Forces and Newton's Laws
Josh punches his open left hand with his right hand. Which statement is true about the forces his two hands exert on each other?
Answer: Josh's right hand and left hand exert equal and opposite forces on each other. Explanation: As Josh punches his open left hand with his right hand, Josh's right hand and left hand exert equal and opposite forces on each other. The interaction between Josh's two hands affects both of them equally. It may be tempting to think that the hand that "does the punching" will somehow exert a larger force, but that would violate Newton's third law.
An object rests on a ramp that is angled at 20º above the horizontal. What direction (relative to the vertical) will the normal force from the ramp point?
Answer: The normal force will be directed 20º to the right of vertical. Explanation: The normal force will be directed 20º to the right of the vertical. Normal forces always point in a direction that is perpendicular to the surface that causes them. We can see that if the ramp were level the normal force would point directly upward. As the ramp tilts up to 20º above the horizontal, the normal force tilts until it is 20º away from the vertical.
Floating in deep space, you find yourself at rest next to a small asteroid. You reach out and tap the asteroid with a hammer. What happens to you in this process?
Answer: You will briefly accelerate away from the asteroid and then drift away at a constant speed. Explanation: As you tap the asteroid with a hammer, you will briefly accelerate away from the asteroid and then drift away at a constant speed. The asteroid and hammer exert equal forces on each other (from Newton's third law), and since you are holding the hammer, you and the hammer will be pushed away from the asteroid during the (very brief) interaction. This will accelerate you away from the asteroid briefly (Newton's second law), after which you will continue to drift away from it at a constant speed (Newton's first law). There is no force present that could slow you down and bring you back to rest, such as would happen with friction or air resistance if you were on a planet.
In order to state the force exerted on one object by another object, you must give .
Answer: both the magnitude of the force and the direction of the force Explanation: In order to state the force exerted on one object by another between two objects you must give both a magnitude and a direction of the force. Force is a vector quantity, so it can be stated as either a magnitude and direction or as vector components (such as stating that the x-component is -5.0 N and the y-component is +8.3 N). The magnitude of the force and "the amount of force" mean the same thing. Acceleration is related to net force by Newton's second law, but the two quantities are not the same.
In general, when a car is driving along a straight road, the force of friction between a car tire and the road _____________.
Answer: can point either forward or backward, depending on what the car is doing. Explanation: When a car is driving along a straight road, the force of friction between a car tire and the road can point either forward or backward, depending on what the car is doing. If the gas pedal is pushed down, then there is a static frictional force between the road and the tires that points forward (helping the car overcome air resistance, and/or making the car speed up). If the car is braking, then there is a static frictional force between the road and the tires that points backwards (making the car slow down). There could also be kinetic friction (forward or backward) if the car is peeling out or skidding to a stop.
Newtons and pounds are __________.
Answer: different units for measuring force Explanation: Newtons (N) and pounds (lbs) are different units for measuring force. They can be converted into each other, with 1 pound equaling roughly 4.45 newtons. Neither of them can be used to measure mass, since mass and force are fundamentally different quantities.
A person is trying to pull a heavy crate across a room with a rope that is attached to the crate, but it doesn't move. The tension in the rope ________________.
Answer: is equal at both ends of the rope Explanation: The tension in the rope is equal at both ends of the rope. If a rope is not accelerating, then the net force on it must be zero. This means that it is being pulled with equal magnitude forces at both ends (using Newton's second law). If each block exerts the same magnitude force on the rope, then the rope must exert the same magnitude force on each block (using Newton's third law). It doesn't matter what is happening to either end of the rope. As long as the rope is not accelerating the tension will always be exactly the same at both ends of the rope.
The mass of an object
Answer: is not a vector and has no direction Explanation: The mass of an object is not a vector and has no direction. An object's weight is a force and has a direction associated with it, but the mass is scalar, with no direction at all.
A box rests on a table. The normal force exerted by the table on the box is equal in magnitude to the weight of the box and points in the opposite direction. This is because ________________.
