Download Chapter 3 Newton`s First Law of Motion

33.
Many auto passengers suffer neck injuries when
struck by cars from behind. How does NL of I apply? Why
headrests?
 The body is accelerated forward with the
seat, but the head remains (behind) at its
current velocity until an unbalanced force
pulls it forward (which is the neck, which can
cause whiplash if forceful enough).
 Headrests provide the unbalanced force
needed to accelerate the head with the body.
31.
Consider two rotating bicycle wheels, one filled with
air and the other with water. Which would be more difficult
to stop rotating? Explain.
 The bicycle wheel filled with water would be
the more difficult wheel to stop rotating
because it has the greater amount of
rotational inertia or resistance to change its
rotational motion.
HW #9
Chapter 3
Newton’s First Law of
Motion
Pages 42-44 (21, 22, 23, 26, 27, 31, 3537, 43, 46-49)
21
 21.
 a. Greatest to least resistance to change in
motion BACD
 b. Greatest to least weight BACD
 c. greatest to least normal support BACD
22
 22.
 a. Greatest to least force required to keep
them going A=B=C (no force)
 b. Greatest to least force required to stop
them in the same time interval CBA
23. W =mg
w = 50 kg x 9.8 m/s2
weight = 490 N or 500 N
26
 1 N = ? lb
 1 N x 1 lb/4.45 N = 0.2 lb
27
 w = mg
 300 N = m x 9.8 m/s2
 300 N / 9.8 m/s2 = 30.6 kg or 31 kg (rounded)
31.
 NO FORCE IS REQUIRED TO KEEP AN
OBJECT MOVING IF NO OTHER FORCES
ARE ACTING ON IT. INERTIA
35.
Suppose you place a ball in the middle of a wagon that is at
rest and then abruptly pull the wagon forward. Describe the motion of
the ball relative to the ground.
 Except for some change in motion due to
friction between the wagon and ball, the will
be no motion of the ball relative to the
ground; but relative to the wagon, the ball will
appear to move toward the back.
36. To pull a wagon across a lawn at a constant velocity, you
have to exert a steady force. Does this contradict Newton’s
first law?
 No. The key is net force. Your steady force
is balancing the rolling friction, which means
that the net force = 0, which means dynamic
equilibrium, which means constant velocity,
which means no acceleration. Removing
friction would allow the wagon to continue
moving at the same velocity without any more
pull force.
37.
When a junked car is crushed into a compact cube,
does its mass change? Its volume? Its weight?
 Mass remains unchanged.
 Volume is reduced.
 Weight remains unchanged (no change in
position within the gravitational field so no
change in gravitational force)
43.
The head of a hammer is loose and you wish to tighten it by
banging it against the top of a workbench. Why is it better to hold the
hammer with the handle down as shown rather than with the head
down?
 The handle stops when it hits the bench, but
the relatively massive head tends to keep
moving towards the handle and tightens.
46-49
 46. Normal Force (equal to and opposite of
weight)
 47. Weight and tension forces in equilibrium
 48. Weight and tension, with tension force
greater than weight. Upward acceleration
 49. Weight only with a slight amount of air
resistance.
Chapter 6.1-6.4
Newton’s 2nd law of
Motion
Pages 99-102 (1, 3-6, 8, 11, 22, 24, 25,
34, 37, 40)
 1. What causes acceleration?
 Unbalanced forces.
 3. Is acceleration directly proportional to mass, or is
it inversely proportional to mass?
 Acceleration is inversely proportional to mass.
 4. If two quantities are inversely proportional to each
other, does that mean as one increase the other
increases also?
 No, it means the opposite (as one increases, the
other decreases)
 5. If the net force acting on a sliding block is tripled,
what happens to the acceleration?
 The acceleration also triples. It’s a direct
relationship!
 6. If the mass of a sliding block is tripled at the same
time the net force on it is tripled, how does the
resulting acceleration compare with the original
acceleration?
 The resulting and original acceleration do not
change. They are the same. 10N/1kg = 10 m/s2 and
so does 30 N/3kg.
 8. Suppose you exert a push on a crate that
rests on a level floor, and it doesn’t move.
How much friction acts compared to with your
push?
 The same magnitude as your push, but
oppositely directed.
 11. When do you produce more pressure on
the ground, standing or lying down?
 Standing. Same force, less area of
application, more pressure!
 22.
 A) D, A=B=C
 B) A=C, B=D
 24. C,D,B, A
 25. Fnet = ma, so a = Fnet/m; 200 N/40 kg = 5 m/s2
 34. A car moves forward as the brakes are applied.
During braking, the velocity and acceleration of the
car are in opposite directions. Agree or disagree?
 Agree!
 37. When you lift the lineman, the force of friction is
reduced, making it easier to move the lineman.
40.
A common saying goes” It’s not the fall the hurts
you; it’s the sudden stop.” Translate this into Newton’s
laws of motion.
 By a = F/m
 A sudden stop means a very large
acceleration (change in motion!). Seeing that
the mass stays constant, the force, being
directly proportional to acceleration, would
also increase greatly.
Chapter 7.1-7.4
Newton’s 3nd law of
Motion
Pages 118-122 (1, 3, 7, 8, 20, 24, 29, 36,
39, 40)
Chapter 7, pg 118-123
 1. Can an action force exist without a reaction force?
 No.
 3. When you walk along a floor, what pushes you?
 The floor! You push on the floor, and the floor
pushes back on you with equal and opposite force.
 7. If the world pulls you downward against your
chair, what is the reaction force?
 The reaction force is the chair pulling upwards on the
Earth.
Chapter 7, pg 118-123
 8. When a cannon is fired, are the forces on the
cannonball and on the cannon equal in magnitude?
Are the accelerations of the two equal?
 Forces are equal in magnitude, but acceleration is
unequal;
 9. When a cannon is fired, why does the cannonball
and the cannon have very different accelerations?
 Equal forces on two different masses produce two
different accelerations (the cannonball is more greatly
accelerated than the cannon)
Chapter 7, pg 118-123
 20. Fill ins: inertia, acceleration, action and
reaction
 24. When you rub your hands together, can
you push harder on one hand than the other?
 No. Each hand pushes equally on the other.
 25. Your weight is the result of the
gravitational force of the Earth on your body.
What is the corresponding reaction force?
 Your body pulling on the Earth with equal and
opposite force.
Chapter 7, pg 118-123
 29. Why is it easier to walk on a carpeted
floor than on a smooth, polished floor?
 One can exert a greater force on carpet than
on a polished floor because of the greater
friction. This, in turn, provides a greater
reaction force to provide traction for walking.
 36. The impact force is the same for both.
The car has less mass (thus, less inertia)
than the van, so the acceleration for the car is
greater than for the van.
Chapter 7, pg 118-123
 39. Consider two carts, one twice as massive as the
other, that fly apart when squeezed together. When
released, how fast does the heavier cart roll
compared to the lighter cart?
 The heavier cart rolls twice as slow (it is twice the
mass, so given the same force, it has half the
acceleration)
 40. What force propels a rocket when it is in a
vacuum (a place with no matter)?
 The action force is the rocket pushing on the exhaust
gases, so the reaction force is the gases pushing on
the rocket with equal force in the opposite direction.
Recoil reaction!