Bryan-Science 8
Homework assignment – Answer key
Energy Calculations Worksheet
1. A shot putter heaves a 7.26kg shot with a final velocity of 7.5m/s. What is the
kinetic energy of the shot?
Ek=½ · m · v2
Ek=½ · 7.26kg · (7.5m/s)2
Ek=204.18 J
2. What is the final velocity of a 5700kg truck, if 2 200 000J of energy is used to
accelerate it?
Ek=½ · m · v2
2200000=½ · 5700 kg · v2
2200000=2850 · v2
2200000/2850=v2
v2=771.93
v=27.78m/s
3. A 2kg textbook is lifted from the floor to a shelf 2.10m above the floor.
(a) What is the gravitational potential energy of the book relative to the
floor?
Ep=m · g · h
Ep=2 kg · 9.8m/s2 · 2.1m
Ep=19.6 · 2.1
Ep=41.16 J
(b) What is its gravitational potential energy relative to the head of a
1.65m tall person?
Ep=m · g · h
Ep=2 kg · 9.8m/s2 · 1.65m
Ep=2 · 16.75
Ep=33.5 J
4. A bike rider approaches a hill with a speed if 8.5m/s. The total mass of the
rider and the bike is 85kg.
(a) Find the kinetic energy of the bike and rider.
Ek=½ · m · v2
Ek=½ · 85 kg · 8.52 m/s
Ek=32.5 · (8.5m/s)2
Ek=3070.63 J
(b) The rider coasts up the hill. Assuming there is no friction, at what
height will the bike come to a stop?
3070.63=85 kg · 9.8m/s2 · h
3070.63=833 · h
3070.63/833=h
h=3.69m
5. Tarzan, mass 85kg, swings down from a tree limb on the end of a 20m vine.
His feet touch the ground 4.0m below the limb. How fast is Tarzan moving
when he reaches the ground?
Ep=m · g · h
Ep=85 kg · 9.8m/s2 · 4
Ep=3332 J
3332=½ · m · v2
3332= ½ · 85 kg · v2 m/s
3332=42.5 · v2 m/s
3332/42.5=v2 m/s
v2 =78.4m/s
v=8.85m/s
6. A 98N sack of grain is hoisted to a storage room 50m above the ground floor
of a grain elevator.
(a) How much work was required?
W=Fd
W=98n · 50m
W=4900J
(b) What is the potential energy of the sack of grain at this height?
Ep=m · g · h
Ep=98/9.8 · 9.8m/s2 · 50m
Ep=10 kg · 9.8m/s2 · 50m
Ep=4900J
(c) The rope being used to lift the sack of grain breaks just as the sack
reaches the storage room. What kinetic energy does the sack have
just before it strikes the ground floor?
Because of conservation of energy the maximum potential energy is the same as
maximum kinetic energy. So, Ep=Ek meaning as found in the last equation kinetic
energy is 4900J
(d) What is the speed of the sack just before it hits the ground?
Ek=½ · m · v2
4900= ½ · 98/9.8 · v2
4900=½ · 10 kg · v2
4900=5kg · v2
4900/5=v2
v2=980m/s
v=31.3m/s