Answer: the net force on the box is zero and, therefore, the forces must be balanced Explanation: When a box is resting on a table, the normal force exerted by the table on the box is equal in magnitude to the weight of the box and points in the opposite direction because the net force on the box is zero and therefore the forces must be balanced. Since the box has zero acceleration, Newton's second law tells us that the net force must be zero. The normal force and the weight do not form an action-reaction pair (from Newton's third law). The weight is due to an interaction between the box and Earth, not the box and the table. The normal force is sometimes equal to the weight, but not always. The magnitude of the normal force will always depend on the situation and on all the forces that act on the object. For example, if there was a string tied around the box and the string was used to slightly lift the box upward, then the normal force would have a magnitude that is less than the weight.
While sparring, Katie punches Arman in the shoulder. Which statement is true about the forces Katie and Arman exert on each other?
Answers: Katie exerts a force on Arman's shoulder and Arman exerts a force on Katie's hand. Explanation: When Katie punches Arman in the shoulder, Katie exerts a force on Arman's shoulder and Arman exerts a force on Katie's hand. The interaction between Katie's hand and Arman's shoulder affects both objects, creating two forces that form an action-reaction pair. It is never possible for one object to exert a force on another without having a force exerted on it in return. This is one of the core ideas of Newton's third law. Only objects can exert forces on other objects. When Katie's hand contacts Arman's shoulder it creates only one force on Arman due to Katie. Her "punch" is not a separate object that can exert a force
Which of these is a valid force that could be exerted on a car racing through narrow city streets?
Answers: The force of friction between the road and the car (its tires) Explanation: A valid force that could be exerted on a car racing through narrow city streets would be the force of friction between the road and the car (its tires). This force is responsible for making the car speed up, slow down, and make turns. Motion, inertia, and acceleration are not forces. Remember that all forces are exerted by one object on another object.
In the figure, a hand is pushing on block B to the right, causing both blocks to move to the right across a rough surface at a constant speed. Compare the force that pushes block B to the right to the force that pushes block A to the right.
Answers: The force that pushes block A to the right is smaller than the force that pushes block B to the right. Explanation: The force that pushes block A to the right is smaller than the force that pushes block B to the right. The force that pushes block B to the right is exerted by the hand pushing on it. The force that pushes block A to the right is exerted by block B pushing on block A. The force from block B on block A has to counteract the force of kinetic friction that would otherwise make block A slow down. However, the force from the hand on block B has to counteract both the force of kinetic friction on block B and the force pushing backwards from block A on block B. Essentially, the force on block B from the hand must counteract the total friction exerted on both blocks, but the force on block A from block B only has to counteract the friction exerted on block A.
Newton's first law is a description of what basic property of our universe?
Inertia: Newton's first Law describes the concept of inertia, stating that an object cannot accelerate unless it is acted upon by a net force. In other words, as long as the net force on it is zero, an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction. The concepts of force, acceleration, velocity, and mass are important for understanding Newton's first law, but they are not its main point.
You are lying on your back on the slope of a grassy hill, watching the clouds go by overhead. Which is the best statement about the force of friction exerted on you by the ground in this situation?
Answer: A force of static friction pointing "up the hill", parallel to the surface of the hill, is applied to you by the ground. Explanation: As you lie on the slope of the hill, a force of static friction pointing "up the hill", parallel to the surface of the hill, is applied to you by the ground. Frictional forces always act in a direction parallel to the surfaces that are interacting. In this case, you are not sliding along the hill, so a force of static friction must be keeping you from sliding down the hill by pointing up the hill. Kinetic friction exists between surfaces that are in contact and moving relative to each other (i.e., sliding past each other).
A person is standing still while leaning against the wall in a classroom. Which is the best answer regarding the number of forces exerted on her?
