# An elephant weighing 40000N stands on one foot of area 0

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```					Physics Calculation Revision - answers
1.   A man lifts a brick of mass 5kg from the floor to a shelf 2 metres high.                 6.    A 2kW fire, a 200W TV and three 100W lamps are all switched on from 6pm until
a] What is the weight of the brick? (g = 10N/kg)                                               10pm. What is the total cost at 8p per kW-h?
m = 5kg                W = m x g = 5kg x 10N/kg = 50N                                          fire P = 2kW
h = 2m                                                                                         TV P = 200W = 0.2kW
W=?                                                                                            lamp P = 100W = 0.1kW
g = 10N/kg                                                                                     total P = 2kW + 0.2kW + (3 x 0.1kW) = 2.5kW
t = 4hr
b] How much work is done?
cost = ?
Wd = ?                 Wd = F x d = 50N x 2m = 100J
unit cost = 8p           cost = P x t x unit cost = 2.5kW x 4hr x 8p = 80p
2.   A weight lifter raises a mass of 200kg up to a height of 2m. How much gravitational
7.    An electric kettle is rated at 2kW. How many joules of energy are transferred in
potential energy does the mass gain? (g = 10N/kg)
10 seconds?
m = 200kg              GPE = m x g x h = 200kg x 10N/kg x 2m = 4000J
P = 2kW = 2000W            P = Wd/t
h = 2m
Wd = ?
GPE = ?
t = 10s                    Wd = P x t = 2000W x 10s = 20000J
g = 10N/kg
8.    An athlete exerts a force of 80N for a distance of 100m, how much work does she
3.   A step down transformer is required to transform 240V a.c. to 12V a.c. for a model             do? She uses up 40000J of food energy, what is her efficiency?
railway. If the primary coil has 1000 turns, how many turns should the secondary coil
F = 80N                    Wd = F x d = 80N x 100m = 8000J
have?
d = 100m
V1 = 240V              N2/N1 = V2/V1
Wd = ?
V2 = 12V
N1 = 1000turns         N2 = (V2/V1) x N1 = (12V/240V) x 1000 = 50turns                         efficiency = (useful E/total E) x 100% = (8000J/40000J) x 100% = 20%
N2 = ?
9.    What voltage is needed to send a 2A current through a 5Ω resistor?
4.   A crane lifts a load weighing 3000N through a height of 5m in 10s. What is the power
V=?                        V = I x R = 2A x 5Ω = 10V
of the crane?
I = 2A
W = 3000N              P = Wd/t = (F x d)/t = (3000N x 5m)/10s = 1500W
R = 5Ω
h = 5m
t = 10s                                                                                  10. A car travels 100m in 5s. What is its average speed?
P=?                                                                                            d = 100m                   s = d/t = 100m/5s = 20m/s
t = 5s
5.   A car is travelling at 20m/s. The driver has a reaction time of 0.7s. How far does she
s=?
travel before she starts to brake?
s = 20m/s              s = d/t                                                           11.   An electric lamp uses 100J of electrical energy each second but only transfers 2J
t = 0.7s                                                                                       to light energy. How efficient is the lamp?
d=?                    d = s x t = 20m/s x 0.7s = 14.0m                                        efficiency = (useful E/total E) x 100% = (2J/100J) x 100% = 2%

