Physics momentum, work/energy, power

Momentum

how difficult it is to bring a moving object to a stop

What is the variable for momentum?

p (lowercase)

units = kg m/s (distance has to be meters and mass has to be kg)

What is the equation for momentum?

p=mv (mass times velocity)

• an objects momentum depends on its mass and velocity

Is momentum a vector?

yes

• will be the same direction as velocity

• have to state which direction the momentum is going

• pay attention to whether or not the velocity is neg or + (up and usually right= +)

How do you convert cm—> m and g—> kg?

• 100 cm in 1 m - move decimal two times to the left

• 1000 g in 1 kg - move decimal three times to left

Law of conservation of Momentum

• total momentum of objects before the collision= total p after collision

• ref table= p before= p after

• fill in mv for each p depending on the type of collision

elastic collision

when two objects hit each other then bounce off one another (no loss of energy)

inelastic collisions

two objects hit each other and stick together

ex. a baseball player catching a ball (starts off as two separate objects then combine into one)

explosion

• a single object splits apart into multiple pieces

• ex. softball player throwing a ball (starts as one entity then splits into two)

elastic collisions equation

pi=pf —> p1+p2 = p1+p2 —> mv+mv=mv+mv

• starts off as two objects/ 2 momentum’s and ends as two objects

example of elastic collisions problem

1 15kg ball going to the right at 3.5 m/s collides with a 22kg ball going to the left at 4 m/s. The first ball’s velocity after the collision is 5.4 mls. What is the velocity of the second ball after the collision?

• make the ball going to the left negative (the velocity)

• the first ball after the collision is going to the left now because its elastic (make velocity neg)

• plug in and you get 2.07 m/s for the second balls velocity

• got a positive number so the ball is going to the right after the collision

inelastic collision equation

p+p= p —> mv+mv= (m1+m2) v

• starts off as 2 separate objects then combines into one so we add their masses

• it is just mv not mv+mv after the collision (right side of equation) because it is only one object after collision

explosion

p=p+p —> (m1+m2)v = mv+mv

• starts as one, ends as two

• inelastic but flipped

Impulse

an objects change in momentum

impulse equation

Ft= change in p (momentum)

change in p= m(change in v)

What is the variable for impulse?

J (uppercase)

• units= kg m/s (same units as momentum because it is a change in momentum)

Work

process of moving an object by applying a force

equation for work

W=Fd (force times distance)

• force is in newtons (also weight)

• mass is in kg

what is the unit for work?

J (joules)

• variable is just W

When is work “done” on an object?

• only if it moves in the direction your applying the force

• direction of displacement= direction of force

• ex. of no work done - a waiter carrying a tray (tray moving side to side, force pushing up)

How much work is done to lift a 10kg box 5m at a constant velocity?

W=Fd d=5m

• applying upwards force of # and the force net is zero so the applied force upwards has to equal the Fg (weight/ the force due to gravity)

• fg=mg (10)(9.81)

• F= 98.1

• then plug in

components with work

if something is being pulled at an angle and you have to find the amount of work done

• find the component (x or y) that is going in the direction of the object

• if its being pulled horizontally, find x component of the force and only use that

How do you calculate work done on a graph?

• find area underneath the line on the graph

• may have to split into square and a triangle

Power

the rate at which work is done

equation for power

P=w/t —> substitute work P=fd/t

• ref table - p=fv (change in velocity) - different way to do it