Download Key Energy Worksheet Day 2

Energy Worksheet Day # 2
Name ______________________________
1. Complete the table.
Force (N)
Distance (m)
Time (s)
Work (J)
Power (watt)
50
2
5
100
100 ÷ 5 = 20
100
2
5
200
40
100
4
10
400
40
50
4
5
200
40
500
10
1
5000
5000
960
1.5
6
1440
240
500
8
10
4000
400
2. For each of the following, state whether the system contains primarily kinetic energy or
potential energy:
Situation
Picture
Energy
Stone in a stretched
slingshot
Potential
energy
A speeding race car
Kinetic energy
Water above a
hydroelectric dam
Potential
energy
3. A moving object has _kinetic erngy__ associated with its motion.
The mallet that the person is swinging has __kinetic energy__. When the
mallet hits the lever, the mallet’s energy is _transfered__ to the puck;
the puck rises to strike the bell, and the bell _kinetic_ energy into
the air as sound. _work__ has been done. Whenever work is done,
energy is _converted__ or is transferred. One definition of energy is “the
ability to do _work_.” Both energy and work are measured in Joules (J).
4. A science student holds a 55 g egg out a window. Just before the student
releases the egg, the egg has 8.0 J of gravitational potential energy with
respect to the ground. How high (height) is the student’s arm above the
ground, in meters? [Hint: Convert the mass to kilograms (divide by
1000g/kg) before solving.]
Mass = 55 ÷ 1000 = 0.055 kilograms {cannot use grams as the SI unit of mass is kilogram)
Potential energy = mass × gravity × height
8.0 = 0.055 × 9.8 × h
8 = 0.539 × h
h = 8 ÷ 0.539 = 14.84 meters
5. What is the work done in lifting 30 kg of blocks to a height of 10 m?
Work done = mass × gravity × height
= 30 × 9.8 × 10 = 2940 joule
6. A frog with a mass of 0.23 kg hops up in the air. At the highest point in the hop,
the frog has a gravitational potential energy of 0.744 J. How high can it hop?
Potential energy = mass × gravity × height
0.744 = 0.23 × 9.8 × h
0.744 = 2.254 h
h = 0.744 ÷ 2.254 = 0.33 meter
7. A 70.0kg man is walking at a speed of 1.5m/s. What is his kinetic energy?
Kinetic energy = ½ × mass × velocity2
= 0.5 × 70 × 1.52 = 35 × 2.25 = 78.75 joule
8. A cart moves with a kinetic energy of 140J, if its velocity is 3 m/s, find its mass?
Kinetic energy = ½ × mass × velocity2
140 = 0.5 × m × 32
140 = 4.5 m
m = 140 ÷ 4.5
m = 31.11 kilograms
Level 3
9. A cart moves with a kinetic energy of 140 J, if its mass is 17 kg, find its velocity?
Kinetic energy = ½ × mass × velocity2
140 = 0.5 × 17 × v2
140 = 4.5 v2
v2 = 140 ÷ 4.5
v2 = 31.11
v = √31.11 = 5.57 m/s
10. A cart moves with a kinetic energy of 60 J, if its speed is doubled, what will its kinetic
energy be?
Kinetic energy = ½ × mass × velocity2
Here, kinetic energy and velocity2 are directly proportional
Kinetic energy α velocity2
KE= 22 = 4 times
KE = 4 times 60 = 240 joule
11. What amount of work can a 500 Watts motor do in 1.5 minutes?
Work = power × time = 500 × 90 {1.5 minutes = 90 seconds}
= 45,000 watt
12. At what height does a 300-kg mass have potential energy of 980 joule relative to ground?
Potential energy = mass × gravity × height
980 = 300 × 9.8 × h
980 = 2940 h
h = 980 ÷ 2940 = 0.33 meter
13. An alkaline AA battery stores approximately 12,000 J of energy. A small
flashlight uses two AA batteries and will produce light for 2 hours. What is
the power of the flashlight bulb? Assume all of the energy in the batteries
is used.