KRa9a5e91a-c400-4944-a0a7-55095686f03b.doc
12. A car accelerates from 5m/s to 25m/s in 10 seconds. What is its acceleration?          17. A car-driver of mass 100kg is in a crash. He decelerates from 30m/s to rest in
2
u = 5m/s                a = (v-u)/t = (25m/s - 5m/s)/10s = 2m/s                            2 seconds. Calculate: a] his deceleration,
v = 25m/s                                                                                  m = 100kg                   a = (v-u)/t = (0m/s - 30m/s)/2s = -15m/s2
t = 10s                                                                                    u = 30m/s
a=?                                                                                        v = 0m/s
t = 2s
13. A sealed syringe contains 60cm3 of air at a pressure of 1.0atm. The piston is pushed       a=?
in slowly until the volume is 40cm3. What is the pressure now?
V1 = 60cm3              P2/P1 = V2/V1                                                      b] the force exerted by his seat-belt.
P1 = 1.0atm             P2 = (V2/V1) x P1 = (60cm3/40cm3) x 1.0atm = 1.5atm                a = -15m/s2                 F = m x a = 100kg x -15m/s2 = -1500N
V2 = 40cm3                                                                                 F=?
P2 = ?
18. A ship sends out a sound wave and receives an echo after 1s. If the speed of sound in
14. A wave has a frequency of 2Hz and a wavelength of 10cm. What is the speed of the           water is 1500m/s, how deep is the water?
wave?                                                                                      t = 1s                      s = d/t
f = 2Hz                 v = f x  = 2Hz x 0.1m = 0.2m/s                                    s = 1500m/s                 d = s x t = 1500m/s x 1s = 1500m
 = 10cm = 0.1m                                                                            d=?                         depth = 1500m/2 = 750m
v=?
19. A current of 1A flows through a lamp for 1 minute. How much charge passes through
15. A 2000W electric fire is used for 10 hrs. What is the cost at 8p per kW-h?                 the lamp?
P = 2000W = 2kW         cost = P x t x unit cost = 2kW x 10hr x 8p = 160p                  I = 1A                      Q = I x t = 1A x 60s = 60C
t = 10hr                                                                                   t = 1min = 60s
cost = ?                                                                                   Q=?
unit cost = 8p
20. If a potential difference of 10V causes a current of 2A to flow for 1 minute.
16. An man has a mass of 60kg. a] What is his mass on the Moon? 60kg                           How much energy is transferred?
b] What is his weight on Earth? (g = 10N/kg)                                               V = 10V                     E = I x V x t = 2A x 10V x 60s = 1200J
m = 60kg                W = m x g = 60kg x 10N/kg = 600N                                   I = 2A
g = 10N/kg                                                                                 t = 1min = 60s
W=?                                                                                        E=?

KRa9a5e91a-c400-4944-a0a7-55095686f03b.doc
21. An electric kettle connected to the 230V mains supply draws a current of 10A.        25. Galileo drops a stone from the leaning tower of Pisa. The tower is 45m high. At what
Calculate: a] the power of the kettle,                                                   speed does the stone hit the ground? (hints: g = 10N/kg; GPE lost = KE gained)
V = 230V                P = I x V = 10A x 230V = 2300W                                   h = 45m
I = 10A                                                                                  v=?
P=?
GPE lost = KE gained
b] the energy transferred in 1 minute.
m x g x h = ½ x m x v2
E=?                     E = I x V x t = 10A x 230V x 60s = 138000J
g x h = ½ x v2
t = 1min = 60s
v = √(2 x g x h) = √(2 x 10 x 45) = 30m/s
22. An elephant of mass 2000kg is travelling at 5m/s. How much kinetic energy does it
26. A car of mass 800kg is travelling at 10m/s. When the brakes are applied, it comes to
have?
rest in 8m. What is the average force exerted by the brakes? (hint: kinetic energy
m = 2000kg              KE = ½ x m x v2 = ½ x 2000 x 52 = 25000J                         lost = work done by the brakes)
v = 5m/s                                                                                 m = 800kg
KE = ?                                                                                   v = 10m/s
23. A potential difference of 4V is applied to two resistors (6Ω and 2Ω ) connected in       d = 8m
series. Calculate: a] the combined resistance and the current flowing,                   F=?
V = 4V                      V=IxR
R = 6Ω + 2Ω = 8Ω            I = V/R = 4V/8Ω = 0.5A                                       KE lost = Wd by brakes
I=?                                                                                      ½ x m x v2 = F x d
F = (½ x m x v2)/d = (½ x 800 x 102)/8 = 5000N
b] the potential difference across the 6Ω resistor.
V = (6Ω /8Ω) x 4V = 3V

24. When a force of 6N is applied to a block of mass 2kg, it moves along a table at
constant velocity. a] What is the force of friction? 6N
b] When the force applied is increased to 10N, what is the acceleration?
m = 2kg                     F=mxa
F = 10N - 6N = 4N
a=?                         a = F/m = 4N/2kg = = 2m/s2

KRa9a5e91a-c400-4944-a0a7-55095686f03b.doc

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