Forces and Energy Review
Forces and Energy test
• 10 multiple choice
• 7 short answer
– Potential and kinetic energy calculations
– Resultant force
– Weight vs mass (w= mg)
– Force calculations (F=ma)
– Forces in Fluids (Pascal’s principle, Archimedes
Principle, Bernoulli’s principle)
Question 1
• A baseball pitcher throws a ball weighing
about 150 g to another player. What is the
kinetic energy of the ball if, at the moment it is
caught, its velocity is 5 m/s?
1
Ek  mv2
2
1
Ek  (0.15kg)(5) 2
2
1
Ek  (0.15kg)(25)
2
Ek  1.875 J
Question 2
• An elevator carries a person weighing 60 kg from the ground
to the eighth floor.
• a) If the eighth floor is 56 m above the ground, what is the
potential energy acquired by the person?
EP  mgh
EP  (60 kg)(9.8 N/kg)(56m)
• b)
EP  32 928 J
Where did the potential energy come from?
• From the work of the elevator
Question 3
• In each of the following situations, does the
force involve pulling or pushing?
• a)the force of a dog on its leash pulling
• b) the force of a person inserting a letter in
an envelope
pushing
• c) the force of a magnet on a paper clip
pulling
• d) the force of a bowstring on an arrow
pushing
Question 4
• The illustration opposite represents the action of
a force.
• a) Is the force applied horizontally or vertically?
• the force is applied at a 25o to the horizontal
• b) Which symbol indicates the direction of the
force?
• the arrowhead
• c) What is the magnitude of the force?
• 20 N
Question 5
• How does the intensity of the Earth’s
gravitational field vary?
• The intensity of the Earth’s gravitational field
decreases as the distance from the centre of
the Earth increases.
Question 6
• What distinguishes mass from weight?
• Mass is a measure of the quantity of matter
in an object, while weight is a measure of the
gravitational force acting on an object.
Question 7
• Plasma or liquid crystal television sets are
much lighter in weight than the old models
with cathode ray screens. What is the weight
of a television set that has a mass of 25 kg at
the Earth’s surface?
w  mg
w  (25kg)(9.8N / kg)
w  245N
Question 8
• On the surface of which planet of the solar
system would you be heaviest? Explain your
answer.
• On Jupiter, because the intensity of the
gravitational field on the surface of this
planet is the greatest of all the planets in the
solar system
Question 9
• A) Will the mass of this explorer vehicle be the
same on Earth and Mars? Explain your answer.
• Yes, the mass of the vehicle will be the same
because the quantity of matter in an object
remains the same regardless of its location
• B) Will the weight of the explorer be the same on
Earth and Mars? Explain your answer.
• No, it will weigh less. Weight depends on the
intensity of the gravitational field and the
gravitational field of Mars is weaker than that
of Earth
Question 10
• A) 10 N
• B) 45 N
Question 11
• Which of the following situations will have the same effect as
a force of 50 N? Explain your answer.
• a) two forces of 25 N applied along the same line of action but
in opposite directions
• b) two forces of 25 N applied along the same line of action
and in the same direction
• c) two forces of 25 N applied along different lines of action
but in the same direction
• Situation b), because two forces applied along the same line
of action and in the same direction are added together. Their
total will therefore be 50 N.
Question 12
• The motor of a boat has a propulsion force of
75 N, while the frictional force of the water
against the hull is 10 N. What is the resultant
force of this system?
• 75 N – 10 N = 65 N
• The resultant force is 65 N.
Question 13
• If you walk at a constant pace (speed), what is
the resultant force of your travel?
• The resultant force is zero because the
motion is not altered.
Question 14
• A) What is a fluid?
• A fluid is a substance with the ability to
spread and take the shape of the container
that holds it.
• B) Name four examples of fluids.
• water, milk, air and helium.
Question 15
• a) How does water pressure act on these objects?
• The pressure is exerted perpendicularly to the
surfaces of the objects.
• b) Where is the pressure greatest? Explain your
answer.
• The pressure is greatest at the lowest points of the
objects in the water because pressure increases with
depth.
• c) If the water is replaced by a liquid of lower density,
how will the pressure change?
• The pressure will be reduced because pressure also
depends on the density of a liquid
Question 16
• Pressure is applied to the plunger of a syringe
containing 30 mL of liquid. The plunger then exerts a
pressure of 30 kPa on the liquid. If the initial pressure
of the liquid was 2 kPa, what will be the pressure of
the liquid expelled at the other end of the syringe?
Explain your answer, specifying the principle of
physics you used to determine the pressure.
• The pressure will be 32 kPa because, according to
Pascal’s principle, an increase in the pressure of an
enclosed fluid is transmitted uniformly in all
directions
Question 17
• What makes a boat float?
• The fact that the buoyant force is equal to or
greater than the gravitational force (the
weight) of the boat.
Question 18
• An object weighing 98 N is immersed in water.
• a)What weight of water must be displaced so
that the object can float? Explain your answer
• The weight of the displaced water must be
equal to or greater than 98 N. For something
to float within the water (not sink to the
bottom) Fb must equal Fg. For an object to
float on top of the water, Fb must be greater
than Fg
Question 18 b
• b) What is the volume of water displaced by
the object, given that the density of water is
1 g/mL?
Fg  mg
Fg  98 N
m?
g  9.8 N / kg
m
Fg
g
98 N
m
9.8 N / kg
m  10Kg or 10 000 g
m  10 000 g
 V  10 000 mL
or 10 L
Question 18 c
• The same object is immersed in a solution with a density of
0.8 g/mL. Given that the object displaces the same volume
of liquid as in b), will it float or not? Show your calculations.
m
D
V
m  DV
F ?
m  8kg
g  9.8 N / kg
m  (0.8g / mL)(10000mL)
m  8000 g or 8 kg
Fg  mg
Fg  (8kg)(9.8N / kg)
Fg  78.4 N
The weight of the object is 98 N
while the weight of the fluid
displaced (=Fb) is 78.4 N. The
object will sink
Question 19
• When the wind blows hard, what happens to
the air pressure? Explain your answer.
• According to Bernoulli’s principle, the higher
the speed of a fluid, the lower its pressure.
When the wind blows hard, the air pressure
drops because the air particles are moving
faster.
Question 20
• a) The hull of a submarine must be very thick,
from top to bottom, to resist the pressure of the
water.
Pascal’s
• b ) To float, a submarine must empty its ballast
tanks.
Archimedes’
• c) Force can be amplified with a hydraulic press.
Pascal’s
• d) The action of the wind makes a flag fly.
Bernoulli’s
• e) Warming the air in a hot-air balloon makes
it rise into the air.
Archimedes’
• f) Birds can fly partly because of the
particular shape of their wings.
Bernoulli’s