Action and Reaction Flashcards
What do forces always occur in?
Pairs
What must the action and reaction of forces be?
- Equal in magnitude
- Opposite in direction
- Act on different objects
- This is Newton’s 3rd law
What is the reaction force of weight and gravity when someone jumps?
The Earth is also attracted to that person but its mass is so huge that it moves by an amount too small for us to notice.
Why does a gun recoil??
The force on the bullet moves the bullet forward so the force on the gun moves it back.
What is conservation of momentum?
- The total momentum before a collision = the total momentum after a collision
- Momentum = mv
- m1u1 + m2u2 = (m1 + m2) v
- Explosion is the opposite of a collision as objects move apart but momentum is still conserved
- m1u1 + m2u2 = m1v1 + m2v2
What is every material made out of?
Tiny moving particles with kinetic energy.
Why is there pressure on the walls of a container?
The particles collide the with walls and create a force on the walls.
How does pressure increase or decrease?
- Changing the volume: if the same number of particles at the same temperature are in a smaller container the particles collide with the walls more often so the pressure increases
- Changing the temperature: if the same number of particles in the same size of container are heated then they have more KE and will move faster, hitting off the walls more often and increasing the pressure
What does a particle have when its moving?
- Mass, m
- Velocity, -v
- Momentum, -mv
What happens to a particle when it collides perpendicularly with a wall?
It will rebound with thee same speed but in the opposite direction so its velocity now changes to +v, changing its momentum to +mv.
How do we calculate how much force is on the wall?
- Force = rate of change in momentum
- Momentum before collision = -mv
- Momentum after collision = +mv
- Change of momentum = mv - (-mv) = 2mv
- Force = 2mv/time
What happens in a rocket to make it move?
As a rocket moves up the hot gases released move down. Momentum is conserved meaning the high momentum of the large massed rocket moving up is balanced by the high velocity of the exhaust gases.
