Physics 2.7.4h - A Rock is Droped off a Cliff

Example high school physics problem dealing with falling bodies and the acceleration of gravity.

The problem: In order to determine the height of a cliff, you drop a rock from the top. You see it splash in the water 2.4 seconds after it is released. How high above the water is the top of the cliff?

From the Physics course by Derek Owens. The distance learning class is available at http://www.derekowens.com

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[ballsoffury4]

C)No forces are doing work on the object.

D)There are no vertical forces acting on the object.

Now if it is at a constant speed the net forces must equal zero and there is no acceleration. First off how does it move if something is pushing and pulling on it with the same force? Second, my book says there is work being done by pushing and going against (friction), how is there work being done if the net force is zero? Thanks and blessings :)

[derekowens]

@ballsoffury4 It can't be A. For friction to be negligible, the coefficient would have to be close to zero.

The answer is B. To keep it moving, something must be acting to overcome the friction. Otherwise it would stop.

It can't be C. The force pushing it, and also friction, are doing work.

And it can't be D. Gravity and the floor holding it up are both veritcal.

[derekowens]

@ballsoffury4 If something is pushing and pulling on it with the same force, then the net force is zero. Note that this means that the acceleration must be zero, but this does not mean that the *velocity* is zero.

When we talk about the work done on an object, we generally refer to the work being done by a particular force, not by the net force.

Hope that helps,

DO

[ballsoffury4]

@derekowens

Thank you Derek, I appreciate it

[ballsoffury4]

Hi Derek I'm very annoyed right now I seemed to understand Physics very well when I took the course but am having a little trouble as I am studying for the Mcat. Anyways I have this question

An object with a mass of 50 kg moves across a level surface with a constant speed of 15m/s. Kinetic friction=.7 which of the following is true.

A)The force exerted on the object by kinetic friction is negligible.

B) There must be some other horizontal force acting on the object.

[Zadose18]

Thanks for this.