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# I would like to be shown how to do problem 5 with all parts shown.

Name: Section: Open Response Homework 18: Motion in a plane Problem 1 You are unloading a refrigerator from a moving van using a rolling dolly. The refrigerator starts 0 . 8m above the ground, and the ramp you use is 5m long. To keep the refrigerator at a manageable speed, you have to push horizontally against it with a constant force of 300N . See the ﬁgure. How much work do you do on the refrigerator during its trip down the ramp? 5m 0.8m F = 300 x v ^ Problem 2 You push a 1kg block against a spring with spring constant k = 100 N m on a horizontal table top. The spring is compressed to 0 . 2m away from its relaxed length, and then the block is released. If the horizontal table top has a coeﬃcient of kinetic friction of 0 . 2 , how much further past the end of the spring does the block go before coming to rest? Problem 3 A child pulls a 2kg red wagon with a constant force by pulling on a handle at an angle of 25 to the horizontal. Unfortunately he has not taken care of his toys and has left it out in the rain so the wheels have rusted into place. Despite this, the child still manages to pull the wagon with a constant speed of 1m / s . Assume the coeﬃcient of kinetic friction between the wheels and the ground is 0 . 4 . (a)Draw a free body diagram for the wagon as it is being pulled, and ﬁnd the magnitude of the force applied by the child on the wagon. (b)What is the work done by the child as he moves the wagon 10m across the ground? (c)If the child were to release the handle, what would be the instantaneous power due to the force exerted on the wagon by the ground? Problem 4 A 5 kg block initially has a speed of 1 . 5 m / s at the top of a 30 incline, and slides a distance of 5 . 0 m down the incline in 1 . 75 s . (a)Find the magnitude of the acceleration of the block. (b)Draw a free-body diagram for the block. You may not neglect friction. Use a coordinate system with the x -axis pointed down the incline, and decompose all vectors into x and y components. Label everything. (c)Find the components of the gravitational force in the given coordinate system. (d)Find the normal force and the force of kinetic friction. (e)Find the coeﬃcient of kinetic friction. (f)What is the ﬁnal speed of the block? Problem 5 You are on the top of a 100m high (approximately 30 story) building. You just ﬁnished building a brick grill so the tenants of the building can have barbecues. You have one brick left and do not want have to lug it back oﬀ the roof. Luckily, you see a garbage truck with its roof open (top of trash pile is 2m above the road) coming down a road that passes by the building. The road is 30m from the building at closest approach. The truck is moving down the road with constant velocity 15m / s . You need to make sure you hit the trashpile, or someone is going to get hurt. You can throw the brick comfortably at an angle of 45 above the horizontal. 1
(a)With what initial speed do you need to throw the brick so that it hits the top of the trash pile when the truck is 30m away from the building? Does this sound like you could do it? (b)How far down the road must the truck be when you throw the brick so that it is at its closest approach when the brick lands in the trash pile in the garbage truck? (c)At what velocity will the brick hit the trash pile? 2

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Subject: Physics, Science

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