Speed, Velocity and Acceleration
In physics, vectors are quantities which have a magnitude and a direction.
Speed is distance travelled/time taken. It is measured in m/s.
Velocity is speed in a particular direction. Velocity is a vector, so it has a direction. It is also measured in m/s.
Acceleration is the change in velocity per unit time. It is measured in m/s/s. An object is accelerating if either its speed or its direction is changing.
Forces and Newton’s Laws
Forces are pushes and pulls. They are interactions between two objects. If I push the table, the table pushes me.
Forces are measured in Newtons. Forces are vectors – they have direction.
Forces cause changes in motion or changes in shape (stretching, compressing, twisting etc.)
Newton’s 1st law (law of inertia): If no unbalanced force acts on an object, the object will continue in the same state of motion, that is, it will either remain at rest, or continue moving in a straight line at constant speed.
Newton’s 2nd law: The unbalanced force on an object is equal to the mass times the acceleration.
Newton’s 2nd law with momentum: Impulse (force x time) = change in momentum.
Newton’s 3rd law: If A pushes B, B pushes back on A with the same force.
Momentum, impulse and collisions
Momentum of an object = mass x velocity. Momentum is a vector and has units of kg m/s or Ns.
Two useful things to do with momentum
Force and motion: An impulse (force x time) changes an object’s momentum. F x t = m Δv. For the same change in momentum, if you increase time, you decrease force. Many car safety features work by increasing stopping time to decrease the forces experienced by humans in cars.
Conservation of momentum: If two objects collide, the total momentum of the objects before the collision is equal to the total momentum of the objects after the collision.
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