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Physics Newtons Laws
Physics Spring Y12
| Question | Answer |
|---|---|
| A 50g ball is travelling at 2.0ms when it hits a wall and rebounds at 1.5ms. Calculate the magnitude of the change in momentum. | Remember it's a vector quantity (check if it's looking for magnitude), so the magnitude of velocity change is 3.5ms. Then turn into momentum |
| Define linear momentum | The vector product of mass and velocity |
| Define N1L | An object will remain at rest (if already at rest) or move with a constant velocity unless acted upon by a resultant force |
| What will a resultant force do | Change an object's speed and/or direction |
| How to show resultant force is zero | Either using vector polygon or resolving forces in multiple directions to show the resultant force is zero |
| Define N3L | If object A exerts a force on object B, then object B exerts a force on object A that is equal in magnitude, opposite in direction, and of the same type |
| Conditions to be a N3L pair | Act on different objects Same magnitude Opposite direction Same type of force |
| Block sitting on the earth. What N3L pairs are there | Weight down from the middle of the block and up from the centre of the earth. N up from bottom of block and N down from bottom of block |
| Explain how newton's laws can be used to explain how a person runs on a road | Friction acting on ground (as in like the entire earth) against direction of movement so as is a N3L pair, friction acting on you in direction of movement Frictional force will act forwards on the person |
| N2L definition | The resultant force on an object is equal to (or directly proportional to) the rate of change of momentum YOU HAVE TO SAY **RESULTANT** |
| N2L equation | F = delta(rho)/delta(t) or F = delta(mv)/delta(t) If mass constant, F = ma |
| 2.5 kgs^-1 water from hosepipe. Speed is 4ms^-1. Calculate force needed to push water out. | (2.5 x 4)/1 = 10 |
| Starting/stopping running | Starting running gives momentum to the earth but slowing down balances it |
| Impulse definition | The product of the force and the time for which the force acts |
| Impulse = | F x delta(t) = change in momentum |
| Area under Force Time graph | Impulse |
| 'Impulse given to the car' | The car + truck stuck together each have a proportion of the total momentum, so only use mass of car for change in momentum |
| 'Impulse on the truck' | Went from 34k to 24k so -10k |
| Why is the impulse given to the car and the impulse on the truck the same | They are equal in magnitude because N3L pair, so equal but opposite force on each other, acting for same time, so as impulse = Ft, impulse equal for both: no change in momentum |
| What is a closed system | Has no mass, energy, or external forces entering or exiting the system |
| What properties are conserved in a closed system (in mechanics) | Momentum, energy (not necessarily still in the form of kinetic energy), mass |
| Elastic collision | KINETIC energy is conserved. All collisions conserve energy in closed system, and inelastic collisions dissipate some energy as thermal energy |
| Arrow going to the left but going in opposite direction as previous positive direction | Most accurate to say '6ms to the left' |
| When calculating conservation of momentum | Have to say which side is positive (do an arrow going left). Make sure to check everything if + or - |
| When doing a collision question working out velocity after collision, you often get two answers. Which is correct? | One is the initial conditions - this is if nothing happened, so not the answer. Phrase as 'this is the initial speed so not the answer' One is the final conditions |
| Collisions in 2 dimensions where the objects bounce off at a right angle. | The momentum at the start and end has to be equal. However don't just do a = b + c. Instead need to do in terms of components. Due to it being a right angle, can just do pythagoras (hypotenuse is the starting momentum) |
| Rule for collision of 2 equal mass objects | If 2 equal mass objects collide at an angle elastically, the angle between them will be 90 degrees after the collision |
| Resolving conservation of momentum in 2 dimensions | x direction: m1*v0 = m1*v1*costheta1 + m2*v2*costheta2 (literally just components) y direction: 0 = m1*v1*sintheta1 + m2*v2*sintheta2 (rember = 0 and REMEMBER that either the v2 or the theta2 has to be given as negative depending on given positive y) |
| Conservation of KE in 2 dimensions when elastic | As normal. Starting E = E1 + E2 |
| Prove the rule for collision of 2 equal mass objects | If elastic, 0.5*m*u^2 = 0.5*m*v1^2 + 0.5*m*v2^2, so divide by 0.5*m, gives u^2 = v1^2 + v2^2, so right angle Momentum and velocity both at 90 degrees so they must move apart at 90 degrees |
| Remember with momentum of 2 objects going towards each other | One has to be negative |
| Momentum in 2D practical | Clamp slow motion camera above measurement sheet with stopwatch in frame. Place one marble, roll the other at it. Reset until 'good' collision. Measure distances, times = velocities (including initial). Weigh marbles if going for momentum. |
| 'Describe the measurements the students will need to take to determine if momentum is conserved' | Also repeats |
| Light gate experiment what measurements to take | Remember to measure interrupt card width |
| Kinetic energy of proton doubles: what effect on wavelength? | Double kinetic energy in 1/2mv^2 formula Divide by sqrt2 |
| If asked to give rebound speed by drawing tangent on graph | Draw the tangent at the point of bouncing |
| ball released from rest above the ground. According to a student, the principle of conservation of p is violated because the ball gains p as it falls. Explain why the student’s observation is incomplete and discuss how p is conserved in this situation | The Earth has (equal and) opposite momentum to the (falling) ball (so momentum is conserved) |
| 2 marker on explaining why something isnt a N3L pair | Look for 2 different reasons |
| Graph to show velocity part way through acceleration | Careful to not put it through origin |
| Mention with momentum | Principle of conservation of momentum |
| What is the relationship between the force acting on a body and the momentum of the body? | Resultant force is directly proportional to the rate of change of momentum |
| Trucks all resisting motion. When some trucks released, what to remember to keep | The resistance to motion of the remaining trucks |
| Force against time graph is identical to what other graph | Acceleration against time. A diagonal line upwards shows increasing acceleration. |
| 3 identical balls. Two stationary with the other fired at them at 1.5ms (all in 1D). How to prove 3rd ball at same speed as initial ball part 1 | Conserve p first (initial = all 3 combined after), then divide by mass. We know the middle ball stays at zero, so initial velocity = sum of velocity of outer 2 afterwards |
| 3 identical balls. Two stationary with the other fired at them at 1.5ms (all in 1D). How to prove 3rd ball at same speed as initial ball part 2 | Conserve KE, removing middle ball. Sub in first equation. Get that after the collision either the first of last ball has to be at rest, so due to not being initial collisions, first ball at rest. Sub in to before to get that initial speed = v of 3rd |
| What do 6 markers need | Experiment, measurements, and clear analysis |
Created by:
Pyrogearos2