Physics Chp6 Newtons 3rd Law

TRUE or FALSE: For an object resting upon a non-accelerating surface, the normal force is equal to the weight of the object.

False Quite surprisingly to many, the normal force is not necessarily always equal to the weight of an object. Suppose that a person weighs 800 N and sits at rest upon a table. Then suppose another person comes along and pushes downwards upon the persons shoulders, applying a downward force of 200 N. With the additional downward force of 200 N acting upon the person, the total upward force must be 1000 N. The normal force supplies the upward force to support both the force of gravity and the applied force acting upon the person. Its value is equal to 1000 N which is not the same as the force of gravity of the person.

Which of Newton's Laws gives the reason for why you can feel things that you touch?

3rd

Pick which of Newton's most governs the situations described below. Heavy objects are not easier to move around in a horizontal fashion on the Moon than on the Earth. a. First Law (inertia) b. Second Law (F = m•a) c. Third Law (action-reaction)

A - All objects have inertia or a tendency to resist changes in their state of motion. This inertia is dependent solely upon mass and is subsequently not altered by changes in the gravitational environment. To move an object horizontally, one must apply a force; this force will be resisted by the mass or inertia of the object. On the moon, the object offers the same amount of inertia as on Earth; it is just as difficult (or easy) to move around.

Pick which of Newton's most governs the situations described below. If you were in an elevator and the cable broke, jumping up just before the elevator hit the ground would not save you. a. First Law (inertia) b. Second Law (F = m•a) c. Third Law (action-reaction)

A - An object moving downwards will continue to move downwards unless acted upon by an unbalanced force. If you make an effort to supply such a force in an attempt to suddenly alter the direction of your motion, then you are creating a greater velocity change than if you merely hit the ground and stopped. If this greater velocity change occurred suddenly (in the same amount of time as the stopping of you and the elevator), then you would experience a greater acceleration, a greater net force, and a greater ouch mark than if you had merely hit the ground and stopped.

Pick which of Newton's most governs the situations described below. You usually jerk a paper towel from a roll in order to tear it instead of pulling it smoothly. a. First Law (inertia) b. Second Law (F = m•a) c. Third Law (action-reaction)

A - The paper towel is at rest and resists changes in its at rest state. So if you apply a sudden force to one of the paper towel sheets, the great mass of the remainder of the roll will resist a change in its at rest state and the roll will easily break at the perforation.

Big Bubba has a mass of 100 kg on the earth. What is Big Bubba's mass on the moon where the force of gravity is approximately 1/6-th that of Earth's? ________ Explain or show your work.

Answer: 100 kg Mass is a quantity which is independent of the location of the object. So if Big Bubba has a mass of 100 kg on Earth, then he also has a mass of 100 kg on the moon. Only the weight would change as Big Bubba is moved from the Earth to the moon. He weighs ~1000 N on Earth and 1/6-th this value (~167 N) on the moon.

The amount of net force required to keep a 5-kg object moving rightward with a constant velocity of 2 m/s is ____. a. 0 N b. 0.4 N c. 2 N d. 2.5 N e. 5 N

Answer: A Net force is always m•a. In this case, the velocity is constant so the acceleration is zero and the net force is zero. Constant velocity motion can always be associated with a zero net force.

4. In the top picture (below), Kent Budgett is pulling upon a rope that is attached to a wall. In the bottom picture, Kent is pulling upon a rope that is attached to an elephant. In each case, the force scale reads 500 Newton. Kent is pulling ... A. with more force when the rope is attached to the wall. B. with more force when the rope is attached to the elephant. C. the same force in each case.

Answer: C Kent is pulling with 500 N of force in each case. The rope transmits the force from Kent to the wall (or to the elephant) and vice versa. Since the force of Kent pulling on the wall and the wall pulling on Kent are action-reaction force pairs, they must have equal magnitudes. Inanimate objects such as walls can push and pull.

For years, space travel was believed to be impossible because there was nothing that rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because ... A. space is void of air so the rockets have nothing to push off of. B. gravity is absent in space. C. space is void of air and so there is no air resistance in space. D. nonsense! Rockets do accelerate in space and have been able to do so for a long time.

Answer: D It is a common misconception that rockets are unable to accelerate in space. The fact is that rockets do accelerate. There is indeed nothing for rockets to push off of in space - at least nothing which is external to the rocket. But that's no problem for rockets. Rockets are able to accelerate due to the fact that they burn fuel and push the exhaust gases in a direction opposite the direction which they wish to accelerate.

As you sit in your chair and study your physics (presuming that you do), the force of gravity acts downward upon your body. The reaction force to the force of the Earth pulling you downward is ___. a. the force of the chair pushing you upward b. the force of the floor pushing your chair upward c. the force of the Earth pushing you upward d. the force of air molecules pushing you upwards e. the force of your body pulling the Earth upwards f. ... nonsense! Gravity is a field force and there is no such reaction force.

Answer: E The most common wrong answer is a - the force of the chair pushing you upward. As you sit in your chair, the chair is indeed pushing you upward but this is not the reaction force to the force of the Earth pulling you downward. The chair pushing you upward is the reaction force to you sitting on it and pushing the chair downward. To determine the action-reaction force pairs if given a statement of the form object A pulls X-ward on object B, simply take the subject and the object in the sentence and switch their places and then change the direction to the opposite direction (so the reaction force is object B pulls object A in the opposite direction of X). So if the Earth pulls you downward, then the reaction force is you pull the Earth upward.

TRUE or FALSE: An object which is moving rightward has a rightward force acting upon it.

