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# Physics Energy question watch

1. A rubber bouncy ball bounces vertically into the air from ground level. You note that when it is 1.5 m above the ground it is travelling at 5 m s−1 upwards. Use conservation of energy to ﬁnd (a) the ball’s speed just as it left the ground and (b) its maximum height, assuming no air resistance.

2. The sum of potential and kinetic energy is always the same. Therefore, E_k + E_p at 1.5 meters = E_k + E_p at 0 meters.

a)
So to find initial velocity, or v at 0 meters
(E= 1/2mv^2+ mgh)
E = 1/2 m(5)^2 + mg(1.5)
E = 12.5m 1.5mg
(Using E_k = 1/2mv^2 to solve for v considering mgh = 0 at 0 meters)

v = 7.4 ms^-1 to 2sf

b)
At max height, the velocity of the ball is 0, therefore the energy is just equal to mgh
(solving for h)
12.5m 1.5mg = mgh
h = 2.8m
3. (Original post by CaptainPegleg)
The sum of potential and kinetic energy is always the same. Therefore, E_k + E_p at 1.5 meters = E_k + E_p at 0 meters.

a)
So to find initial velocity, or v at 0 meters
(E= 1/2mv^2+ mgh)
E = 1/2 m(5)^2 + mg(1.5)
E = 12.5m 1.5mg
(Using E_k = 1/2mv^2 to solve for v considering mgh = 0 at 0 meters)

v = 7.4 ms^-1 to 2sf

b)
At max height, the velocity of the ball is 0, therefore the energy is just equal to mgh
(solving for h)
12.5m 1.5mg = mgh
h = 2.8m
Thank you, very helpful. (btw I'm currently in the 1st year of a natural sciences degree and I haven't done physics A-level so I'm a little bit behind)

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Updated: October 7, 2017
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