance / average speed = .75/10 = .075 seconds
    so
    F = .5 * 19 / .075 = 127 N

    6. A 10.0kg crate initially at rest sitting on the floor with a static and kinetic coefficient of 0.20 is pushed with a force of 15N for 4.0s. What is the work done?
    my answer: I'm not sure. I think it's 60 J
    -------
    weight = m g = 98.1 N
    friction force = .2*98.1 = 19.6 N
    total force = 15 -
    LOL - another trick question
    the crate DOES NOT MOVE
    therefore NO WORK DONE

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  3. thanks a lot!

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  4. You are welcome. Nasty questions !

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