Work, energy and momentum Flashcards
W=Fd
work (J) = force (N) x distance moved (m)
Friction
Forces opposing the movement of one surface over another
Gravitational potential energy
Energy of an object due to its position in a gravitational field
E=mgh
energy (J) = mass (kg) x gravitational field strength (N/kg) x change in height (m)
Power
The energy transformed or transferred per second (W)
P=E/t
power (W) = energy (J) ÷ time (s)
E=1/2 mv^2
kinetic energy (J) = 1/2 x mass (kg) x speed squared (m/s)
Momentum
Mass times velocity - has a size and a direction
p=mv
momentum (kg m/s) = mass (kg) x velocity (m/s)
Conversion of momentum
In a closed system, the total momentum before an event is equal to the total momentum after the event
Impact time
Time taken for a collision to take place
Crumple zone
Region of a vehicle designed to crumple in a collision to reduce the force on the occupants
What do we mean by “work” in science?
Work is done on an object when a force makes the energy move
What is the relationship between work and energy?
Energy transferred = work done
How happens to the work done to overcome friction?
Work done to overcome friction is transferred as energy that heats objects that rub together and the surroundings
What does the gravitational potential energy of an object depend on?
The object’s weight and how far it moves vertically
What happens to the gravitational potential energy of an object when it moves up or down?
The gravitational potential energy of an object increases when the object goes up and decreases when it goes down
What does the kinetic energy of an object depend on?
The kinetic energy of a moving object depends on its mass and its speed
What is elastic potential energy?
Elastic potential energy is the energy stored in an object when work is done on the object
What is the unit of momentum?
kg m/s
What happens to the total momentum of two objects when they collide?
Momentum is conserved whenever objects interact, provided the objects are in a closed system so that no external forces can act on them
What does momentum have a direction as well as a size?
Momentum is mass x velocity, and velocity is speed in a certain direction
When two objects push each other apart, how do they move apart?
They move with different speeds if they have unequal mass, and with equal and opposite momentum so that their total momentum is zero
When vehicles collide, what does the force of the impact depend on?
When vehicles collide, the force of the impact depends on mass, change of velocity and the duration of the impact
How does the impact force depend on the impact time?
The longer the impact time is, the more the impact force is reduced
What can we say about the impact forces and the total momentum when two vehicles collide?
When two vehicles collide, they exert equal and opposite forces on each other, and their total momentum is unchanged
Why do seat belts and air bags reduce the force on people in car accidents?
Seat belts and air bags spread the force across the chest and increase the impact time
How do side impact bars and crumple zones work?
Side impact bars and crumple zones “give way” in an impact, so increase the impact time
How can we work out if a car in a crash was “speeding”?
We can use the conservation of momentum to find the speed of a car before an impact
