Moments And Momentum Flashcards
What is the definition of centre of mass
The centre of mass is defined as the point on an object where all the mass acts through.
Where is the centre of mass in a regular shaped object
The centre
Describe a practical to find the centre of mass of an irregular shaped piece of card
Hang a pendulum from the cork. Place the pin in a hole in the sheet, then put it into the cork. Allow the card to rest, without touching it, so it can be used to measure. Draw a line on the card to show the position of the string behind the card. Repeat this 3 times, then view the point at which the lines intersect. This is the centre of mass of the card.
Why does an object topple
The centre of mass/gravity acts outside the base of the object
What factors make an object less stable (2)
- A smaller base
- A higher centre of gravity
When does toppling happen relative to moments
When the clockwise and anticlockwise moments are not balanced
Centre of gravity vs centre of mass
Centre of gravity depends upon the gravitational field, but centre of mass does not.
What is the principle of momentum
The momentum before and after a collision gets conserved unless an external force acts on the system
What is the principles of momentum formula
Pbefore = Pafter
M1U1=M2U2=M1V1+M2V2
M - mass
U - initial velocity
V - final velocity
What are the three types of collisions and what do they mean
Elastic - momentum and kinetic energy are conserved
Inelastic - momentum is conserved, but kinetic energy is not conserved
Explosion - momentum is conserved, initial momentum is always zero. Kinetic energy is conserved.
What is the definition of momentum
A conceptual measure of how difficult it is to stop an object, taking into account mass and velocity to find out the energy required to stop it
What is the equation for momentum
Momentum = mass x velocity
What are the units for momentum
kg m/s (Kilograms metres per second)
Is momentum a vector or a scalar
Vector
When does momentum change
If a force acts on a body which is moving or is able to move
Is kinetic energy a vector or a scalar
Scalar
What is:
1. Scalar x scalar
2. Scalar x vector
3. Vector x vector
- Scalar
- Vector
- Scalar
What is the definition of the moment of a force
The size of the force multiplied by the perpendicular distance to the pivot
What is the equation for moments
Moment = force x perpendicular distance to the pivot
What are the units for moments
Nm (newtonmeters)
What is the sum of the clockwise moments and the anticlockwise moments in equilibrium
They are equal
What is the moment size if the force is on the pivot
0Nm
What is the moment size if the force is on the pivot
0Nm
Are moments scalar or vector
Vector
What is the definition of impulse
A measure of the change of momentum
Does impulse look at the whole system or just one object
One object only e.g. one car in a multi-car crash
What are the units for impulse
KG m/s OR Ns (newtonseconds)
What letter is used to represent impulse
ΔP (change in momentum)
What is one equation used for impulse
ΔP = M(V-U)
M - mass
V - final velocity
U - initial velocity
What equation links force, impulse and time
ΔP/ΔT = F
Note that ΔT is the same as just using T
What are 3 safety features in a car
- Seatbelt
- Airbag
- Crumple zone
How do car safety features protect humans in the event of a crash? Use an equation to aid your answer
They increase the time taken for the momentum of the person to reach zero, so decreases the force on them using the equation:
ΔP/ΔT = F
