Force & Motion Flashcards
Confused by free-body diagrams, pulleys, and inclined planes? Use these cards to see how all of these topics connect, and you can master force, motion, and gravity as tested on the MCAT.
Define:
force
It is the change in velocity per unit time that is experienced by a given mass. Force can also be thought of as a pull, push, or other action exerted upon an object.
Note that force can be positive or negative, depending on the direction of the velocity.
What quantity is measured in newtons?
- Newton (N) is the SI unit for force.
- 1 Newton = 1 kg*m/s2.
How is force related to change in momentum?
Force and change in momentum are directly proportional.
Change in momentum, or Δp, is equal to the product of force and time.
Describe:
Newton’s first law of motion
Also called the Law of Inertia, it states that an object in motion will continue with constant velocity unless acted on by a net force.
Similarly, an object at rest will continue to remain at rest until acted on by a net force.
What must be true about the acceleration of an object, if all forces acting on it cancel?
The object has zero acceleration.
Since all forces cancel, there is no net force and velocity will not change. No change in velocity means no acceleration.
In Newton’s second law of motion, what relationship exists between force, mass, and acceleration in?
Fnet = ma
Net force and acceleration are both vectors, and will be pointing in the same direction.
What change in force is required to make an object move with twice its original acceleration?
Twice the original force must be applied.
According to Newton’s second law, F=ma, net force and acceleration are directly proportional.
How does Newton’s third law of motion describe the forces between two objects?
F1on2 = -F2on1
For every force exerted by one object on a second, an equal and opposite force is exerted by the second back on the first.
An orange exerts a 5N force on an apple in free space. What magnitude of force must be exerted by the apple on the orange?
5N
From Newton’s third law, every force exerted must have an equal and opposite force. The negative sign is already factored in, since the question specified direction.
What formula is used to calculate gravitational force?
Fg = Gm1m2 / r2
Where:
G = gravitational constant (N*m2/kg2)
m1 and m2 = masses (kg)
r = distance between masses (m)
What change in gravitational force between two objects must take place for the distance between them to double?
Force must be decreased to one-quarter of its previous value.
Since F is proportional to 1/r2, doubling r will reduce the force by a factor of 4.
What change in gravitational force will occur between two objects if both the distance between them and the mass of each object halves?
No change in force will occur.
Since each mass is directly proportional to force, halving each mass will result in 1/4 the original force. But, since force is proportional to 1/r2, halving r will result in a force that is 4 times greater. The net change in F is the product of both factors: 4 (1/4) = 1, or no change.
On Earth, what convenient relationship can be demonstrated for the force on a mass due to gravity?
F = mg
Where:
F = force (N)
m = mass (kg)
g = acceleration due to gravity (9.8 m/s2)
What is the magnitude of the force acting downward on an apple with a mass of 0.2 kg on Earth?
2N
F = mg= (0.2)(9.8) ≈ 2N
Note that if the apple is resting on the ground, it will also be subject to an equivalent normal force pointing upwards.
Define:
weight
It is the term for the force on an object due to gravity.
W = mg
Weight is often confused with mass; an object with one weight on Earth will have a different weight on the moon, but its mass will remain constant.
What will the proportional weight of an object be on the moon, if the moon has 1/6 of Earth’s gravity?
The object will have 1/6 the weight it had on Earth.
Since W is proportional to the acceleration due to gravity, moving to an area with 1/6 g will produce 1/6 W.
How could an object’s percieved (or instantaneous) weight seemingly change?
It is the force pushing back on an object against gravity. This is a specific type of normal force.
A person might experience a decreased instantaneous weight if the ground falls away beneath them, such as in an elevator accelerating downwards.
How will the instantaneous weight compare for a person in an elevator that is accelerating upwards vs. at rest?
Instantaneous weight while accelerating upwards will be greater than at rest.
This happens because the upward acceleration adds to the force that the person experiences pushing up on him.
If the elevator were accelerating downwards, the person would feel less weight. In fact, if the elevator could accelerate downwards at 9.8 m/s2, the person would experience weightlessness.
Define:
normal force
When two objects are touching, an opposing force exists between the two objects and perpendicular to the surface in contact.
Often, on the MCAT, normal force opposes the force due to gravity.
What is the normal force for a book with a mass of 0.5kg sitting stationary on a table?
5N directed from the table on the book
The force pulling down on the book due to gravity is F = mg = (0.5)(10) = 5 N downwards on the table. Since the book is stationary, we know that normal force is equal and opposite; hence it must also be 5 N in magnitude.
What relationship exists between normal force and friction for a static object?
It is a force that opposes any two surfaces in contact from sliding over each other. The relationship involved is:
fs ≤ μsFN
Where:
fs = force of static friction (N)
μs = coefficient of static friction (unitless)
FN = normal force (N)
Describe what happens to a box if a person pushes on it horizontally with a force of 1/2 μsFN, μsFN, and 2μsFN, respectively.
- In the first case, no motion occurs. 1/2 μsFN will be exerted back on the box by static friction.
- In the second case, no motion occurs. μsFN will be exerted back on the box by static friction.
- In the last case, the box will accelerate forward and begin sliding. 2μsFN is greater than the maximum force with which static friction can resist motion.
What relationship exists between normal force and friction for a sliding object?
Kinetic friction is a force that opposes the motion of any two surfaces in contact that are already sliding relative to each other. The relationship involved is:
fk = μkFN
Where:
fk = force of kinetic friction (N)
μk = coefficient of kinetic friction (unitless)
FN = normal force (N)
Describe what would happen to an already-sliding box, if a person continues to push horizontally on it with a force of 1/2 μkFN, μkFN, and 2μkFN, respectively.
- In the first case, the box will eventually come to rest. 1/2 μkFN is less than the force of kinetic friction acting against the box.
- In the second case, the box will continue its sliding motion. μkFN will exactly cancel the force of kinetic friction against the box and no net force will exist.
- In the last case, the box will accelerate forward. 2μkFN is greater than the force of kinetic friction acting against the box.