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3 Questions that have wracked my brain for two weeks

by frdave20
Tags: brain, solved, weeks, wracked
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frdave20
#1
Sep17-03, 09:50 AM
P: n/a
If anyone can help this physics novice out, I have been working on these problems for 2 weeks. I just started looking at an old Physics book, trying to brush up.... so if you can help, here are the 4 questions:

1.) When you life a bowling ball with a force of 82 N, the ball accelerates upward with an acceleration, a. If you life with a force of 95 N, the balls acceleration is 4 x a. What is the weight of the ball? What is the acceleration, a?

2.) A 23kg suitcase is being pulled by a handle that is at an angle of 25 degrees above the horizontal. If the Normal force exerted on suitcase is 150 N, what is the force F applied to the handle?

3.) A shopper pushes a 8.9 kg shopping cart up a 13 degree incline. Find the horizontal force, F, needed to give the cart an acceleration of 1.35 m/s^2.

4.)On vacation, your 1300 kg car pulls a 540 kg trailer away from a stop light with an acceleration of 1.90 m/s^2. What is the net force exerted by the car on the trailer? what is the net force acting on the car?

I didn't realize that it would be this hard... I dont remember having this difficulty in Physics when I was younger...

If you can help, please leave a little blurb for an explanation....

thanx
frdave20
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HallsofIvy
#2
Sep17-03, 01:57 PM
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PF Gold
P: 39,502
A "little blurb"? I don't know- I'm not good at "blurbing"!

1.) When you life a bowling ball with a force of 82 N, the ball accelerates upward with an acceleration, a. If you life with a force of 95 N, the balls acceleration is 4 x a. What is the weight of the ball? What is the acceleration, a?

You would have to support the bowling ball with a force equal to it's weight in order just to keep it from accelerating downward. Any force more than than causes the acceleration. Letting w be the weight of the ball and F the force upward, m the mass, and a the acceleration, F- w= ma. In the first case, F= 82 N so
82- w= ma. In the second, F= 95 and the acceleration is 4 times as great: 95- w= m(4a)= 4ma. You can treat ma as a single variable and solve the two equations for w.

2.) A 23kg suitcase is being pulled by a handle that is at an angle of 25 degrees above the horizontal. If the Normal force exerted on suitcase is 150 N, what is the force F applied to the handle?

Here, oddly, you don't need to know the mass of the suitcase. I take it that, by "Normal force", you mean vertical: normal to the ground. Break the force F into horizontal and vertical components:
horizontal is F cos[theta] and the vertical is F sin[theta]. Since you are told the vertical force is 150 N, you can find F from that.

3.) A shopper pushes a 8.9 kg shopping cart up a 13 degree incline. Find the horizontal force, F, needed to give the cart an acceleration of 1.35 m/s^2.

The acceleration, up the slope is 1.35 m/s^2. F= ma so there must be a NET force, up the slope of (8.9)(1.35) Newtons. Again, you need to separate the force into horizontal and vertical components. Once you've done that, you can determine the horizontal force.

4.)On vacation, your 1300 kg car pulls a 540 kg trailer away from a stop light with an acceleration of 1.90 m/s^2. What is the net force exerted by the car on the trailer? what is the net force acting on the car?

Again, force= mass times acceleration. Just calculate the force necessary to accelerate 540 kg (the trailer) at 1.9 m/s^2. That's the force the car needs to exert on the trailer. You could find the force necessary to accelerate the whole assembly (car and trailer) and then subtract the force the car exerts on the trailer to find the NET force, but there is no reason to add and then subtract: just find the force necessary to accelerate 1300 kg at 1.9 m/s^2.


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