# Assignment 3: Newtons second and third laws of motion – Flashcards

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The direction of the force of friction on a sliding crate is _______.
opposite to the direction of sliding
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How does the force of friction for a sliding object vary with speed?
The force of friction is approximately independent of speed.
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If Dr. Hewitt applies 3 N of force to keep the block moving at a steady rate, what must be the force of friction, and why?
Equal to 3 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.
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You are dragging a block on a surface with friction at a steady speed of 2 m/s and exert a force of 5 N to do so. What is the force of friction? Why?
Equal to 5 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.
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What general rule can you conclude about the force needed to keep an object in motion at a steady rate?
The force that you need to apply to keep an object moving at a steady rate is equal to the force resisting the motion of the object.
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Which depends on gravity?
Weight
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In the string-pull illustration in Figure 4.8, a sharp jerk on the bottom string results in the bottom string breaking. Does this occur because of the ball's weight or its mass?
Mass
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Fill in the blanks: The Standard International unit for mass is _____. The Standard International unit for force is _____.
Mass is kilograms; force is newtons.
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If the mass of a sliding block is somehow tripled at the same time the net force on it is tripled, how does the resulting acceleration compare with the original acceleration?
The acceleration remains the same.
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Air resistance on a parachutist at terminal speed _______.
is greater for a heavier person
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What is the net force that acts on a 10-N falling object when it encounters 4 N of air resistance? When it encounters 10 N of air resistance?
The net forces are 6 N and 0 N, respectively.
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In the absence of air resistance, if a ball is thrown vertically upward with a certain initial speed, on returning to its original level it will have the same speed. When air resistance is a factor, will the ball be moving faster, the same, or more slowly than its throwing speed when it gets back to the same level? Why? (Physicists often use a "principle of exaggeration" to help them analyze a problem. Consider the exaggerated case of a feather, not a ball, because the effect of air resistance on the feather is more pronounced and therefore easier to visualize.)
When the ball is coming back down it will be moving more slowly than its throwing speed when it gets back to the same level. This is because of the air resistance. When the ball is coming back down uses some of its energy and it heats up the air around it. This could sort of be visually compared to when a rocket returns to earth. You can visually see that the air is being heated around the rocket. This is also because of air resistance.
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What happens to the weight reading on a scale you stand on when you toss a heavy object upward?
The scale reading will increase during the throw.
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A boxer cannot exert much force on a piece of tissue paper suspended in air because _______.
the tissue paper has too little mass to exert as much force on the boxer's fist.
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In the apple-pulling-the orange sequence in this chapter, what is the force that accelerates the system across the floor?
Friction between the apple and the floor.
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Consider the system of a single football. If you kick it, is there a net force to accelerate the system? If a friend kicks it at the same time with an equal and opposite force, is there a net force to accelerate the system?
If you kick it, there is a net force; if both of you kick it, there is not.
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How does the force on the rifle compare with the force on the bullet, and why?
The magnitude of the force of the rifle is equal to the magnitude of force on the bullet and the forces are in the opposite direction, because for every action there is an equal and opposite reaction.
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How does the magnitude of acceleration of the rifle compare with the magnitude of acceleration of the bullet, and why?
The acceleration of the rifle is smaller than the acceleration of the bullet. They experience the same magnitude of force, but the mass of the rifle is greater, and so the rifle will experience a smaller acceleration than the bullet.
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How does the distance traveled by the bullet compare with the distance traveled by the rifle in the same time, and why?
The bullet will travel farther than the rifle because it has a greater magnitude of acceleration.
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Which of Newton's three laws focuses on interactions?
Newton's third law
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If you stand next to a wall on a frictionless skateboard and push the wall with a force of 50
50 N
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a = 0.63 ms2
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In a tug-of-war between Sam and Maddy, each pulls on the rope with a force of 250 N. What is the tension in the rope? If both remain motionless, what horizontal force does each exert against the ground?
In a tug-of-war between Sam and Maddy the tension in the rope is 250 N. Also, if both remain motionless, the horizontal force is also at 250 N against the ground.
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Within a book on a table, there are billions of forces pushing and pulling on all the molecules. Why is it that these forces never by chance add up to a net force in one direction, causing the book to accelerate "spontaneously" across the table?