AS revision U1-U4 and practicals

U1 MECHANICS Only one statement is correct or incorrect. Identify which one.

• 
• The kinetic energy of a 9±1kg ball travelling at 6±2m/s^2 is 162±0.77J.

• 
• The gravitational acceleration on Mars is 8m/s^2. An object is dropped and hits the ground after 1.5 s. It was dropped from a height of 9m.

Answer explanation

Only false is the efficiency = work in direction of movement which is LESS than F·d because force is at an angle from the ramp so ε<Fd/mgh=12·2/3·10·sin30=62.5%; kinetic energy=0.5·9·6^2 ± 1/9+2·2/6 J (0.5 makes no difference in relative uncertainty); the picture W=2Tsinθ and tgθ=sinθ/cosθ so W/tgθ=2Tcosθ; s=0.5·gt^2=9m

U2 MATERIALS Only one statement is correct or incorrect. Identify which one.

• The unit of Young´s modulus is kg/ms

• As I heat honey, all else being equal, the terminal velocity of a Stoke´s law object will be reached sooner.

• Below the limit of proportionality the power needed to stretch an object a given distance is constant.

• A small regular shaped object falling through a liquid at a constant temperature can create a turbulent movement.

Answer explanation

Youngs modulus=stress/strain=(N/m^2)/(m/m)=(kgm/s^2)/(m^2)=kg/ms^2 not kg/ms; viscosity reduces with heat so terminal velocity is bigger so it is reached later (ma=mv/t=6πrηv less η more time); P=E/t=0.5FΔx/t=0.5kΔx^2/t so it is not directly proportional rather depends on the square; CORRECT ONE turbulence can alse be caused by high velocities.

U3 WAVES AND LIGHT Only one statement is correct or incorrect. Identify which one.

• A string has to be pulled four times as hard to create an equal wave that is twice the amplitude.

• To cancel a sound that is three times the frequency of my fundamental wave on a guitar string, I must shorten the string to a sixth and use the first harmonic.

• A prism changes the colour of white light depending on how much the light slows down.

• On a photocell a stopping voltage is applied. This makes no photoelectrons be emitted.

Answer explanation

v=-Awsinwt=√T/μ so T=μ(-Awsinwt)^2 so T∝A^2 (TRUE); f(n=1,L=L)=v/λ=v·n/2L=v/2L and f(n=1,L=6L)=v·1/2·(L/6)=6·v/2L=6f(original) so FALSE; white has all colours, they separate due to speed and wavelength change (n=c/v=c/fλ) but frequency remains constant hence colour too so FALSE; the stopping voltage avoids photoelectrons reaching the cathode, but they are still emitted so FALSE.

U4 ELECTRICAL CIRCUITS Only one statement is correct or incorrect. Identify which one.

• When measuring the internal voltage of a circuit compared to the current, the emf is the Y-intercept and the internal resistance is the gradient.

• Electrons travelling down a current in a conductor go through the Conduction Band

• The total resistance of parallel branches is equal to the sum of each inverse of the resistance.

• To use a rheostat as a potentiometer we need to disconnect one of it's terminals.

Answer explanation

The INVERSE of the total resistance of parallel branches is equal to the sum of each inverse of the resistance, so it is FALSE. All the rest true.

Practicals. Only one statement is correct or incorrect. Identify which one.

• An experiment that measures time taken to freefall depending on the height plots t² over h. Gradient should be 0.2.

• To find out the fundamental frequency of a guitar string with a fixed wave generator we need to modify it's length or tension.

• The only relevant safety measure when investigating the modulus of Young is cushoning the fall of the weight.

• If we don´t have a signal generator we cannot assess the speed of sound with a continuous wave.

Answer explanation

Freefall suvat g=9.81=2h/t²=2/gradient so gradient=2/9.81=0.203 so True; Fixed wave (f) and string (μ) we can only change tension (which alters the frequency over the string) or length (f=√T/μ=2L) to calculate the f(fundamental) so True; Speed of sound is done either with a pulse (no signal generator needed) or with a known frequency continuous sound (needs signal generator) so True; Safety feature missing is goggles to rpotect eyes from flailing wires if they snap so FALSE.