Force Review

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Chapter 3

• Force Review

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1) A 0.15-kilogram baseball and a 7.00-kilogram
bowling ball are both rolling along at 3 m/sec.
Which object is easier to stop and why?

• The baseball has a momentum of 0.45kgm/s and the
  bowling ball has a momentum of 21kgm/s. The
  baseball has less momentum so it is easier to
  stop.

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• 2) An astronaut with a mass of 100 kilograms is
  floating in outer space. If the astronaut throws
  a 2-kilogram wrench at a speed of 10 meters per
  second, his motion would best be described as
• Moving away from the wrench with an equal, but
  opposite momentum. Total momentum before the
  action has to be equal to the momentum after the
  action. Nothing was moving before the action, so
  the total momentum is zero before the action.
  After the wrench is thrown, the wrench has a
  velocity and a mass, so it has a momentum. The
  astronauts momentum must be equal in size, but
  opposite in direction. The wrench has 20kgm/s
  momentum, then the astronaut must have -20kgm/s
  momentum. 20kgm/s 20kgm/s 0 kgm/s Total
  momentum before and after the action are equal.

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3) A stationary object has what inertia and what
momentum?

• Momentum equals mass multiplied by velocity
  (pmv) If an object is stationary, then its
  velocity is zero. Any mass multiplied by zero
  will give a momentum of zero.

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• 4) Newton's third law of motion involves
• a force pair acting on two different objects.

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5) A rocket can fly into space because

• The gasses pushed out of the rocket also push
  back on the rocket with an equal and opposite
  force. Causing the gasses to accelerate in one
  direction, and the rocket to accelerate on the
  opposite direction.

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6) The force of friction acting on a cart on a
ramp is exactly balanced by the force of gravity
acting on the cart down the ramp. What is the
carts motion?

• The net force acting on the cart is the sum of
  all the forces acting on the cart. If the forces
  are balanced, then the net force equals zero.
  When the net force equals zero the will be no
  acceleration. Newtons first law states that
  when the is no net force, and object at rest will
  stay at rest, and an object in motion will stay
  in motion at a constant speed and direction. The
  net force is zero on the cart, so if it was
  moving it will move at the same speed, if it is
  not moving, it will stay put.

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7) What is the approximate weight in newtons of
a 10.0-kilogram bowling ball?

• Weight is a force. The force of weight is equal
  to the mass of an object multiplied by the
  acceleration of gravity. Fwmag or Fwmg. G on
  Earth equals 9.8m/s/s or approximately 10m/s/s.
• The weight of the ball 10.0kg 10m/s/s The
  weight of the ball 100N

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Acceleration is the change in speed divided by
the time it took to change the speed. When you
know how long something has been accelerating,
the you know how fast it is moving. The ball
accelerates at 10m/s/s for 4s, then the speed
would equal 40m/s. 10m/s/s 4s 40m/s

• 8) The acceleration due to gravity is
  approximately 10 m/sec2. If a ball is dropped
  from the tenth floor of a building, ignoring air
  resistance, after falling for 4 seconds the speed
  of the ball will be

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9) A cart is rolling down a special ramp where
there is no friction at all. What will happen if
the amount of mass on the cart is increased?

• Force equals mass multiplied by acceleration.
  The acceleration of the ramp remains constant.
  If the mass is increased the force will increase.
  

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10) As a box is sliding down a ramp, a 20 newton
force of gravity is pulling the box down the ramp
while a 4 newton force of friction acts to stop
the box. What is the net force accelerating the
box down the ramp?

• The net force is the sum of all the forces acting
  on an object. The force of friction always
  opposes motion. There are 20 newtons of force
  pointing down the ramp, and 4 newtons of force
  pointing up the ramp, then the net force is 16
  newtons acting to accelerate the box down the
  ramp. The net force equals
• Fnet 20N 4N
• Fnet 16N

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11) A car is traveling down a straight stretch
of highway at 100 kilometers per hour. The driver
suddenly slams on the brakes and brings the car
to a quick stop. As a result, the unbelted
passengers will at first

• Act under the influence of Newtons first law.
  The car stops. The passengers will continue to
  move in a straight line at a constant speed until
  they hit something. Just like the objects left
  on the rear dashboard in the Myth Busters video
  clip we watched.

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12) A potato launcher uses a spring that can
apply a force of 25 newtons to potatoes. A
physics student who is studying projectiles
launched a 100-gram potato, a 150-gram potato,
and a 200-gram potato with the launcher. Which
had the greatest acceleration?

• Force equals mass multiplied by acceleration. If
  we rearrange the formula to solve for
  acceleration, then we get F/ma. If the force
  remains constant, then the acceleration will get
  larger when the mass gets smaller. The smallest
  potato will have the greatest acceleration.

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• In the picture below, a girl is sitting
  motionless on a swing. What is the net force on
  the girl?
• The sum of all the forces are
• 245N245N-490N0N
• (Up) (up) (down)
• Net force zero

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• A cart is rolling down a ramp. If the angle of
  the ramp is increased to make the ramp steeper,
  the cart will
• Accelerate faster

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• Objects tend to resist changes in motion. This
  property is called
• Inertia

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• When constructing a graph, the y-axis is most
  closely related to
• Dependent variable