# How do I find the speed?

• max1020

#### max1020

some children go tobogganing on an icy hill. They start from rest at the top of the hill as shown in the diagram. The toboggan and children have a combined mass of 90 kg. If friction is small enough to be ignored, determine:
The speed of the toboggan at B
A is at 10m
B is at 0
C is at 3m
I only know the potential energy for A, the mass, and initial velocity

I only know the potential energy for A, the mass, and initial velocity

You know the potential energy for A relative to where? Relative to B?

If so, then that's enough information.

You know about conservation of energy, right? All of the potential energy of A relative to B will have been converted into kinetic energy (at point B).

I know the total mechanical energy for A which is 8820J

That is the sum of the kinetic and gravitational potential energy at point A?

If so, then that sum will have the same value at point B.

But how do I use that to find speed? Is there a formula for that

Conservation of energy: the total of potential energy and kinetic energy is constant. You know the potential energy at the top where the kinetic energy is 0. At a lower point where the potential energy is lower, the kinetic energy is the difference between the two potential energies. You can find the velocity from the kinetic energy.

But how do I use that to find speed? Is there a formula for that

Do you know a formula that relates the K.E. of an object to its velocity?

Do you know a formula that relates the K.E. of an object to its velocity?

Is it
V=sqrt2k/m

Is it
V=sqrt2k/m

No, not quite right...but you could make it right be adding a pair of parentheses. 