Power = work ÷ time = 12,000 ÷ 7200
{2 hours = 2 × 60 × 60 = 7200 seconds}
Power = 1.66 watt
14. What is the gravitational potential energy of a 61.2 kg person standing on the roof of a 10storey building relative to (a) the tenth floor, (b) the sixth floor, (c) the first floor? (Each
storey is 2.50 m high.)
(a) Gravitational Potential Energy = mass × gravity × height
= 61.2 × 9.8 × (10 × 2.50)
= 14994 joule
15. A 45.0kg cheetah has a kinetic energy of 18,000J. How fast is the cheetah running?
Kinetic energy = ½ × mass × velocity2
18,000 = 0.5 × 45 × v2
18,000 = 22.5 × v2
v2 = 18,000 ÷ 22.5
v2 = 800
v = √800 = 28.28 m/s
Level 4
16. A coconut falls out of a tree 12.0 m above the ground and hits a bystander 3.00 m tall on
the top of the head. It bounces back up 1.50 m before falling to the ground. If the mass
of the coconut is 2.00 kg, calculate the potential energy of the coconut relative to the
ground at each of the following sites:
(a) while it is still in the tree,
(b) when it hits the bystander on the head,
(c) when it bounces up to its maximum height,
(d) when it lands on the ground,
(e) when it rolls into a groundhog hole, and falls 2.50 m to the bottom of the hole.
(a) while it is still in the tree,
Gravitational Potential Energy = mass × gravity × height
= 2 × 9.8 × 12
= 235.2 joule
b) when it hits the bystander on the head,
Gravitational Potential Energy = mass × gravity × height
= 2 × 9.8 × 9 {12 – 3}
= 176.4 joule
(c) when it bounces up to its maximum height,
Gravitational Potential Energy = mass × gravity × height
= 2 × 9.8 × 7.5
{12 – (3 + 1.5 ) = 7.5}
= 147 joule
(d) when it lands on the ground,
Gravitational Potential Energy = mass × gravity × height
= 2 × 9.8 × 0
= 0 joule
(e) when it rolls into a groundhog hole, and falls 2.50 m to the bottom of the hole.
Gravitational Potential Energy = mass × gravity × height
= 2 × 9.8 × (-2.5) {negative because below the zero level}
= - 49 joule
17. At the moment when a shotputter releases a 5.00 kg shot, the shot is 3.00 m above the
ground and travelling at 15.0 m/s. It reaches a maximum height of 8.00 m above the
ground and then falls to the ground. If air resistance is negligible,
(a) What was the potential energy of the shot as it left the hand relative to the
ground?
(b) What was the kinetic energy of the shot as it left the hand?
(c) What was the total energy of the shot as it left the hand?
(d) What was the total energy of the shot as it reached its maximum height?
(e) What was the potential energy of the shot at its maximum height?
(f) What was the kinetic energy of the shot at its maximum height?
(g) What was the kinetic energy of the shot just as it struck the ground?
(a) What was the potential energy of the shot as it left the hand relative to the ground?
Potential Energy = mass × gravity × height
= 5 × 9.8 × 3
= 147 joule
(b) What was the kinetic energy of the shot as it left the hand?
Kinetic energy = ½ × mass × velocity2
= 0.5 × 5 × 152 = 2.5 × 225 = 562.5 joule
(c) What was the total energy of the shot as it left the hand?
Total Energy = Potential energy + kinetic energy
= 147 + 562.5 = 709.5 joule
(d) What was the total energy of the shot as it reached its maximum height?
Total Energy = 709.5 joule {Total energy will be same at every height}
(e) What was the potential energy of the shot at its maximum height?
Potential Energy = mass × gravity × height
= 5 × 9.8 × 8
= 392 joule
(f) What was the kinetic energy of the shot at its maximum height?
Kinetic energy = ½ × mass × velocity2
= 0.5 × 5 × 02 = 0 joule { no velocity at the top of its height}
(g) What was the kinetic energy of the shot just as it struck the ground?
Total Energy at ground = Potential energy + kinetic energy
709.5
= 0 + Kinetic Energy
Kinetic Energy = 709.5 joule