Download Science 10 Assignment U2L7 Key

Science 10 Assignment U2L7 (44 marks)
1. Why is potential energy not as obvious as kinetic energy ? (1 mark)
Because it potential energy cannot be easily seen and may remain a static
form of energy for long periods of time.
2. Identify the type of potential energy in the following situations : (5 marks)
a) a rubber ball striking a wall is compressed and deformed at the exact
moment it strikes the wall.
Elastic Potential energy
b) An elevator in an office building slowly rises to the 20th floor and then
stops.
Gravitational Potential energy
c) A bow string is slowly drawn back, bending a fiberglass bow :
Elastic Potential energy
d) An arrow rises vertically into the air after a bow string is released.
Elastic potential becoming gravitational potential energy
e) Natural gas burns in a fireplace.
Chemical Potential energy.
3. State two differences between kinetic energy and gravitational potential
energy. (2 marks)
- Kinetic energy is related to velocity and not position.
- Kinetic energy is not measured relative to any specific position but
rather it is related to a certain frame of reference.
4. A force of 32.0 N is required to lift a box 3.00 m vertically against the force of
Earth’s gravity.
a) Calculate the work done against Earth’s gravitational field. (3 marks)
Fg = 32.0N, d=3.00 m
W=F•d = 32.0N • 3.00 m = 96.0 J
b) Calculate the gravitational potential energy stored in the box. (3 marks)
Fg(mg)= 32.0N, h=3.00 m
Ep=mgh = 32.0N • 3.00 m = 96.0 J
5. An object gains 155 J of gravitational potential energy when it is lifted 1.30 m
above the surface of Earth. Calculate the force exerted on the object. (3
marks)
W= 155J, d= 1.30 m, f=?
W=f•d , F=W/d = (155J)/1.30m = 1.19 x 102 N
6. A person jumping on a trampoline exerts an average force of 500 N in
stretching the trampoline a distance of 0.750 m. Calculate the elastic
potential energy stored in the trampoline. ( 3 marks)
F= 500N, d=0.750 m. Ep= ?
W=F•d = (500N)(0.750m) = 375 J
7. A 60.0 kg person climbs up a ladder to the roof of a building that is 3.50 m
above the surface of Earth. Calculate the gravitational potential energy
stored in the person. (3 marks)
m=60.0 kg, h=3.50m,g=9.81 m/s2 Ep=?
Ep=mgh = (60.0kg)(9.81 m/s2)(3.50m) = 2.06 x 103 J
8. What is mechanical energy ? (1 mark)
Since Me= Ek + Ep and Ek=0 J Me=Ep = 2.06 x 103 J
9. A 60.0 kg athlete jumps vertically upward from the ground to a height of
0.910 m above the ground. What was the athlete’s initial vertical speed ? (3
marks)
m=60.0 kg, h= 0.910 m, g=9.81 m/s2,
Ek=Ep
½ mv2 = mgh
v=(2gh) ½ = (2•9.81 m/s2 • 0.910m)½ = 4.23 m/s
10. A 0.300 kg billiard ball is propelled from a table at a horizontal speed of 1.50
m/s. If the table is 1.30 m above the floor, what is the mechanical energy of
the ball at the instant it leaves the table ? (3 marks)
m=0.300 kg, v=1.50 m/s, h=1.30 m, g=9.81 m/s2
Me = Ek + Ep = ½ mv2 + mgh
= ½ (0.300kg)(1.50m/s)2 + (0.300kg)(9.81m/s2)(1.30m) = 4.16 J
11. An average force of 40.0 N is needed to compress a spring 0.200 m. A 1.00 x
10-2 kg ball is placed on the spring.
a) Calculate the work done compressing the spring. (2 marks)
F=40.0N, d=0.200m W=F•d = (40.0N)(0.200 m) = 8.00 J
b) What happens to the work done on the spring ? (1 mark)
The work done on the spring becomes a type of elastic potential energy.
c) If the spring is released, what happens to the energy of the spring ? (1
mark)
The energy of the spring is converted into kinetic energy when it is
released.
d) Calculate the energy the ball has at the instant that the ball leaves the
spring (3 marks)
F=40.0N, d=0.200m W=F•d = (40.0N)(0.200 m) = 8.00 J
The ball will have the same energy that is released from the spring.
e) What will be the speed of the ball as it leaves the spring ? (3 marks)
Ep=Ek Ep= 8.00 J m=1.00 x 10-2 kg
Ek= ½ mv2, v = (2Ek) ½ = (2•8.00J) ½ = 40 m/s
m 1.00 x 10-2 kg
f) If the ball is fired up into the air by the spring, how much gravitational
potential energy will it gain ? (1 mark)
Esp = Egp = 8.00 J
g) What will be the maximum height that the ball will rise ? (3 marks)
Ep= 8.00 J, m=1.00 x 10-2 kg g= 9.81 m/s2 h= ?
Ep=mgh
h=Ep = ___ 8.00 J
= 81.5 m
-2
mg (1.00 x 10 kg)( 9.81 m/s2)