Answer: At least four Explanation: There are at least four forces on her at this moment. The person is at rest, so the net force on her must equal zero. The four forces that we know are exerted on her are: 1) the force of gravity on her by the earth, 2) the normal force on her by the floor, 3) the normal force on her by the wall, and 4) a frictional force on her by the floor (which keeps her feet from slipping). They are each in different directions, allowing them to add up to zero (which must be true since the person is standing still). There may be other forces (such as friction with the wall), but the statement doesn't tell us much about how she is interacting with the wall. The friction with the wall could be pushing up or down on her, or even be zero, depending on the specifics of how she is leaning on it.
Michael leans on a shopping cart, causing it to start moving. Which statement is true about the forces Michael and the cart exert on each other?
Answer: Michael and the cart exert equal and opposite forces on each other. Explanation: As Michael leans on a shopping cart, causing it to start moving, Michael and the cart exert equal and opposite forces on each other. The interaction between Michael and the cart affects both objects, creating two forces that form an action-reaction pair. The forces in an action-reaction pair are always exactly equal in magnitude.
Miguel leans against a wall. Which statement is true about the forces Miguel and the wall exert on each other?
Answer: Miguel and the wall exert equal and opposite forces on each other. Explanation: As Miguel leans against a wall, Miguel and the wall exert equal and opposite forces on each other. The interaction between Miguel and the wall affects both objects, creating two forces that form an action-reaction pair. The forces in an action-reaction pair are always exactly equal in magnitude.
Acceleration can be expressed in units of m/s2 or __________
Answer: N/kg Explanation: Acceleration can be expressed in units of m/s2 or N/kg. Newton's second law can be rearranged to show that acceleration equals net force divided by mass. In other words, acceleration can be thought of as a measure of how strong the force must be, per kg of mass that the object has. If we make errors in manipulating Newton's second law we might come up with either "N kg" or "kg/N." If we mistakenly think the units for mass are kg2 and make an algebraic error, we might come up with "kg2/N."
Jack and Luis are pushing on opposite ends of a shopping cart. At the moment, the cart is staying at rest. We know that the forces each person exerts on the cart are equal in magnitude and opposite in direction. What is the reason this must be true?
Answer: Newton's second law tells us these two forces must be equal and opposite Explanation: As Jack and Luis push on the cart, Newton's second law tells us these two forces must be equal and opposite. The fact that the cart is not moving tells us that the net force on it must be zero. From that we can deduce that the two people must be exerting equal and opposite forces on the cart. Newton's third law cannot be applied to these two forces because they do not form an action-reaction pair (a.k.a., a force pair). The two forces are both exerted on the cart (by Jack and Luis, respectively) and they do not represent two aspects of the same interaction.
An arrow is in mid-flight on its way to a target. At this moment, what force keeps the arrow going forward?
Answer: No force is pushing the arrow forward. Explanation: As an arrow is in mid-flight on its way to a target there is no force pushing the arrow forward. The concept of inertia (Newton's first law) tells us objects will always keep doing what they have been doing unless a net force acts on them, forcing them to change their motion. Gravity and air resistance are real forces that the arrow experiences during its flight, but gravity pulls the arrow downward and air resistance pushes backwards against the arrow. The force from the bowstring was what originally accelerated the arrow, but once the arrow loses contact with the bowstring that force no longer exists.
If you know all of the forces acting on an object and you know the mass of that object, what quantities can you figure out from that information?
Answer: The acceleration of the object Explanation: If you know all of the forces acting on an object and you know the mass of that object, you can figure out the acceleration of the object. The core idea of Newton's second law is that the forces on an object determine that object's acceleration. Knowing the acceleration doesn't let you deduce what the velocity or position is at that moment, but the acceleration can tell you how the velocity will be changing in the immediate future.
Which of the following is a legitimate way to describe the force between a skydiver and the air she is falling through as the skydiver is falling toward the ground?
