1. ENERGY EQUATIONS
By the end of this presentation you should be able to:
Calculate kinetic energy, work and power.

2. Energy Equations Mechanical energy
_________ energy is the energy an object has because of its speed (or _________ ).
The faster it is moving, the _________ its _________ energy will be.
KINETIC
VELOCITY
GREATER
KINETIC
KINETIC ENERGY (KE) = 1 x m x v
Where m= mass (kg) , v = speed or velocity (m/s)

3. Question 1 How much kinetic energy does this car have? Mass = 2000 kg
Velocity = 40m/s
Mass = 2000 kg
Since k.e. = ½ mv2, then k.e. = ½ x 2000 x 40 x 40
So the car has J of kinetic energy (1600kJ)

4. The remaining equations
The equations (and the units used for all the quantities) to calculate…
Work done =
Power =
Force x distance
J N m
Energy transformed / time
W J s

5. Work done question 5
Calculate how much work is done in pushing the crate with a force of 40N up the ramp.
W = F x d = 40 x 5 = 200J 5m
40N

6. Work done question 6
Calculate how much work is done in lifting the crate, if its weight is 45N, to the top of the ramp (4m high).
W = F x d = 45 x 4 = 180J
The ramp supports some of the weight of the
crate so allows a person to move it with a
force smaller than the total weight, but the
length of the ramp is greater than its
height so more work is done using
the ramp. The ramp does
make the job easier
however! Energy
is also
wasted
while pushing the crate up the ramp due to friction. 4m
45N

7. Power and efficiency qu 7 and 8
The same crate is lifted to the top by an electric motor and pulley. It takes 20 seconds to do the job (180J of work). What is the power output of the motor?
P = W / t = 180 / 20 = 9W (watts)
The electric motor is actually rated at 15W (this means it is taking in 15J of electrical energy every second). What is the efficiency of this motor? 4m
Efficiency = power output = 9
power input
= 0.6 = 60%
45N