Matter & Interactions 2nd ed. Practice Problems
Aaron Titus | High Point University
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1260001     Force by the road on a truck as it slows down.     1260001
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A 1200-kg truck travels in a straight line and has a speed 25 m/s in the +x direction when the driver hits the brakes. The truck slows down to 5 m/s in 10 s. Assume that the net force on the truck is constant.

  1. What is the force by the road and air on the truck as it slows down?
  2. What is the displacement of the truck during this time interval?

 

1260002     Forces on a Moon lander as it is landing.     1260002
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A 500-kg Moon lander has a velocity of 10 m/s downward when it is a certain height above the ground. 1 minute later, it is 1 m from the ground and its speed is 0.5 m/s downward. Assume that the thruster on the lander fires upward with a constant force. Moon's gravitational field strength near its surface is 1.6 N/kg.

  1. What is the force by the exhaust on the lander (i.e. thrust)?
  2. What was its initial height, when its speed was 5 m/s downward?

 

1250001     Projectile motion with no air resistance     1250001
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A 0.4-kg soccer ball is kicked from a location m (on the ground) with initial velocity m/s. The ball's speed is low enough that air resistance is negligible. Therefore, you can assume that the net force is constant. (Note: the +y direction is defined to be perpendicular to the ground and upward, and the x-z plane is the soccer field. t = 0 is defined to be the instant the ball leaves the foot.)

  1. What is the velocity and position of the ball at t = 0.3 s after being kicked?
  2. What is the maximum height reached by the ball?

 

1250002     Momentum and position for an electron moving between charged plates with a constant electric force on the electron.     1250002
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An electron of mass and initial speed in the –y direction enters a region between charged plates in which the force on the electron is constant, in the –x direction with a magnitude . After the electron travels 5 cm in the -y direction, it exits the region between the plates. At this instant when it exits, what is its momentum and how far has it deflected to the left from where it started?


Figure: An electron travels between two charged plates.

 

1230001     Force by a merry-go-round on a child     1230001
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A 20-kg girl rides on a merry-go-round. She is sitting near the edge of the merry-go-round and is rotating counterclockwise. The +z axis is defined to be perpendicular to the merry-go-round, and the +x and +y axes are in the plane of the merry-go-round. (See figures below.)


Figure: A merry-go-round.

At , the velocity of the girl is m/s, and 0.15 s later, her velocity is m/s.

  1. Sketch the location of the girl on the merry-go-round, her initial momentum, her final momentum, and her change in momentum.
  2. What is the net force on the girl during this time interval? Calculate and sketch this vector.
  3. What is the force by the merry-go-round on the girl during this time interval?

 

1230003     Net force on a model rocket; momentum update.     1230003
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Ad a certain instant, the velocity of a 0.050-kg model rocket is , and all significant forces on the rocket are:

  1. What is the net force on the rocket?
  2. Assuming a constant net force during a small time interval, what will be the rocket's momentum 0.02 s later?

 

1230002     Force on a box that is pushed in equilbirum across a floor.     1230002
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You push a 15-kg box across a somewhat smooth concrete floor as shown below.


Figure: A box being pushed across the floor.

You push with a force of 20 N at an angle, with respect to the horizontal, of . If the box moves in the +x direction with a constant speed, what is the force by the floor on the box? (Note: the gravitational force of Earth on the box is .)

 

1240001     Momentum and position of a proton accelerated across charged plates     1240001
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In a mass spectrometer (an instrument used in chemistry to measure the mass and therefore identity of unknown atoms) positively charged ions are accelerated between two charged plates. Suppose that a proton is nearly at rest at the positively charged plate on the left. It is accelerated toward a negatively charged plate on the right, as shown below. The plates are 5 cm apart.


Figure: A proton accelerated between charged plates.

The electric force by the charged plates on the proton is in the +x direction. This is the only significant force acting on the proton.

  1. If the initial velocity of the proton is zero, what is the velocity of the proton after a time interval of ?
  2. If you define the origin to be at the initial location of the proton, what is the position of the proton at ?

 

1240002     Momentum and position of a nerf basketball moving through the air     1240002
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You toss a nerf basketball of mass 0.002 kg into the air. At t = 0, the ball leaves your hand. At t = 0.11 s, the net force on the ball is , its velocity is , and its position is . What is its position at t = 0.13 s?

 

1290001     Change in momentum of Earth as a result of bouncing a superball.     1290001
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Suppose that you measure the velocity of a 0.015-kg superball just before it hits the floor and find that its velocity is downward with a magnitude of 4.5 m/s. You also measure its velocity just after it hits the floor and find that it is upward with a magnitude of 3.9 m/s. What is the change in momentum of Earth as a result of the collision with the superball?

 


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