Mechanics Flashcards
What is Newton’s first law?
An object will remain at rest or constant velocity unless a resultant force is acting on it.
What is a moment?
The turning force of an object about a pivot.
Or
A turning moment is the force x perpendicular distance of the force from the pivot.
How is a moment calculated?
Moment = Force x Distance (perpendicular)
What is the relationship between clock and anti-clock moments when an object is at equilibrium?
sum of anti-clock moments = sum of clock moments
Where does weight act through on an object?
Its centre of mass.
For an object on pivot to be in eqilibrium, what relationship should all forces acting on it have?
Sum of all moments (clock and anti-clock) around pivot must equal 0.
Sum of all vertical forces must equal 0.
What is a torque?
When a couple, a pair of equal and opposite forces acting on different lines, produce a turning moment it is called a torque.
How do you calculate torque?
T = Fd Torque = Force x distance
What is stable equilibrium?
If the object is displaced, it will return to equilibrium when released.
What is unstable equilibrium?
If an object is displaced, it will move further away from equilibrium when released.
What is Newtons 2nd law?
Force is equal to the rate of change of momentum. F = ma
What is Newton’s 3rd law?
For every action there is an equal and opposite reaction.
What is kinetic energy?
The energy due to the motion of an object.
What is potential energy?
The energy due to position. As an object rises it gains potential energy.
When does ½mv² no longer hold for finding kinetic energy?
At relativistic speeds (close to the speed of light)
When does mgΔh not hold for finding potential energy?
When the gravitational field is not uniform
What is the equation for power (mechanics)?
P = w/t P = Fd/t P = Fv
What is the equation for efficiency?
Useful power output / total power input
How does a rocket engine work?
The rocket engine is decreasing in mass but the momentum is staying the same so the velocity must increase.
What is an elastic collision?
A collision where kinetic energy is conserved
What is an inelastic collision?
A collision where kinetic after collision is less than before the collision.
What are the 8 energy stores?
Thermal Electrostatic Chemical Kinetic Magnetic Elastic potential Nuclear Gravitational potential
What are the types of energy transfer?
Mechanical (work done)
Radiation (waves)
Electrical
Heating
What’s the difference between power, work done and energy?
Work done is energy transferred.
Power is rate of energy transferred.
Energy is the capacity of an object to do work.
Why do seatbelts work?
F = (mv - mu) / t
So increasing time during the change in p reduces the force exerted on the person.
What is the conservation of momentum?
Momentum is always conserved in a closed system.
Momentum before event = momentum after event
How can efficiency be improved?
Use oil on moving parts
Fluorescent lamp instead of bulb
Streamline
Insulate
What happens in terms of forces when a powered object increases its speed?
It’s output force exceeds the resistive force.
How do you find the speed of trolley collision with a block just before impact?
Place a piece of card on the trolley.
Use a light gate just in front of block and find the time it was obscured.
Find v using:
v = length of card / time
What is the centre of mass of an object?
The point at which the weight of an object acts through.
The point through which a force produces no turning effect.
