2. Mechanics Part 1

Question 1

Explain Newtons first law.

Answer 1

Newtons first law is the law of inertia. This explains that an objects velocity will not change, unless a net force acts on it.

Inertia = an objects natural resistance to change its state of motion.

Question 2

What is Newtons second law?

Answer 2

Fnet = ma

When Fnet = 0, acceleration = 0 which reiterates Newtons first law of inertia.

Question 3

What is Fnet?

Answer 3

The sum of all forces acting on an object. A force exerted by the object itself is not considered in Fnet.

Question 4

t or f, if Fnet = 0, than velocity is 0.

Answer 4

False, acceleration is 0 which means velocity is constant.

Question 5

What is the unit for force?

Answer 5

Recall, Fnet = ma = Kg m/s^2

This unit is called a Newton (N)

Question 6

What is Newtons third law?

Answer 6

Forces exist in action-reaction pairs where each force has equal magnitude exerted in opposite directions.

“For every action there is an equal and opposite reaction”

Note: the two opposing objects must be acting on each other to satisfy this law (not on a third object).

Question 7

If a 100kg skater standing still pushes a 50kg skater with 50N of force, what happens?

Answer 7

The 50kg skater experiences 50N of force.
The 100kg skater experiences -50N of force (third law)

The 50kg skater will accelerate more than the 100kg skater (second law, F = ma)

Question 8

What is the formula for weight? What is Newtons law of gravitation? What is a consequence of these two formulas?

Answer 8

w = mg (g = 10m/s^2)

Newtons law of gravitation - all objects in the universe exert a gravitational pull on all other objects.

Fg = w = G(mM / r^2)

We can set these two equations to equal each other. One of the masses cancels out. We do this to find the value of g when we are not on earths surface.

Question 9

t or f, in the formula, Fg = w = G(mM / r^2), r is to the center of each object.

Answer 9

True (i.e. r = the radius of each object + the distance between them)

Question 10

What is static and kinetic friction?

Answer 10

If there is no relative motion between two objects, they experience static friction.
If there is relative motion between two objects, they experience kinetic friction.

Question 11

t or f, a rolling object experiences static friction.

Answer 11

True, since its not sliding along another surface.

Question 12

t or f, the normal force (Fn) is the perpendicular component of the contact force exerted by a surface onto an object.

Answer 12

TRUE, it does not have to just oppose gravity

however, normally speaking
Fn = Fyg
i.e. the normal force opposes the y-dimension of gravity

Question 13

What is the formula for the force of kinetic friction?

Answer 13

Ffk = μk x Fn

μk = the coefficient of kinetic friction
Fn = normal force  

note that this is just a magnitude equation (the direction of friction is always opposite the movement of an object)

Question 14

What is the maximum coefficient of static friction?

Answer 14

Static friction opposes movement of an object. Thus, μs is the maximum coefficient of static friction that will prevent movement from occuring

Question 15

t or f, μk > μs.

Answer 15

FALSE, μs > uk

think: its harder to get something to move then to keep it moving.

Question 16

What is the formula for static friction?

Answer 16

Ffs = μs x Fn

Note however, this only gives us the maximum Force of static friction.

essentially, static friction matches the necessary force to prevent an object from moving up until a maximum. we can only calculate this maximum.

Question 17

Estimate the squareroot of 2 and 3.

Answer 17

squareroot of 2 = 1.4

squareroot of 3 = 1.7

Question 18

If an object is sliding down an inclined plane, we must split the gravitational force into its x and y components (one downwards, and one perpendicular the plane). How do we find these values?

Answer 18

Remember that Sine Slides down the Slope.

to calculate the Fg parallel with the incline, we take mg x sin0
to calculate the Fg perpendicular with the incline, we take mg x cos0

Question 19

For an inclined plane question, how do we know what theta is?

Answer 19

Theta will always equal whatever the incline to the plane is (it just so happens to work out this way because of geometry).

So if a crate slides down a 30 degree plane, use 30 degrees to calculate Fgx and Fgy.

Question 20

How do pulley systems work?

Answer 20

Pulley systems decrease the amount of force needed to lift an object.

pulley systems multiple the tension force by the number of of strings / ropes pulling upwards on the object (not the number of pulleys).

If we have two pulleys, we require half the force to lift the object.