P5 - Forces & Newton's Third Law & Ideas about Momentum Flashcards
What was newton’s third law of motion?
• If object A exerts a force on object B, then object B exerts and equal and opposite force on object A.
What is an interaction pair?
• When an object exerts a force on another object it always experiences a force in return, the two forces are sometimes called an “interaction pair”.
What are some examples of newton’s third law of motion in action?
- If you push against a wall, the wall will push back against you in the opposite direction with exactly the same force and as soon as you stop pushing, so does the wall.
- Same is true in a collision, colliding objects exert equal and opposite forces on each other.
How is Newton’s third law of motion applied to gravity?
- If you put a book on a table, the book pushes down on the table with a force equal to its weight, the table exerts an equal and opposite force upwards on the book.
- The upwards force is called a reaction force, because it is the table’s “response” to the “force” exerted by the book. If the book weighs 10 N then the tables reaction force is 10 N
Why do objects exert a downwards force?
• Objects exert a downwards force due to gravity.
How does recoil work in a gun?
- When a gun is fired, the bullet exerts a force on the gun equal and opposite to the force exerted by the gun on the bullet.
- The bullet travels out of the barrel and the gun recoils in the opposite direction.
- The bullet travels much faster forward than the guns does backwards because the bullet is much lighter.
How do rockets move?
- When gas particles collide with things they exert a force on them
- In a rocket engine, particles of hot gas collide with the walls, exerting a force on the walls, and the wall exerts an equal but opposite force on them.
- Force from the wall pushes the gas particles out of the exhaust.
- Force from the gas on the wall pushes the rocket forwards.
What conditions are required for a rocket to move?
- The force pushing the rocket upwards must be larger than the force of gravity & air resistance, otherwise it won’t take off.
- So for large rockets, used to lift satellites into orbit above the Earth, you need a large number of particle moving at high speed to produce enough force to lift the rocket.
What is the correlation between mass and velocity?
• The greater the mass of an object, and the greater the velocity of that object, the greater the momentum of that object.
What is the formula for momentum?
Momentum (kg m/s ) = Mass ( Kg ) x Velocity ( M/S )
When is momentum conserved?
- In a collision when no other ( external ) forces act, momentum is conserved.
- The total momentum after, is the same as before.
How is the momentum reserved when a gun is fired?
Momentum conservation can be used to explain the recoil action of guns.
• before a shot is fired, the gun and bullet have no velocity, so they have no momentum either.
• When a shot is fired, the bullet travels forward ( positive momentum, and the gun recoils backwards ( with negative momentum ).
• The combined momentum of the gun and bullet will be zero, they have the same amount of momentum, but in opposite directions.
How does conservation of momentum explain rocket propulsion and explosions?
- Rockets work in the same way as guns, they chuck a load of exhaust gases out backwards, and since momentum is conserved, the rocket moves forward.
- Before an explosion, total momentum is zero, when something explodes particles are thrown out at different speeds and in all directions, so they have different momentums, all of these add up to zero.
What is the momentum for two objects that combine and coalesce?
Total momentum of objects before the collision = momentum of the combined objects after the collision.
