Units and Vectors
Consider a person pulling a cart on the Earth's surface. According to Newton's 3rd law, the gravitational force of the Earth on the person is equal in magnitude to the gravitational force of the person on the Earth.
TRUE Newton's 3rd law states that every action will have an equal and opposite reaction.
What pieces of information do you NOT need to know in order to solve for the gravitational force felt by the Earth from a butterfly pollinating flowers? Select all that apply:
A. Universal Gravitational Constant B. Mass of the Earth C. Distance between the center of mass of the butterfly and the Earth
Which of the following scenarios best describes centripetal force due to friction?:
A Race house running a practice run on a race track.
Free Body Diagram (FBD)
A diagram that shows those forces that act upon an object/body.
Electromagnetic Force
A force that a charge will experience in an electric field. Force that a moving charge will experience in a magnetic field.
Electrostatistic force
A type of electromagnetic force between two charges that are not moving relative to each other.
What force needs to be applied in order to accelerate a 9 μg sample of blood at 50 m/s2?:
F = (9 x 10-6 g)(50 m/s2) = (9 x 10-9 kg)(50 m/s2) = 450 x 10-9 N = 4.5 x 10-7 N
Static and Kinetic Fiction are
the two non conservative forces
Force is measured in Newtons (N) and equal to the mass (kg) of an object multiplied by the acceleration (m/s2) of that object. Therefore, 1 N = (kg)(m / s2). Power is measured in (newton)(meter) / (second). Following standard SI notation, the units for power would be [(kg)(m/s2)(m)] / (s)= kg(m2 / s3).
(m/s) Force is measured in Newtons (N) and equal to the mass (kg) of an object multiplied by the acceleration (m/s2) of that object. Therefore, 1 N = (kg)(m/s2). Power is measured in (newton)(meter)/(second). Following standard SI notation, the units for power would be: [(kg)(m/s2)(m)]/(s) = kg(m2/s3). The question wants us to identify what units we need to multiply by a newton to get the unit for power. If we were to multiply (m/s) by a newton, we would get kg(m2/s3), which is the unit for power.
An object is experiencing a force that causes it to accelerate at 17 m/s2. Which of the following interventions will change its acceleration? Select all that apply:
- Newton's second law says that acceleration is directly proportional to force, or specifically to the net force. Changing the force by a factor of 1.5 (either increasing it or decreasing it) will cause the acceleration to change to a value higher or lower than 17 m/s2. - According to Newton's second law, F = ma, mass, and acceleration are inversely proportional (consider rearranging the equation as a = F/m). Changing the mass by a factor of 0.43 (either increasing the mass or decreasing it) will cause the acceleration to change, - Constant velocity implies zero acceleration, which is much less than the object's current acceleration of 17m/s2.
An emergency room physician is designing a new textile to be used for special pants and jackets in order to protect motorcycle riders involved in crashes from severe road rash. Which µk value for the coefficient of kinetic friction would you select in order to minimize the adverse effects of kinetic friction?:
0.05
An emergency room physician is designing a new textile to be used for special pants and jackets in order to protect motorcycle riders involved in crashes from severe road rash. Which of these values for the coefficient of kinetic friction would be the worst choice if you wanted to minimize the adverse effects of kinetic friction?:
1.2 Explanation: The force of kinetic friction is directly proportional to the coefficient of kinetic friction. Kinetic friction is the force opposing the motion of the rider as they slide across the highway in the event of a bad crash. The force of kinetic friction is responsible for tearing up the rider's clothes and ultimately their skin, which is known as road rash. In order to minimize these negative effects of kinetic friction, we want the smallest possible coefficient of kinetic friction between the material and the highway. This means we want to stay away from material that has a very high coefficient of kinetic friction with the highway. This is the largest value of the choices listed, so this answer choice gives the coefficient of kinetic friction we most want to avoid.
Your dog, still chasing after that bouncing rubber ball on the tile floor, suddenly goes sliding across the tile as she attempts to turn around and run back to you. Which of these forces is opposing the forward movement of your dog? (GOT QUESTION RIGHT)
ANSWER: DRAG FROM THE AIR AS YOUR DOG RUNS Explanation: Drag from an object moving through the air is known as air resistance. Air resistance is really just friction between the air molecules and whatever is trying to move past them. Air resistance opposes your dog's forward movement.
