# Physics Chapter 5

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How many forces are required for an interaction?
Two forces, an action and a reaction, are needed for an interaction
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The force that propels a rocket is that provided by _______.
The expelled gas pushing on the rocket
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To produce a net force on a system, must there be an externally applied net force?
Yes, there must be an externally applied force
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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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A Volkswagen Bug and a Volvo truck have a head-on collision. Which statement is true?
The magnitudes of both forces are the same
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Earth pulls down on you with a gravitational force that you call your weight. Do you pull up on Earth with the same amount of force?
Yes, you pull up on Earth with the same force
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If the forces that act on a cannonball and the recoiling cannon from which it is fired are equal in magnitude, why do the cannonball and cannon have very different accelerations?
Remember F = ma, and note that the cannon has a much greater mass than the cannon ball, so the cannon accelerates less for the same force
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Can you physically touch a person without that person touching you with the same amount of force?
No, when you touch a person, they must touch you with an equal and opposite force
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A 7-N vector at an angle of 45° to the horizontal has a vertical component that is about _______.
5 N
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How does the magnitude of the vertical component of velocity for a ball tossed at an upward angle change as the ball travels upward? How about the horizontal component of velocity when air resistance is negligible?
The vertical component decreases in magnitude until it reaches the maximum height and then increases; the horizontal component is constant
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The length of a force vector indicates the ________.
Magnitude of force
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Fill in the blanks: Newton's first law is often called the law of ____; Newton's second law is the law of ____; and Newton's third law is the law of ____.
Inertia, acceleration, action -reaction
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When a boxer hits a punching bag, the strength of his punch depends on how much force the bag can
Exert on the boxer's fist
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Your friend says that the heavyweight champion of the world cannot exert a force of 50 N on an isolated piece of tissue paper with his best punch. You
Agree that it can't be done
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One end of a rope is pulled with 100 N, while the opposite end also is pulled with 100 N. The tension in the rope is
100 N
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When you walk, you push on the floor to the left and the floor
Pushes you to the right
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Harry pulls on the end of a spring attached to a wall. The reaction to Harry's pull on the spring is
The spring pulling on Harry
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Action and reaction pairs of forces
Always act simultaneously
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For every action force, there must be a reaction force that
Is equal in magnitude
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To produce an acceleration to a system there
Must be a net force on the system
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While you stand on the floor you are pulled downward by gravity, and supported upward by the floor. Gravity pulling down and the support force pushing up
Do not make an action-reaction pair of forces
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The force exerted on the tires of a car that directly accelerate it along a road is exerted by the
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Neglecting air resistance, once a tossed ball leaves your hand
Only the force due to gravity acts on it
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An automobile and a golf cart traveling at the same speed collide head-on. The impact force is
The same for both
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A Mack truck and a Volkswagen traveling at the same speed have a head-on collision. The vehicle that undergoes the greatest change in velocity will be the
Volkswagen
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As a ball falls, the action force is the Earth's pull on the ball. The reaction force is the
Ball's pull on Earth
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The force with which Earth pulls on the Moon is
Equal in magnitude to the force that Moon pulls on Earth
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When a cannonball is fired from a cannon, which undergoes the greater acceleration?
