Action and reaction

Question 1

The action and reaction pairs of forces:

Answer 1

• Are equal in magnitude (size).
• Are opposite in direction.
• Act on different objects.
• This is Newton’s third law.

Question 2

Earth and boy.

Answer 2

• The boy has weight because he is attracted to Earth due to gravity.
• The Earth is also attracted to the boy; this is the reaction force.
• If the boy jumps in the air he is attracted to Earth and moves down to it.
• Earth is also attracted to the boy with an equal force but its mass is so great that it only moves a small amount, much too small for us to notice.

Question 3

Guns and recoil.

Answer 3

When a gun is fired, there is a force on the bullet from the gun so it moves forward and the gun recoils due to the force from the bullet.

Question 4

Conservation of momentum

Answer 4

• The momentum of an object depends on its mass and velocity.
• In any collision, the total momentum before the collision is equal to the total amount after the collision.
• An explosion is the opposite of a collision. Momentum is still conserved.

Question 5

Momemtum equations

Answer 5

momentum = mass x velocity

m₁ + u₁ + m₂ + u₂ = (m₁ + m₂) v

Question 6

Kinematic theory of gases

Answer 6

• Every material is made of tiny particles with kinetic energy.
• The higher the temperature the more kinetic energy the particles have and the faster they move.
• In a gas the particles are far apart and free to move.
• The particles collide with the walls of the container, creating a force, and hence a pressure, on the walls.

Question 7

Changing volume of gas particles

Answer 7

If the same number of particles, at the same temperature, are in a smaller container they hit the walls more often, increasing the pressure.

Answer 8

If the same number of particles in the same container are heated, the particles move faster hitting the walls more often and with a greater force, increasing the pressure.

Question 9

What does the force equal?

Answer 9

Rate of change in momentum

Question 10

Consider a particle moving in a sealed box:

Answer 10

• The particle has a mass, m, moving with velocity, -v, and momentum -mv
• Each time the particle collides perpendicularly with a wall it rebounds with the same speed but in the opposite direction. Its velocity is +v and its momentum is +mv.
  
  • Change in momentum is m-(-mv) = 2mv
  • Force on wall, F = 2 mv/t where t = time between collisions with wall.

Question 11

As a rocket moves up.

Answer 11

The hot gases released moves down. Momentum is conserved. This means that the high momentum of the large massed rocket moving up is balanced by the high velocity of the exhaustt gases.