What will happen to a ball that is rolling forward along the ground, assuming that friction forces are present?: It will eventually slow to a stop
According to Newton's first law, in the absence of an external force, a body at rest will remain at rest and a body in motion will remain in motion with constant velocity. The question specifies the ball is rolling along the ground subject to friction, so there is an external force acting on the ball here. Because there is a net force acting on the ball, the force of friction opposing the ball's motion, Newton's second law tells us that the ball must be undergoing acceleration. In this case, its velocity is decreasing due to friction until it comes to a stop.
Spring Force
Can provide centripetal force: Centripetal force is any force that causes something to move in a circular or elliptical path. If you imagine attaching a ball to one end of a spring and whirling it around in a circle, the force of the spring pulling back in as the ball stretches it out would serve as the centripetal force. It also pushes back against compression and pulls back against stretching.
Spring forces are conservative forces.
Conservative forces are those that conserve energy and are what we call path-independent. That is, the total work they do only depends on the starting state and ending state of the object, and the path traveled from start to finish doesn't matter. The common conservative forces are gravity, electrostatic force, magnetic force, and spring force. On the MCAT, functionally we also treat general pushing and pulling forces - that is forces applied by a rope or a person pushing on a box, for example- as conservative forces.
Conservative Forces
Conserve Energy and Path Independent which means the amount of work performed will be the same regardless of the path taken. For example, to raise a ball from its initial height to its final height. Electrostatic, Gravitational, and Conservative Forces do not dissipate energy.
A patient in a rehab hospital wants to develop better core strength by pushing various objects from a stationary position across the floor of the physical therapy suite as quickly as possible. List the following objects in order from fastest to slowest acceleration when a 300 N net force is applied: a 9 kg medicine ball, a 1 kg exercise ball, a 17 kg stationary bike, and a 6000 g kettlebell: (GOT CORRECT_)
First: A 1 kg exercise ball Second: A 6000 g kettlebell Third: 9 kg medicine ball Fourth: a 17 kg stationary bike
Non Conservative Forces
Dissipate energy and Path-dependent in the amount of work they do: more energy will be lost if a car skidded to a halt over a windier, longer path. Friction, Air Resistance, and Viscosity can be a factor; in these cases: a moving object will slowly lose energy as it works against any of these non-conservative forces. For Example: If I were to take my foot off the car, my energy would continue to be lost as a result of friction between the tires and the road, and the car would have eventually slowed to a halt.
Your dog is chasing after a bouncing rubber ball and goes running across a tile floor. What are all of the forces acting on your dog?:
Drag from Air as your dog runs through it. The Normal Force Between the Floor and the Dog. The force of the Static Friction Between the Floor and Your Dog.
What is the gravitational force a 35 kg person standing on the Earth's surface exerts on the Earth? (The gravitational constant G = 6.67 x 10-11 m3/kg*s2)
Explanation: Gravity is an attractive force that exists between any two masses. Every piece of matter in the universe exerts some amount of gravitational attraction on every other mass. This means the Earth exerts an attractive force on us, and we also exert an equal and opposite attractive force on Earth. When discussing the gravitational attraction between the Earth and an object on its surface, we can calculate gravitational force as Force=(mass)(acceleration due to gravity) = (35 kg)(10 m/s2) = 350 N. The Earth exerts a 350 N force on this person and this person exerts a 350 N force on the Earth.
Spring Force
F = -kx This means the force of a spring depends on k, which defines the stiffness of the spring. x = how far the spring has been deformed from rest position.
Magnetic Forces Equations
F = iLB sin (current running through a wire, suspended in a magnetic field) or F = qvB sin (point charge moving through a magnetic field)
Non-Uniform Electric Field
F = k(Q1q2)/(r^2) This is called Coulomb's Law
Gravitational Force Equation
F = mg: Used for objects at or near the surface of the Earth: This is a modification of the second law in which G is the constant for gravitational acceleration, which is about 9.8 meters per second squared. F = (Gm1m2)/(r^2): m1m2 are the masses of the two objects exerting and experiencing the force, along with the universal gravitational constant, or Big G, whose value would be provided if you needed to use it. r^2 is the distance between the objects. F is the Gravitational Force and it is Directly Proportional to the mass of each object and inversely proportional to the square of the distance between them. This means the more massive the two objects are, or the smaller the distance between them, the stronger the gravitational force.
Uniform Electric Field
F = qE F is the force experienced by a particle with charge q in an electric field E.