Answer: The drag force exerted by the air on the skydiver Explanation: The drag force exerted by the air on the skydiver is a legitimate way to describe the force between a skydiver and the air she is falling through as the skydiver is falling toward the ground. As the skydiver falls toward the ground she is pushed upward by the drag force from the air, also called the force of air resistance. Gravity is a legitimate force, but it is the force that the earth exerts on the skydiver. Tension is a legitimate force, but it refers to a force exerted by a rope (or similar object) on an object it is attached to. Friction is a legitimate force, but it is the force exerted by one object on another object as the two try to slide past each other. This is distinct from the drag force exerted by air (or any fluid) when an object moves through it.
Allison shoves her brother Brody away from her, causing him to fall down. Which of the following is an action-reaction pair (from Newton's third law)?
Answer: The force from Allison on Brody and the force from Brody on Allison form an action-reaction pair. Explanation: As Allison shoves Brody, the force from Allison on Brody and the force from Brody on Allison form an action-reaction pair. An action-reaction pair, described by Newton's third law, is a pair of forces that arise due to a single interaction between two objects. It is the two forces that form a pair, not the two objects that are interacting. There are force-pairs that exist between Allison and Brody, between Brody and the ground, and between Allison and the ground. But those forces were not described in the choices given.
Which of these is a valid force that could be exerted on a car racing through narrow city streets?
Answer: The force of friction between the road and the car (its tires) Explanation: A valid force that could be exerted on a car racing through narrow city streets would be the force of friction between the road and the car (its tires). This force is responsible for making the car speed up, slow down, and make turns. Motion, inertia, and acceleration are not forces. Remember that all forces are exerted by one object on another object.
Which of the following is a legitimate way to describe the force between a skydiver and the earth as the skydiver is falling toward the ground?
Answer: The force of gravity exerted by the earth on the skydiver Explanation: The force of gravity exerted by the earth on the skydiver is a legitimate way to describe the force between a skydiver and the earth as the skydiver is falling toward the ground. Regardless of its circumstances all objects on or near the surface of the earth experience the force of gravity from Earth. This force is a "long-range" force, which does not require physical contact to exist (unlike most macroscopic forces). Normal force is a legitimate force, but it describes the force exerted by a surface on an object touching that surface. Tension is a legitimate force, but it refers to a force exerted by a rope (or similar object) on an object it is attached to. Drag is a legitimate force, but it is the force exerted by a fluid on an object that is moving through the fluid (also called air resistance).
When trying to analyze the forces in a situation we sometimes invent forces that don't actually exist. Which of these forces is NOT a valid force that could be exerted on a sled being pulled along a snowy path by a girl?
Answer: The force of motion pushing the sled forward Explanation: The force of motion pushing the sled forward is NOT a valid force under any circumstances. Any object that interacts with the sled will create a force on the sled, but concepts (like motion) or other physics quantities (like acceleration) are not forces. All valid forces are exerted by one object on another object.
Blocks A and B have equal masses and are connected by a rope that passes over a frictionless pulley, as shown. At this moment, the system is at rest. Compare the magnitudes of the force of tension exerted on block A and the force of tension exerted on block B.
Answer: The force of tension exerted on block A is equal in magnitude to the force of tension exerted on block B. Explanation: As blocks A and B hang over the pulley, the force of tension exerted on block A is equal in magnitude to the force of tension exerted on block B. Since the rope that connects the two blocks is at rest, the force each block exerts on the rope must be equal in magnitude (using Newton's second law). If each block exerts the same magnitude force on the rope, then the rope must exert the same magnitude force on each block (using Newton's third law). It doesn't matter what is happening to either end of the rope. As long as the rope is not accelerating the tension will always be exactly the same at both ends of the rope.
Blocks A and B have equal masses and are connected by a rope that passes over a frictionless pulley, as shown. At this moment, the system is in motion, with block B moving downward and A moving upward at a constant speed. Compare the magnitudes of the force of tension exerted on block A and the force of tension exerted on block B.
