17 Momentum, N3L and suvat Flashcards
What effect does a cushion on the inside of a helmet have on the impact force felt by a cyclist if they collide with a windscreen?
- cushion increases the collision time
- F = (change in momentum)/ time
- Larger collision time for the same change in momentum
- decreases the impact force
- Less force, less injury
What is different about N3L pair forces?
- they act in different directions
- they act on different objects
What is the equation for impulse?
Is momentum a vector quantity?
Yes- it has both magnitude and direction
Use the conservation of momentum to explain why a cannon would have a recoil velocity when a cannon ball is fired.
pbefore = pafter
pbefore = 0
Therefore pafter = 0
Momentum of cannonball is positive after the collision
Momentum of cannon must be negative, cannon therefore moves backwards
- momentum’s cancel out*
If the action and reaction force in the image below are equal an opposite.
Can these forces cancel? Why does the small boat move backwards if these forces are equal and opposite?
The reaction force acts on the boat
The action force acts on the person
They cannot cancel!
Therefore the reaction force on the boat makes the boat move to the right.
Define momentum
The product of its mass and velocity
mass in motion!!!
Name the N3L pair force for the force named below
The water pushes up and forward on the swimmer
What effect does making a hammer of hard steel have on the impact force on a nail?
- hard steel hammer decreases the collision time
- F = (change in momentum)/ time
- smaller collision time for the same change in momentum
- increases the impact force
- more force, easier to drive in nail
Explain why the velocity of the cart decreases after the mass is dropped onto it.
momentum is conserved
p before = p after
If the mass of the system increases after the collsion the velocity must decrease to keep momentum the same.
Define force in terms of momentum
Force is the rate of change of momentum
What does the area of a force-time graph represent?
F t = change in momentum
This is called impulse
The area of a force-time graph is the impulse
An object undergoes a change in motion indicated as Case A and Case B.
In which case is the change in momentum the greatest?
Case A- Since mass is the same we can look at the change in velocity.
The change in velocity is found by using
(v-u)
(-3 m/s) - (5m/s)
= -8m/s
If a person on a skateboard pushes on a wall. Name the N3L pair force of….
Persons’ force on wall [left]
Walls’ force on person [right]
What is the relationship between momentum, mass and velocity?
p=mv
Momentum = mass x velocity
When a person runs they exert a back and downwards force on the road surface. Name the N3L pair of this force….
Runner pushes back and down on the road
Road pushed up forwards and up on the runner
What is the unit for momentum?
kgm/s
What is the total momentum before the explosion?
What is the total momentum after the collsion?
zero before and after!!
What effect does a crumple zone have on the impact force felt by a driver of a car?
- crumple zone increases the collision time
- F = (change in momentum)/ time
- Larger collision time for the same change in momentum
- decreases the impact force
- Less force, less injury
Is mass a vector quantity?
NO-it is a scalar quantity- it only has magnitude (You can’t cancel mass)
What effect does bending your legs when landing a parachute jump have on the force felt by the persons legs?
- bending legs increases the collision time
- F = (change in momentum)/ time
- Larger collision time for the same change in momentum
- decreases the impact force on the legs
- Less force, less injury
Name the N3L pair force
air exerts an upward force on the wings
What is the unit for mass?
kg
A person and an elephant are having a tug-o-war. They both pull with 500N.
If the person pulls on the rope with 500N to the left, name the N3L pair force and determine the tension in the rope?
The rope pulls on the person with 500N to the right
The tension in the rope is still only 500N! (It is no different if the rope is tied to a wall)
Is velocity a vector quantity?
Yes- it has both magnitude and direction
What is the unit for velocity?
m/s
Define impulse
Impulse is the change in momentum of an object
In the suvat equation:
v2=u2+2as
what do the symbols stand for?
What equation can we use which links initial and final speed, acceleration and distance travelled.
State Newton’s Third Law
If a object A exerts a force on object B, Object B exerts an equal and opposite force on object A.
What is the same about N3L pair forces?
- same magnitude
- same type of force
- they act along the same line