Two objects exerting an equal and opposite force on each other will experience the same magnitude of acceleration because force and acceleration are directly proportional:
FALSE This statement is false. Two objects exerting equal and opposite forces on each other will experience different accelerations if their masses are different. This concept is represented by Newton's second law: Force=(mass)(acceleration). The accelerations will only be the same if the masses are equal. Because this question does not specify what the mass of each object is, we cannot assume that the acceleration will be equal.
Newton's Third Law
For every action force, there is an equal and opposite reaction force. The Force of Object A exerted on Object B, is equal to and opposite in sign, by Object B on Object A.
Newton's Second Law
Force = mass x acceleration
Which of the following forces does nonzero total work in the situation described?: (GOT CORRECT)
Friction is a non-conservative force, so the path an object takes from the starting point to the ending point does affect the total work it does. In this case, friction does negative work on the table as you slide it to the right (i.e., slowing it down), and more negative work as you slide it back to the left, so the total work done is negative, not zero.
hich of the following scenarios best represents friction in action?:
Friction is the force that resists movement and pushes back against the direction of movement any time an object is moving while in contact with another object. Because this motorcycle racer is in the middle of a turn, they are experiencing centripetal force. In this case, it is the force of friction of the racetrack against the motorcycle's tires that keeps the motorcycle moving in a circular path around the track. If you were to imagine a racetrack with a frictionless surface, the racer would instead slide straight off the track when they tried to make the turn. Friction is what typically provides the centripetal force for a vehicle to turn, so turns and circular motion are impossible without some other source of centripetal force.
Which of the following scenarios involves a NONconservative force? (GOT RIGHT)
Friction, Air Resistance, and Viscosity ARE FACTORS THAT AFFECT NON CONSERVATIVE. In this question, the sky reaches a constant, terminal velocity due to air resistance.
TWO ON MCAT ONLY:
GRAVITY FORCE AND ELECTROMAGNETIC FORCE
Which of the following can provide centripetal force? Select all that apply
Gravitational Force: Centripetal force is any force that causes something to move in a circular or elliptical path. The Moon being kept in orbit by Earth's gravity is an example of gravity providing the centripetal force. Friction: Centripetal force is any force that causes something to move in a circular or elliptical path, like a race car following a circular path around the curved parts of a racetrack; if the track is flat, this force comes from the friction present between the track and the wheels of the car. In this context, if there was no friction, there would be no centripetal force. Tension: Centripetal force is any force that causes something to move in a circular or elliptical path. Tension can provide centripetal force when a weight on a string is spun around in a circle, for instance. Normal Force: Centripetal force is any force that causes something to move in a circular or elliptical path. The normal force from the walls of a rapidly spinning centrifuge or washing machine are an example of normal force providing the centripetal force - if the contents of a centrifuge were able to pass through the outer wall, they would go flying straight out of the machine, but the normal force preventing them from passing through the wall as they try to escape is what keeps them in a circularly spinning path.
For Example
If a sky driver is in free fall, there is conservative force of gravity, pulling down on the skydiver. Non-Conservative Force of air resistance pushing up, slowing the rate of descent. At the start of the jump, the forces are unbalanced: The force of gravity is larger than that of air resistance, resulting in a net downward force. Whenever there is a net force, acceleration must be occurring, so our skydiver starts accelerating downwards. Opening the parachute causes air resistance, to increase significantly, and the skydiver slows down and reaches a new equilibrium. If the force of gravity pulling down on the sky driver is equal in magnitude to the air resistance slowing the skydiver's descent, then there will be no net force. Newton's Second Law: No Net Force means that the sky driver is no longer accelerating toward the Earth. Newton's First Law: The sky driver stops falling; rather simply that the sky driver is now falling at a steady and much safer velocity.
What force will need to be applied to start sliding a 60 kg patient from rest in one bed to the next if the coefficient of static friction is 0.75?
In order to determine how much force needs to be applied, we need to calculate the force that must be overcome. Since we are concerned about starting to move the patient from rest, we need to overcome the force of static friction. We can use the following equation: Fstatic = μN. Fstatic is the force due to static friction, μ is the coefficient of static friction, and N is the normal force. Fstatic = μN = (0.75)(60 kg)(10 m/s2) = (3/4)(600 N) = (1800 N/4) = 450 N If the patient is going to be moved, this 450 N force of static friction must be overcome, so physicians and nurses moving the patient must apply a force greater than 450 N.
What will happen when you stop pushing a moving grocery cart in the absence of friction?