Answer: The force of tension exerted on block A is equal in magnitude to the force of tension exerted on block B. Explanation: As blocks A and B hang over the pulley, the force of tension exerted on block A is equal in magnitude to the force of tension exerted on block B. Since the rope that connects the two blocks is moving at constant speed, the force each block exerts on the rope must be equal in magnitude (using Newton's second law). If each block exerts the same magnitude force on the rope, then the rope must exert the same magnitude force on each block (using Newton's third law). It doesn't matter what is happening to either end of the rope. As long as the rope is not accelerating the tension will always be exactly the same at both ends of the rope.
Stephanie pulls on a rope that is attached to a large crate. She is pulling the crate along at a constant speed as it slides across rough carpet. How does the magnitude of the force of tension exerted on Stephanie compare to the magnitude of the force of tension exerted on the crate?
Answer: The force of tension exerted on the crate has the same magnitude as the force of tension exerted on Stephanie. Explanation: As Stephanie pulls the crate along at constant speed, the force of tension exerted on the crate is equal in magnitude to the force of tension exerted on Stephanie. Since the rope is moving with a constant velocity, it has no acceleration. This means that the forces exerted on the rope by Stephanie and by the crate must be equal (using Newton's second law). Therefore, the forces of tension the rope exerts on Stephanie and on the crate will also be equal in magnitude (using Newton's third law). If the force of tension exerted on the crate were smaller in magnitude than the force of tension exerted on Stephanie that would mean Stephanie was pulling harder on the rope, which would make the rope accelerate to the left and speed up. This would, in turn, make the rope pull harder on the crate, forcing the two tension forces to once again be equal. If the force of tension exerted on the crate were greater in magnitude than the force of tension exerted on Stephanie that would mean the crate was pulling harder on the rope, which would make the rope accelerate to the right and slow down. This would, in turn, make the rope pull harder on Stephanie, forcing the two tension forces to once again be equal.
Which of the following is a valid force that could be exerted on a baseball?
Answer: The force on a baseball being hit by a bat Explanation: A valid force that could be exerted on a baseball is the force on a baseball being hit by a bat. Forces exerted on a baseball must be caused by some other object and act on the baseball. Forces exerted by the baseball are not forces exerted on the baseball.
In the figure, a hand is pushing on block B to the right, causing both blocks to move to the right across a rough surface at a constant speed. Compare the force that pushes block B to the right to the force that pushes block A to the right.
Answer: The force that pushes block A to the right is smaller than the force that pushes block B to the right. Explanation: The force that pushes block A to the right is smaller than the force that pushes block B to the right. The force that pushes block B to the right is exerted by the hand pushing on it. The force that pushes block A to the right is exerted by block B pushing on block A. The force from block B on block A has to counteract the force of kinetic friction that would otherwise make block A slow down. However, the force from the hand on block B has to counteract both the force of kinetic friction on block B and the force pushing backwards from block A on block B. Essentially, the force on block B from the hand must counteract the total friction exerted on both blocks, but the force on block A from block B only has to counteract the friction exerted on block A.
Blocks A and B are connected by rope 1 and are pulled at constant speed across a rough surface by rope 2, which is attached to block B, as shown. Compare the magnitude of the tensions in ropes 1 and 2.
Answer: The magnitude of the tensions in the two ropes will not be equal Explanation: As the two blocks are pulled along, the magnitude of the tensions in the two ropes will not be equal. The tension in rope 2 must be large enough to counteract the total force of friction on both block A and block B. The tension in rope 1, in contrast, need only be strong enough to counteract the force of friction on block A. Newton's third law cannot be applied to the tension in two separate ropes. Newton's third law only applies to a pair of forces that exist between two objects. These two tension forces involve two separate ropes and multiple other objects.
If you could somehow increase the mass of an object, what would happen to the net force on the object?
Answer: The net force on an object doesn't depend on the object's mass. Explanation: Forces on an object come from other (external) objects, so the net force on an object doesn't depend on the object's mass; there is no reason to expect that changing the mass of an object would change the net force on it. One force that does depend on an object's mass is the force of gravity, but even then the net force wouldn't be dependent on the mass of the object.
