Force and Newton's Law

electrostatic force

A type of electromagnetic force between two charges that are not moving relative to each other.

Which of the following statements are true? Select all that apply. A. A bird exerts gravitational force on the Earth just as the Earth exerts gravitational force on the bird. B Only more massive object will exert gravitational force C. The gravitational force we exert on the Earth is equal in magnitude to the gravitational force the Earth exerts on us. D. Gravitational force only applies to object with mass.

A. A bird exerts gravitational force on the Earth just as the Earth exerts gravitational force on the bird. ->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. C. The gravitational force we exert on the Earth is equal in magnitude to the gravitational force the Earth exerts on us. The magnitudes of the gravitational forces between two objects are equal, although the direction of these forces will be opposite. This may seem unintuitive, but is given by the expression: Fgrav= G(m1m2/r^2) Note that in this expression, reversing which object you call "mass #1" and which one you call "mass #2" will not change the magnitude of the force, since it's always proportional to the product of both masses. In other words, the math backs up the notion that the force of gravity that object 1 exerts on object 2 is always equal in magnitude to the force object 2 exerts on object 1. Note also that the acceleration experienced by two objects attracting each other need not be the same, since their acceleration depends on both the force and their individual masses (a = F/m). This is the case between the Earth and a human body, in which the Earth is far more massive and only accelerates imperceptibly while the human body accelerates at ~10 m/s2 towards the Earth. D. Gravitational force only applies to objects with mass. Gravity is an attractive force that exists between any two masses. The amount of gravitational force exerted is dependent on mass. In cases where there is no mass, there is no gravitational force.

Which of the following can provide centripetal force? Select all that apply. A. Gravitational force B. Friction C. Tension D. Normal force

A. 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. B. 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 C. 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. D. 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.

Which of the following events is due to tension? A. The moon orbits the Earth B. A bike coasting along flat ground eventually comes to a stop C. A dog on a leash pulls a person a skateboard from a dead stop to a final velocity of 5 m/s. D. A patient's leg is elevated using an inclined wedge to raise their foot above their heart.

A. The moon orbits the Earth: The moon orbits the Earth is due to the centripetal force caused by gravity. B. A bike coasting along flat ground eventually comes to a stop. A bike coasting along flat ground will eventually come to a stop due to the force of friction opposing the bike's motion. There is no indication that tension is at all involved in slowing this bike because the answer choice does not say the bike is tied to anything that could be slowing it down. C. A dog on a leash pulls a person a skateboard from a dead stop to a final velocity of 5m/s. When the dog moves ahead of the skater, it creates tension in the leash. This forward force from the dog is transmitted back to the skater via tension in the leash, therefore causing the skater to move forward as well. D. A patient's leg is elevated using an inclined wedge to raise their foot above their heart. The patient's leg is elevated due to the normal force applied form the wedge to the patient's leg where it is in direct contact with the wedge.

Which of the following scenarios best represents friction in action? A. A flaming ball attached to a chain is swung around in a circle. B. A patient is safely moved from the operating table to a gurney using an intricate pulley system. C. A drink sits on a perfectly level table D. A motorcycle racer makes a sharp turn around the track.

D. A motorcycle racer makes a sharp turn around the track. 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.

Newton's Third Law

Every action has an equal and opposite reaction

True or false: Normal force only exists in the presence of gravitational force.

False, Any object in contact with and exerting a force on surface will generate a reaction force equal to that force and perpendicular to the surface - this is called the normal force. The reason an object is exerting a force on a surface doesn't need to be gravity - for instance, a car crashing into a solid wall will experience a normal force from the wall.

True or false: 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, 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.

Newtons Law

For every action there is an equal and opposite reaction

Newton's Second Law

Force equals mass times acceleration (f=ma)

Which of the following are examples of electromagnetic force exclusively?

Friction: Friction is a manifestation of electromagnetic force. This 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 body. This force is electromagnetic since it fundamentally results from electromagnetic interactions between the atoms of the objects in contact with each other. Tension: Tension is an example of an electromagnetic force. Tension is a pulling force exerted by a string or a rope that allows a pulling force to act at a distance. Essentially the force is applied to one end of the rope and is transmitted undiminished throughout the entire rope, allowing the force to act at a different angle, or along a path more complicated than a simple straight line. This force is electromagnetic since, fundamentally, the electromagnetic interactions between the atoms of the string are what hold it together. Normal force: Normal force is an example of electromagnetic force. Normal force is the force exerted between two contacting surfaces, at an angle perpendicular to those surfaces, that prevents them from passing through each other. For example, if your cat is sitting on your computer keyboard, the keyboard would be exerting a normal force upward on the cat. This force is electromagnetic since it fundamentally results from electromagnetic interactions between the atoms of the two surfaces being brought into contact.

Which of the following are true about spring force? Select all that apply.

It 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. Spring force pushes back against compression and pulls back against stretching. Springs and other elastic materials exert a force by being either stretched or compressed, causing them to push back or pull against that compression or stretching.

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 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 friction due to friction until it comes to a stop.

What will happen when you stop pushing a moving grocery cart in the absence of friction?

The cart will continue to roll at a constant velocity indefinitely. 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.

Newton's First Law

an object at rest will stay at rest, an object that is moving will stay moving unless disturbed by an unbalanced force.

Friction

caused by the electrostatic repulsion between the object and the surface it is on

What are the units for force?

kg(m/s^2) Force is measured in newtons. Force is calculated as mass multiplied by acceleration, so the units for a newton must be equal to kilograms (the units for mass) multiplied by m/s2 (the units for acceleration).

Which of the following is acceleration inversely proportional to?

mass 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.

electromagnetic interactions

mutual force interactions between particles with electric charge

Force

push or pull that causes an object to accelerate. measured in newtons

True or false: 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, newtons 3rd law states that every action will have an equal and opposite reaction.