In the absence of external forces, like you pushing the cart or the force of friction opposing the cart's motion, a body at rest will remain at rest and a body in motion will remain in motion with constant velocity. This is Newton's first law, the law of inertia. Velocity will be constant because no net force means there can be no acceleration. This concept is represented mathematically as force = (mass)(acceleration), also known as Newton's second law.
Centripetal force is any force that causes something to move in a circular or elliptical path by pushing or pulling it toward the center of the circle:
In this case, the frictional force of the race track on the horse's hooves as it pushes against the dirt provides the centripetal force allowing the horse to turn. (If friction were not present here, like on a slippery, iced-over track, the horse would instead go sliding off the track when it tried to make the turn.)
Which of the following will cause the normal force between a cat and the surface it is sitting on to decrease?
Increasing the angle of incline for the surface the cat is sitting on to 15 degrees. Explanation: The normal force is the force exerted between two surfaces in contact, at an angle perpendicular to those surfaces, that prevents them from passing through each other. If the surface the cat is sitting on is horizontal, the normal force must be equal to the force of gravity on the cat, since gravity pulls the cat perpendicularly into the surface. However, if the surface is at an angle, the force of gravity does not point perpendicularly to the surface; you could imagine splitting it into two components, one pointing perpendicularly into the surface and the other pointing parallel to the surface, and only the component pointing into the surface will be opposed by the normal force. At such an angle of incline, the component of gravity pointing into the surface (which will have the same magnitude as the normal force) will be equal to (mass)(acceleration due to gravity)(cosθ). Because cosθ is equal to 1 at no incline and is less than 1 at any nonzero angle of incline up to 90°, the normal force will decrease when the angle of incline is increased. Note that at 90°, the normal force goes to zero; this is the case where the surface is completely vertical, so it doesn't support the cat at all!
Which of the following will cause the force of friction to increase?:
Increasing the coefficient of friction and Increasing the weight of the object.
An object is pushed forward by a constant force which accelerates it at 17 m/s2. Which of the following interventions will keep the object's acceleration at 17 m/s2? Select all that apply.
Increasing the force applied by a factor of 2, and increasing the mass of an object by a factor of 2. Explanation: According to Newton's second law, F = ma, an object's acceleration is directly proportional to the net force acting on it but inversely proportional to its mass (this is perhaps more intuitive if rearranged as a = F/m). If the force is doubled but the mass is also doubled, the acceleration will remain unchanged.
Newton first law
Law of Inertia: in the absence of an external force, a body at rest will remain at rest and a body in motion will remain in motion. Interia = resistance to acceleration; moving objects want to just keep going in a straight line. More Massive = More Interia it has that is why it is hard to push a heavy dresser across the floor than a small side table.
Which of the following is acceleration inversely proportional to?:
Newton's second law says that acceleration is directly proportional to force, or specifically to the net force, and inversely proportional to the mass. This is represented by the equation Force=(mass)(acceleration), or alternatively, acceleration = force/mass.
What force needs to be applied in order to accelerate a 9 mg sample of blood at 40 m/s2?
Newton's second law says that acceleration is directly proportional to the net force. This is expressed by F = ma, where "m" represents mass "a" represents acceleration. For our answer F to come out in newtons (kg*m/s2), "m" must be in kilograms and "a" must be in m/s2. F = (9 x 10-3 g)(40 m / s2) = (9 x 10-6 kg)(40 m / s2) = 360 x 10-6 N = 3.6 x 10-4 N
Which of the following will increase an object's acceleration when subjected to a force? Select all that apply:
Newton's second law states that acceleration is directly proportional to force, or specifically to the net force. This is expressed by the equation force = (mass)(acceleration), or alternatively, acceleration = force/mass. Decreasing the mass by half will cause acceleration to double. Newton's second law states that acceleration is directly proportional to force, or specifically to the net force. This is expressed by the equation force = (mass)(acceleration), or alternatively, acceleration = force/mass. Increasing the force will cause the acceleration to increase. Newton's second law says that acceleration is directly proportional to force, or specifically to the net force. This is expressed by the equation force = (mass)(acceleration), or alternatively, acceleration = force/mass. These two effects cancel each other out to some extent, since decreasing the mass will cause an increase in acceleration while decreasing the force will cause a decrease in acceleration, but decreasing them by different factors will still result in the acceleration changing. In this case, decreasing the mass by more than you decrease the force will result in an increase in the acceleration, so this answer is correct.
