Document 8-2 PHYSICS ial and Kinetic Energies Kinetic energy (KE) of an object i

Question

Document 8-2 PHYSICS ial and Kinetic Energies Kinetic energy (KE) of an object is the energy due to its motion KE- Potential energy (PE) of an object is the energy due to its position. PE -mgh UNITS: When mass is in kg, ois in m/s, h is in meters, and g-9.8 m/s, then KE and PE are in joules (J). (1) A 6 kg object moving at 10 m/s has a kinetic energy (KE) of (2) A 1000 kg vehicle moving at 20 m/s (about 45 mph) has a KE of (3) An object moving at 7 mis has a KE of 200 J f its mass is (4) A 50 kg child has a KE of 100 J when she runs at (5) The potential energy of a 4 kg mass held 2.5 m above the ground is (6) An 18 kg object having a PE of 220.5 J must be at a height of (7) An object with a PE of 135 J at a height of 4.5 m must have a mass of (8) The skier starts from rest. What is the skier's speed once at ground level ? m- 80 kg 35 m (9) The skier starts from rest. (a) What is the skier's speed once at ground level (b) What is the skier's speed once at the 8 m level ? N- 50 kg 22 m 8 m

Explanation / Answer

1.

KE = 0.5*m*V^2

KE = 0.5*6*10^2 = 300 J

2.

KE = 0.5*1000*20^2 = 200000 J

3.

KE = 0.5*m*V^2

m = 2*KE/V^2

m = 2*200/7^2 = 8.16 kg

4.

KE = 0.5*m*V^2

V = sqrt (2*KE/m)

V = sqrt (2*100/50) = 2 m/sec

5.

PE = m*g*h

PE = 4*9.8*2.5 = 98 J

6.

PE = mgh

h = PE/mg = 220.5/(18*9.8)

h = 1.25 m

7.

PE = mgh

m = PE/gh

m = 135/(9.8*4.5)

m = 3.06 kg

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