# Physics EBIs

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What I got wrong and why

Forces

• Pay attention to units! 250N is not the same as 250kg.
• A negative value for acceleration means something is decelerating or slowing down, not necessarily moving backwards.
• Acceleration (according to Grade Gorilla) is not a type of force.
• Magnetic and electrosatic are types of force.
• When something falls over, it loses kinetic energy (doesn't increase then decrease)
• Gravitational potential energy decreases if something has a lower center of gravity (you can only say it's closer to the ground if you mention MGH equation)
• When talking about moment and something is closer to one end, both forces increase because of the additional mass on the plank but force on one side will be greater than the other (unless object is half way)

Momentum

• If a bullet is fired and 20% of energy is lost, it's because some of it is lost due to high fiction and air resistance (therefore speed lost quickly)
• KE = 1/2 x M x V^2
• When calculating kinetic energy lost after collision between 2 objects, remember to add the initial / end momentum of both objects.
• A rocket changing direction: momentum is conserved. Spent fuel has momentum to the right and gets equal momentum to the left, resulting in an increase in speed/velocity
• Momentum is defined as "the product of mass and velocity" in a past paper mark scheme
• Some energy lost is transferred to other forms i.e. heat and sound
• When a bullet hits a wooden block, it decelerates because work is being done to deform both the block and and bullet, so energy is being transferred to other forms (away from kinetic)
• to get a greater moment in a smaller distance, more force is required (if mass is closer to one end) and moment needs to be equal on both sides for it to be balanced
• If variables not controlled, the momentum of a cloth + 1 tin can cannot equal momentum of cloth + 2 tin cans

Astronomy

• If there is a staelite in stable orbit and there's a single force acting on it, the force is gravitational. Even though gravity isn't technically a force, that is what…