The cannonball
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The force that propels a cannonball when fired from a cannon is
Equal and opposite to the force the ball exerts on the cannon
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When a rocket forces exhaust gases downward, the exhaust gases
Exert an upward force on the rocket
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An astronaut of mass 70 kg weighs 700 N on Earth's surface. His weight on the surface of Mars, where the acceleration due to gravity is 3.7 m/s2, would be about
260 N. (70*10 On earth) (70*3.7)
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You stand on your skateboard and exert a 50-N push on the wall next to you. If your mass is 60 kg you'll momentarily accelerate from the wall at about
0.8 m/s2. (50/60)
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A 10-unit vector at 60° from the vertical has a vertical component with a magnitude
Less than 10 units
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When Nellie hangs suspended from a pair of ropes that are not vertical, the tension in each rope is
More than half her weight
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Three kids pull on a toy. Kim pulls with a force of 30 N north, Jim pulls with a force of 30 N south, and Tim pulls with a force of 15 N east. The acceleration of the toy is
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Nellie tosses a ball upward at an angle. Neglecting air resistance, the horizontal component of the initial velocity
Remains constant
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Nellie tosses a ball upward at an angle. Neglecting air resistance, the vertical component of the initial velocity
Decreases with time to reach the top
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The airspeed of an airplane is 200 km/h. When it is caught in a 200-km/h crosswind, its speed across the ground below is
283 km/h (Pythag. Theorem)
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A river 100 m wide flows due south. A boat that goes 1 m/s relative to the water is pointed due east as it crosses from the west bank. The boat crosses in
100 s
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A river 100 m wide flows due south at 1 m/s. A boat that goes 1 m/s relative to the water leaves the west bank. To land at a point due east of its starting point, the boat must be pointed
Nowhere- it can't be done
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Calculate the magnitude of the resultant of a pair of 100-km/h velocity vectors that are at right angles to each other
141 km/h
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How does the magnitude of F relate to the vector sum of mg and N when the shoe is in equilibrium?
Vector F will have the same magnitude as the vector sum of mg and N
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What occurs if F is less than this sum?
If F is less, then a net force acts on the shoe and it accelerates down the incline
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If air drag is negligible, how does the horizontal component of velocity relate to Newton's first law of motion?
No force acts horizontally on the ball so the initial horizontal velocity remains constant as the ball moves through the air in accord with Newton's first law of inertia
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As a tossed ball sails through the air, a force of gravity mg acts on it. Identify the reaction to this force.
The ball's upward pull on Earth
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As a tossed ball sails through the air, a force of gravity mg acts on it. Identify the acceleration of the ball along its path, even at the top of its path.
g (9.8m/s^2 or will remain constant)
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If you stand next to a wall on a frictionless skateboard and push the wall with a force of 50 N , how hard does the wall push on you?
50 N
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If you stand next to a wall on a frictionless skateboard and push the wall with a force of 50 N , how hard does the wall push on you? If your mass is 84 kg , find your acceleration
.60 m/s^2 (a=F/m)
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The video identifies the force pair produced when an apple falls through the air. Which force belongs in a free-body diagram of the apple?
Force of Earth on apple (The video describes a gravitational force pair: the force of the apple on Earth and the force of Earth on the apple. The force of Earth on the apple appears in the apple's free-body diagram)
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A force pair is produced when a tennis racket strikes a tennis ball. Which of the following best explains why the tennis ball does not have zero net force acting on it?
Each half of the force pair acts on a different object (The forces in the force pair are equal in size, act in opposite directions, and act on different objects. One half of the force pair acts on the tennis ball and the other half acts on the racket and both objects individually experience a non-zero net force)
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A force pair is created when you push on a large crate that rests on the floor. The crate does not move when pushed. Which free-body diagram correctly represents the forces acting on the crate?
Four forces act on the crate as shown. Note that the other force in the force pair, the force of the crate on the hand, acts in the free-body diagram of the hand
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What explains the dramatically different magnitudes of accelerations that result when a mosquito collides head on with a moving truck?
Unequal masses of the bus and the mosquito
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Predict how the horizontal component of the velocity will change with time after the projectile is fired
It stays constant
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Predict how the vertical component of the velocity will change with time after the projectile is fired
It first decreases to zero and then increases in the opposite direction
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Predict how the time of flight would change if you increased the angle of the projectile, keeping the initial speed constant
It increases
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Predict how the range of the projectile will change when you change the angle from 45∘, keeping the speed constant
It decreases with either an increase or a decrease in angle
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Why does the vertical component of velocity for a projectile change with time, whereas the horizontal component of velocity doesn't change?
Gravity is a purely vertical force
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A stone is thrown upward at an angle. What happens to the horizontal component of its velocity as it rises? As it falls?
Rising or falling, it does not change
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A stone is thrown upward at an angle. What happens to the magnitude of the vertical component of its velocity as it rises? As it falls?
It decreases while rising, but it increases while falling
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A ball rolls off the edge of a table at the same time another ball drops vertically from the same table. The ball to hit the floor first is the