You are standing in an elevator as it starts to move downward, accelerating for a moment. Which of these is true at that moment?
Answer: The net force on you changes from zero to downward as the elevator begins accelerating. Explanation: The net force on you changes from zero to downward as the elevator begins accelerating. You move with the elevator, meaning that when it accelerates you do as well. If you are accelerating, then your net force must be in the same direction as the acceleration. Therefore, you go from having a net force of zero to having a downward net force (momentarily). Your mass never changes; it is a property of your body and not a property of your environment. Your weight does not change at this moment because the Earth still pulls on you (via gravity) just as hard as it was before. The motion of the elevator does not change the force of gravity on you. Because the elevator does not drop you, you do not have free fall acceleration.
Which of the following is a legitimate way to describe the force between a road and a car driving on the road?
Answer: The normal force exerted by the road on the car Explanation: The normal force exerted by the road on the car is a legitimate way to describe the force between a road and a car driving on the road. When an object makes contact with a flat surface of some kind, the contact force exerted on the object by the surface is called a normal force. The name "normal force" comes from the fact that the force is always perpendicular to the surface exerting it. Gravity is a legitimate force, but it is the force that the earth exerts on the car. Tension is a legitimate force, but it refers to a force exerted by a rope (or similar object) on an object it is attached to. Buoyant force is a legitimate force, but it is the force exerted by a fluid on an object that is immersed in it
Consider a car speeding up as it drives along a level road. Which of the following is an action-reaction pair (from Newton's third law)?
Answer: The normal force exerted on the car by the road and the downward force exerted on the road by the car. Explanation: As a car speeds up while driving along a level road, the normal force exerted on the car by the road and the downward force exerted on the road by the car form an action-reaction pair. An action-reaction pair is always a pair of forces that come about due to a single interaction. In this case, the interaction is between the car and the road. In each of the other examples, the two forces are not forces that come from a single interaction. For most of them, three different objects are involved. For all of them, it is possible to imagine a situation in which one of these forces exists without the other.
Consider a car driving at a constant speed on a level road. Which of the following is an action-reaction pair (from Newton's third law)?
Answer: The normal force exerted on the car by the road and the downward force exerted on the road by the car. Explanation: For a car driving at a constant speed on a level road, the normal force exerted on the car by the road and the downward force exerted on the road by the car form an action-reaction pair. An action-reaction pair is always a pair of forces that come about due to a single interaction. In this case, the car and road make contact with each other, creating both of the forces, which must be exactly equal in magnitude and opposite in direction. In each of the other examples, the two forces are not forces that come from a single interaction. For most of them, three different objects are involved. For all of them, it is possible to imagine a situation in which one of these forces exists without the other.
As you accelerate upward in an elevator, which force exerted on you has the greatest magnitude?
Answer: The normal force from the elevator floor pushing up on you Explanation: As you accelerate upward in an elevator, the normal force from the floor is the force on you with the greatest magnitude. Because you are accelerating upward, the net force on you must also be upward and the normal force from the floor must be stronger than the force of gravity from the earth. The question asks about forces that affect you, so any forces that are not exerted on you cannot be the answer. Other objects, such as the elevator motor, are not in contact with you, so you have no direct interaction with them.
Imagine standing in an elevator that is moving upward. Which of the following is the most complete list of the forces that belong on your free-body diagram (FBD) in that moment?
Answer: The normal force on you by the elevator floor and the force of gravity on you by the earth Explanation: The most complete list of the forces that belong on your free-body diagram (FBD) is the normal force on you by the elevator floor and the force of gravity on you by the earth. Free-body diagrams always focus on one object and they should only include forces exerted on that object. Forces exerted by the object in question that act on other objects should not be included. Aside from the force of gravity by the earth, only objects in direct contact with you can exert forces on you. Other quantities (such as acceleration) are not forces.