Will the acceleration of a mobile X-ray machine be constant if a constant, nonzero net force is applied to push it between inpatient rooms in the hospital? Why?: Yes. According to Newton's Second Law, Force = Mass times Acceleration, so acceleration will be constant when a constant net force is applied.
Newton's second law states that acceleration is directly proportional to the net force applied. If a constant net force is applied to the machine, its acceleration will be constant as well.
Kinetic Fiction
No matter how fast an object is moving, kinetic friction will always be the same. Formula: Fk = uN It depends on the coefficient of friction so the slipperiness of the surface which is u and the normal force which is N. Kinetic Friction does not go up if you push harder.
Newton
SI unit of force is 1N = 1kg m/s^2
Which of the following will cause the coefficient of static friction to increase?:
Select Materials With More Textured Surfaces Explanation: The coefficient of friction is unitless, and as you can probably imagine, depends on the material. A rough piece of sandpaper has a much higher coefficient of friction than that of, say, a freshly resurfaced ice-skating rink.
Some biological proteins have a natural ability to be stretched and compressed in response to certain forces acting on them. Which of the following forces best describes this phenomenon?:
Springs exert a force by being either stretched or compressed, causing them to push back or pull against whatever compressed or stretched them. "Stretching" and "compression" are key terms that should make you suspect a spring force is involved, and this type of force indeed matches the description of the proteins in this question being stretched and compressed in response to a force.
At a molecular level, tension is caused by which type of force?
The electromagnetic force is the fundamental force at work when tension is transmitted through an object. On the molecular level, the electromagnetic interactions between the protons and electrons of the atoms making up the rope are what cause the rope to hold together when pulled on and transmit force from one end to the other.
A force is something that makes something accelerate, if you push or pull on something for it to accelerate, the thing has to have mass: mass is the kilogram part of the newton.
UNDERSTOOD PERFECTLY
What is the coefficient of kinetic friction between the shoe covers used in the operating room and the tile floor, if an 80 kg surgeon goes sliding across the floor at a constant velocity when acted upon by a force of 2300 N?
We can calculate the coefficient of kinetic friction using the equation Fkinetic = μN. The surgeon is sliding at constant velocity, which means they are not accelerating, so there is no net force. No net force means that the forward force causing the surgeon to slide must be perfectly balanced by the force of kinetic friction, which opposes the surgeon's motion. Because of this, we can set Fkinetic = 2300 N. Fkinetic = μN 2300 N = μ (80 kg)(10 m/s2) 2300 N = μ (800 N) 2300 N / 800 N = μ ~3 = μ
What is the gravitational force a 75 kg person standing on the beach exerts on the Earth? (The gravitational constant G = 6.67 x 10-11 m3/kg*s2):
We can simply estimate the value of this force based on our knowledge of the acceleration experienced by objects at the surface of the Earth (10 m/s2). Explanation: Force = (mass)(acceleration due to gravity) = (75 kg)(10 m/s2) = 750 N. The Earth exerts a 750 N force on this person and this person exerts a 750 N force on the Earth
Which of the following are NOT an underlying force of tension? Select all that apply.
While gravitational force may be involved in contributing to tension for objects suspended off the floor, it is not the underlying force behind the tension.
Gravity Force
an attractive force that exists between any two masses.
Newton's First Law
an object at rest will stay at rest, and an object that is moving will stay moving unless disturbed by an unbalanced force.
Electromagnetic Force
express different ways at the macroscopic level. 1. Normal Force: This is the force exerted between two contacting surfaces at an angle perpendicular to those surfaces that prevents an object from falling through the surface it rests upon due to gravity. 2. Friction: This is the force that resists movement and pushes back against the direction of movement any time, that any object is moving while in contact with another body. 3. Tension: A pulling force exerted by a string or a rope. that allows a pulling force to act at a distance. 4. Centripetal Force: Force that causes something to move in a circular or elliptical path. 5. Spring exerts force by either stretched or compressing, causing them to push back compressing or stretching.
Static Fiction Forumla
f = ùN F is the coefficient of friction and it is unit less. N = Normal Force: this is the force perpendicular to the surface upon which the object rests.
Friction
is called electrostatic repulsion, between the object and the surface it is on. It is resistant to movement In order for there to be friction, there needs to be an applied force. It is a non-conservative force, it is path-dependent: it depends on its path which means more energy will be lost to friction as heat and noise if you follow a long winding path rather than a short, straight one.
