Problem Set

Energy
1. A man climbs on to a wall that is 5 m high and gains 2200 J of potential energy. What is the mass of the man?

Given: Formula: h = 5 m m = PE/gh
PE = 2200 J

Solution: m = 2200 J / (9.8 m/s2 x 5 m) m = 49 kg

2. Calculate the kinetic energy of a 500 kg car travelling at 50 m/s.
Given: Formula: m = 500 kg KE = mv2/2 v = 50 m/s

Solution:
KE = 500 kg x (50 m/s)2 / 2
KE = 625 000 J

Power

3. Riley climbs a flight of stairs in 3 minutes. If he weighs 700 N and the stairs is 20 m from the ground, how much power will she develop?

Given: Formula: t = 3 mins = 180 s P = Wd/t
W = 700 N d = 20 m

Solution:
P = 700 N x 20 m / 180 s
P = 77.8 J/s

4. How much power is developed by a car that runs 80 m/s when the force is equal to 50 000 N.

Given: Formula:
V = 80 m/s W = Fv
F = 50 000 N

Solution:
W = 50 000 N x 80 m/s
W = 4 000 000 J/s

Electricity

5. Charges of 6 and + 4 nC are 3 m apart. Determine the force they exert on each other, and the electric field at a point midway between them.
Given: Formula: k = 9 x 109 Felec = k q1 q2 /r2 q1 = 6 x 109 C q2 = 4 x 109 C r = 3 m Felec = (9 x 109)(6 x 109)(4 x 109)/32 Felec = 24 x 109 N

6. A 50 cm piece of nichrome wire is used in a home heating element. The cross sectional area of the wire is 0.05 mm2. What is the resistance of the heating element? If it is operated on household current (110 V), how much heat is produced in one minute? ( Nichrome = 100 x 10-8 Ω-m)
Given:
A = .05 mm2 = (.05)(10-3)2 = .05 x 10-6 m2 , and
L = 50 cm = .5 m .

Formula:
R = ρL/A

Solution:
R = (100 x 10-8)(.5)/(5 x 10-8)
R = 10 ohms

7. A nine volt battery supplies power to a cordless curling iron with a resistance of 18 ohms. How much current is flowing through the curling iron?
Given:
R = 18 ohms Formula:
V = 9 volts I = V/R

Solution:
I = 9 volts / 18