Imagine a car sitting empty in the parking lot. Which of the following is the most complete list of the forces that belong on a free-body diagram (FBD) of the car?
Answer: The normal force on your car by the pavement it rests on and the force of gravity on your car by the earth Explanation: For a car sitting empty in a parking lot, the forces that belong on a free-body diagram (FBD) of the car are the normal force on your car by the pavement it rests on and the force of gravity on your car by the earth. FBDs always focus on one object and they should only include forces exerted on that object by external objects (not by one part of the object on another part of the object). Forces exerted by the object in question that act on other objects should not be included.
Allison pulls her little brother, Brody, in a sled on a snow-covered path at a constant speed. Which of the following is an action-reaction pair (from Newton's third law)?
Answer: The pulling force that Allison exerts on the sled and the pulling force that the sled exerts on Allison. Explanation: As Allison pulls Brody in the sled, the pulling force that Allison exerts on the sled and the pulling force that the sled exerts on Allison form an action-reaction pair. An action-reaction pair is always a pair of forces that come about due to a single interaction. In this case, the interaction is between Allison and the sled. In all the other cases, the two forces do not come from the same interaction and, therefore, cannot be Newton's third law action-reaction pairs.
You are about to check out at the grocery store and you put a box of cereal on the conveyer belt at the register. The cashier turns on the conveyor belt, causing the box of cereal to speed up momentarily. At this moment, what can we say about the frictional force acting on the box of cereal?
Answer: There is a static frictional force pointing in the box's direction of motion, parallel to the conveyer belt. Explanation: As the box of cereal speeds up with the conveyer belt, there is a static frictional force pointing in the box's direction of motion, parallel to the conveyer belt. The frictional force is static because the box does not slip or slide on the conveyer belt. The direction of the static frictional force will be in the direction of the box's acceleration because the frictional force is acting against the natural tendency for the belt to slide underneath the box. The force of friction cannot be kinetic because there is no slipping or sliding between the box and the belt. The force of friction cannot point in the opposite direction of the motion because it is only the force of friction that makes the box accelerate with the conveyor belt. Friction is not trying to slow down the box.
Somewhere in the universe an object is moving in a slow arcing turn without changing its speed. From just this information, what can we say about the net force on that object?
Answer: There must be a net force on the object. Explanation: Somewhere in the universe an object is moving in a slow arcing turn without changing its speed. From just this information we can tell that there must be a net force on the object. The object is changing its motion (specifically its direction of motion), and Newton's first law (the law of inertia) tells us that the only way an object can change its velocity (its speed and direction) is for it to be acted on by a net force from external objects. If there were no net force on the object, it could not change any aspect of its velocity.
Under which circumstance is there a force exerted on your hand by the apple?
Answer: When the apple is at rest in your open hand Explanation: A force is exerted on your hand by the apple when the apple is at rest in your open hand. The force exerted on your hand by the apple is a contact force, so it can only occur when the two objects are in contact. When your hand is not in contact with the apple (as in the other two choices) the apple cannot exert a contact force on your hand.
Under which circumstance is a force exerted on an apple by your hand?
Answer: When the apple is at rest in your open hand Explanation: A force exerted on an apple by your hand occurs when the apple is at rest in your open hand. The force exerted on the apple by your hand is a contact force, so it can only occur when the two objects are in contact. When the apple is not in contact with your hand (as in the other two choices) your hand cannot exert a contact force on the apple. If we ignore any force from air resistance, then the only force exerted on the apple in the other two cases is the force of gravity from Earth.
Under which circumstance is there a force exerted on the apple by your hand?
Answer: When you are picking the apple up off of the floor Explanation: A force is exerted on the apple by your hand when you are picking the apple up off of the floor. The force exerted on the apple by your hand is a contact force, so it can only occur when the two objects are in contact. When the apple is not in contact with your hand (as in the other two choices) your hand cannot exert a contact force on the apple.