Answer: False An object which is accelerating rightward must have a rightward force and a rightward net force acting upon it. But an object which is merely moving rightward does not necessarily have a rightward force upon it. A car that is moving rightward and skidding to a stop would not have a rightward force acting upon it.

A golf pro places a ball at rest on the tee, lines up his shot, draws back his club, and lets one rip. During the contact of the golf club with the golf ball, the force of the club on the ball is ____ the force of the ball on the club and the acceleration of the club is ____ than the acceleration of the ball. a. greater than, greater than b. greater than, equal to c. greater than, less than d. less than, less than e. less than, equal to f. less than, greater than g. equal to, equal to h. equal to, greater than i. equal to, less than

Answer: I For every action, there is an equal and opposite reaction force. In this case, the force on the club is equal to the force on the ball. The subsequent accelerations of the interacting objects will be inversely dependent upon mass. The more massive club will have less acceleration than the less massive ball.

Little Billie weighs 360 N on Earth. What is Little Billie's mass on the moon where the force of gravity is approximately 1/6-th that of Earth's? ________ Explain or show your work.

Answer: ~36 kg The mass of an object is related to weight by the equation W = m•g where g = ~10 m/s/s on Earth and one-sixth this value (~1.67) on the moon. So if Billy weighs 360 N on Earth, then his mass is approximately ~36 kg. His mass on the moon will be the same as his mass on Earth. Only his weight changes when on the moon; rather than being 360 N, it is 60 N. His weight on the moon could be found by multiplying his mass by the value of g on the moon: (36 kg) • (9.8/6 m/s/s) = ~60 N

Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backward upon the rifle. The acceleration of the recoiling rifle is ... A. greater than the acceleration of the bullet. B. smaller than the acceleration of the bullet. C. the same size as the acceleration of the bullet.

B. Smaller than the acceleration of the bullet. The force on the rifle equals the force on the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional.

Pick which of Newton's most governs the situations described below. The stronger, heavier team in a tug-of-war does not create a larger tension in the rope than the weaker, lighter team. a. First Law (inertia) b. Second Law (F = m•a) c. Third Law (action-reaction)

C - A rope encounters tension when pulled on at both ends. The tension in the rope is everywhere the same. If team A were to pull at the left end, then the left end would pull back with the same amount of force upon team A. This force is the same everywhere in the rope, including at the end where team B is pulling. Thus team B is pulling back on the rope with the same force as team A. So if the forces are the same at each end, then how can a team ever win a tug-of-war. The way a stronger team wins a tug-of-war is with their legs. They push upon the ground with a greater force than the other team. This force upon the ground results in a force back upon the team in order for them to pull the rope and the other team backwards across the line.

Pick which of Newton's most governs the situations described below. A student desk changes the amount of force it puts on other objects throughout a school day. a. First Law (inertia) b. Second Law (F = m•a) c. Third Law (action-reaction)

C - As a student sits in the seat, they are applying a downward force upon the seat. The reaction force is that the seat applies an upward force upon the person. A weightier person will apply more downward force than a lighter person. Thus, the seat will constantly be changing the amount of reaction force throughout the day as students of different weight sit in it.

Pick which of Newton's most governs the situations described below. A helicopter must have two sets of blades in order to fly with stability. a. First Law (inertia) b. Second Law (F = m•a) c. Third Law (action-reaction)

C - As the helicopter blades spin and push air in one direction, the air pushes the blades in the opposite direction; the result is that the helicopter can begin to rotate about the axis of the blade. To counteract this rotation, a second set of blades is required.

According to Newton's third law, every force is accompanied by an equal and opposite reaction force. The reason that these forces do not cancel each other is ____. A. the action force acts for a longer time period B. the two forces are not always in the same direction c. one of the two forces is greater than the other D. the two forces act upon different objects; only forces on the same object can balance each other. E. nonsense! They do cancel each other. Objects accelerate because of the presence of a third force.

D. the two forces act upon different objects; only forces on the same object can balance each other. Action and reaction forces always act upon the interacting objects for the same amount of time with the same magnitude. So if object A pushes on object B, then object B simultaneously pushes on object A with the same amount of force. The force on object B will be one of perhaps many forces which will govern its motion. But the reaction force is on object A and cannot contribute to object B's motion since it is not acting upon object B. Action-reaction forces can NEVER cancel each other.

According to Newton's third law, in a tug-of-war match, both sides pull equally on each other as the tension in the rope must be the same at both ends. How is it possible then to ever win a tug-of-war?

So if the forces are the same at each end, then how can a team ever win a tug-of-war. The way a stronger team wins a tug-of-war is with their legs. They push upon the ground with a greater force than the other team. This force upon the ground results in a force back upon the team in order for them to pull the rope and the other team backwards across the line.

A horse pulls forward on a carriage with a given force. By Newton's Third Law, the carriage must be pulling on the horse backward with an equal and opposite force. Given this, what explains why the horse and carriage can move forward?

The cart is rolling on wheels while the horse's hooves have traction with the ground

You and a friend are pulling on a rope in opposite directions as hard as you can. What is the "equal and opposite force" to the force of your hand pulling on the rope described by Newton's Third Law?

The force of the rope pulling on your hand in the opposite direction

Which best explains why we are able to accelerate forwards when starting to run?

The striking foot pushes backwards against the ground. The friction with the ground provides an equal and opposite force forwards.

While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

Trick Question! Each force is the same size. For every action, there is an equal ... (equal!). The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction. Besides, fireflies have guts and bug guts have a tendency to be splatterable. Windshields don't have guts. There you have it.

The two forces are

action and reaction

Newton realized that a force is not a thing itself but part of a mutal action, or

interaction