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Laws of Motion Review
Vocabulary
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Acceleration
Equilibrium
Force
Inertia
Kinetic Friction
Mass
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Motion
Net Force
Newton
Static Friction
• Terminal Velocity
• Vector
Types of Forces
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Pushing a crate across the floor –
Applied
Air Friction –
Drag
Opposes motion –
Friction
Support force –
Normal
Created in a rope, etc when connected to another mass
Tension
Force due to gravity –
Weight
Free Body Diagrams
• What is the net force if
the applied is 15N and
friction is 5N?
• Applied is 22N and
friction is 6N?
• What is the acceleration
for each if the weight is
10N?
1st Law of Motion
• Law of Inertia
• An object at rest will stay at rest, an object
in motion will stay in motion unless acted
upon by a net force greater than zero
• Ex. A car rider continues forward when the
driver suddenly applies the brakes
2nd Law of Motion
• ∑F = ma
W = mg
Ff = µFn
• What is the net force?
• The sum of the forces
• Remember: Mass must be in kilograms
2nd Law Practice
• A 20 N block is being pushed across a
horizontal table by an 18 N force. If the
coefficient of kinetic friction between the
block and the table is 0.4, find the
acceleration of the block.
A. 0.5 m/s2
B. 1 m/s2
C. 5 m/s2
D. 7.5 m/s2
E. 9 m/s2
2nd Law Practice
• If all of the forces acting on an object
balance so that the net force is zero, then
A. the object must be at rest
B. the object’s speed will decrease
C. the object will follow a parabolic path
D. the object’s direction of motion can change,
but not its speed
E. none of the above
2nd Law Practice
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A block of mass m is at rest on a frictionless horizontal table placed
on a laboratory on the surface of the Earth. An identical block is at
rest on a frictionless horizontal table placed on the surface of the
Moon. Let F be the net force necessary to give the Earth-bound
block an acceleration of a across the table. Given the gmoon is onesixth of the gEarth the force necessary to give the Moon-bound block
the same acceleration a across the table is
A. F/12
B. F/6
C. F/3
D. F
E. 6F
2nd Law Practice
• A cart of mass 10 kg moves on a frictionless
table surface. A 10 N force pulls on the cart
horizontally from the right. At the same time, a
30 N force acts at an angle of 30 degrees to the
horizontal to the left. What is the magnitude of
the horizontal acceleration of the cart?
A. 0.5 m/s2
B. 1.6 m/s2
C. 2.0 m/s2
D. 2.5 m/s2
E. 2.6 m/s2
2nd Law Practice
• What is the vertical component of a
resultant F acting at an angle?
A. 1/2F
B. F
C. F cosӨ
D. F sinӨ
E. F tanӨ
2nd Law Practice
• Two astronauts tug on opposite sides of a satellite. The
first astronaut tugs to the left with a force of 30 N. With
what force does the second astronaut tug in order to
keep the satellite moving toward him at a constant
velocity of 20 m/s?
A. 30 N
B. – 30 N
C. 0 N
D. 50 N
E. 1.5 N
2nd Law Practice
• Two people, one of mass 100 kg and the other of mass
50 kg, stand facing each other on an ice-covered
(essentially frictionless) pond. If the heavier person
pushes on the lighter one with a force of F, then
A. the force felt by the more massive person is – 1/2F
B. the force felt by the more massive person is -2F
C. the magnitude of the acceleration of the less massive
person will be ½ of the magnitude of the acceleration
of the more massive person
D. the magnitude of the acceleration of the less massive
person will be twice the magnitude of the acceleration
of the heavier person
E. None of the above
2nd Law Practice
• The maximum (or terminal) velocity of a
skydiver (m=70kg) with arms and legs
extended is about 120 mi/h (= 53.6 m/s).
What force does air resistance exert on
the skydiver?
A. 0 N
B. 13 N
C. 70 N
D. 700 N
E. 1000 N
2nd Law Practice
• An airplane has a mass of 3.1 x 104 kg and
takes off under the influence of a constant
net force of 3.7 x 104 N. What is the net
force that acts on the plane’s 92 kg pilot?
2nd Law Practice
• Two forces act on a 6.00 kg object. The
first force has a magnitude of 25 N and is
directed west. The second force has a
magnitude of 72 N at an angle of 47
degrees south of west. What is the
horizontal acceleration (magnitude and
direction) of the block?
2nd Law Practice
• Weight and mass are
A. always the same
B. inversely proportional
C. based on location
D. none of the above
2nd Law Practice
• A helicopter is lifting a 50 kg flood victim out from a
rushing stream of water. How much tension is there in
the rescue line pulling the victim up if the victim is
accelerating upwards at a rate of 12 m/s2?
• a.1300 N
• b.1250 N
• c.1600 N
• d.1100 N
2nd Law Practice
• A large (150 kg) refrigerator needs to be moved
across the kitchen. Fred pulls with a force of
100 N and Wilma pulls with 75 N in the same
direction. If the force of friction was 40 N, what
will be the acceleration of the refrigerator?
• a.1.17 m/s2
• b..90 m/s2
• c..43 m/s2
• d.135 m/s2
2nd Law Practice
• A box of mass 10kg slides down a frictionless
inclined plane that forms a 40º to the horizontal.
• A. Find the magnitude of the parallel force
• B. Find the magnitude of the perpendicular force
• C. What is the acceleration of the block?
• D. How do these values change if a 7N force of
friction is present?
3rd Law of Motion
• For every action, there is an equal and
opposite reaction
3rd Law Practice
• A mass is suspended on a spring. The
reaction force to the force of the gravity
from the Earth acting on the mass is the
force exerted by the
A. mass on the Earth
B. mass on the spring
C. spring on the mass
D. spring on the Earth
E. Earth on the mass
3rd law Practice
• A bucket is being supported by 2 ropes,
each at an angle of 35o with the vertical.
What is the tension force on each rope?