Energy,Work and Momentum

You have to apply a force to move something. So the work you do causes transfer of energy.

ENERGY TRANSFERRED = WORK DONE

work done = force applied x distance moved in the direction of the force. ( W = F x d )

Work done to overcome friction is mainly transferred into energy by heating.

•  If you rub your hands together vigorously, they become warm. Your muscles do work to overcome the friction between your hands. The work you do is transferred into energy that warms your hands

Every time you lift an object up, some of your muscles transfer chemical energy from your muscles into gravitational potential energy of the object.

• When an object is moved up, it's GPE increases. The increase of it's GPE is equal to the work done on it by the lifting force.
• When an object moves down, it's GPE decreases. the decrease of it's GPE is equal to the work done by the force of gravity acting on it.

The change of it's GPE = it's weight x it's change of height

Therefore, when an object is lifted or lowered, beacuse it's change of  GPE is equal to it's weight x it's change of height.

Change of GPE x mass x gravitational field strength x change in height

The momentum of a moving object = mass x velocity ( p = m x v)

Momentum has a size and direction

Whenever objects interact, momentum is conserved, provided the objects are in a closed system so that no external forces act on them.

In general.... In a closed system , the total momentum before an event is equal to the total momentum after the event.

If a vehicle crashes into the back of a line of cars, each car turn is 'shunted' into the one in front. Momentum is transferred along the line of cars to the one at the front.

Momentum before explosion = momentum after explosion

• Any stored energy will cause an explosion wheni it is released
• Explosions can be chemical or elastic

( mass of A x velocity of A) + (mass of B x velocity of B)

( mass of A x velocity of A) = - ( mass of B x velocity of B)

Seat belts

• A seat belt stops the wearer from continuing forwards when the car stops
• The time taken to stop someone in a car is longer if they are wearing a seat belt than if they aren't. The decelerating force is reduced by wearing a seat belt.
• The seat belt spreads the force across the chest without a seat belt the force would act on the head

Air bags

• Air bags are at the front and side to protect you from the impact.

Child Car Seat

• Babies seats must face backwards so they do not face the force head on,.
• Child seats have saved thousands of lives since introduced as they protect the child.