Work, Energy And Momentum Flashcards
What is work?
Work done on an object is when a force makes the object move.
What is the relationship between work and energy?
Energy transferred = work done
How do we calculate the work done?
Work done (J) = force applied (N) x distance moved in the direction of the force (m)
What 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 is gravitational potential energy?
Energy stored in an object because of its position in the Earth’s gravitational field.
How do we work out the change of the gravitational potential energy?
Change of gravitational potential energy (J) = weight (N) x change of height (m)
What does the gravitational potential energy of an object depend on?
- it’s weight
* how far it is moved 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 the object goes down.
How can we calculate the change of gravitational potential energy of an object when it moves up or down?
Change of gravitational potential energy (J) = mass (kg) x gravitational field strength (N/kg) x change of height (m)
What is power?
The energy transferred per second.
How do we calculate the power?
Power (W) = energy (J) / time (s)
What does the kinetic energy of an object depend on?
- mass
* speed
How can we calculate kinetic energy?
Kinetic energy (J) = 1/2 x mass (kg) x speed2 (m/s2)
What do kinetic energy recovery systems (KERS) do?
They store energy in vehicles when the vehicle brakes and use it later.
What so elastic potential energy?
The energy stored in an elastic object when work is done on it to change its shape.
How can we calculate momentum?
Momentum (kg m/s) = mass (kg) x velocity (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 act on them.
What is the law of conservation of momentum?
In a closed system, the total momentum before an event is equal to the total momentum after the event.
Why does momentum have a direction as well as size?
Momentum is mass x velocity and velocity is speed in a certain direction.
What happens when two objects push each other apart?
They move apart:
- with different speeds if they have unequal masses.
- with equal and opposite momentum so their total momentum is zero.
When vehicles collide, what does the force of the impact depend on?
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?
- they exert equal an opposite forces on each other
* 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 they also increase the impact time.
How do side impact bars and crumple zones work?
They give way in an impact so increasing 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.
