Booklet 1A - Dynamics (Worked Examples) Flashcards
Example
At the 2008 Beijing Olympics, Usain Bolt ran 100m in 9.69 seconds. Calculate his average speed.
d = 100 m t = 9.69 s v = ?
d = v t
100 = v x 9.69
v x 9.69 = 100
v = 100/9.69
v = 10.32 ms-1
Example of how to work out average speed and average velocity using scale diagram or calculation:
A walker travels 12km South then 5km West in 4 hours. Calculate:
a) the distance travelled
b) the resultant displacement
c) the average speed
d) the average velocity
(See Image)
a) d = 12 + 5 = 17 km
b) a2 = b2 + c2
a2 = 52 + 122 = 25 + 144 = 169
a = sqrt(169) = 13 km
tan(x) = Opp/Adj = 5/12
x = tan-1(5/12) = 27º
Bearing is clockwise from North i.e. 180 + 27 = 207º
Resultant Displacement is 13 km bearing 207º
c) d = 17 km; t = 4 h; v = ?
d = vt
17 = v x 4
v x 4 = 17
v = 17/4 = 4.25 kmh-1
d) s = 13 km (207º); t = 4 h; v = ?
s = vt
13 = v x 4
v x 4 = 13
v = 13/4 = 3.25 kmh-1 (207º)
An arrow is fired vertically upwards. It reaches a maximum height of 65m.
a. State the velocity of the arrow at its maximum height?
b. Calculate the initial velocity of the arrow.
Interpret displacement-time graphs.
Interpret velocity-time graphs.
a. Draw a free body diagram for a skydiver accelerating after they jump out of an aeroplane using a drogue chute to slow their descent.
b. Find an expression for the unbalanced force on the skydiver.
A load of bricks of mass 1000kg is being lifted by a crane at constant speed. Calculate the tension in the crane cable.
A cyclist of mass 100kg is accelerating at 1.0ms-2 along a level road. The total frictional force due to air resistance and rolling resistance is 20N. Calculate the forward force produced by the cyclist.
A rocket of mass 1000kg produces a constant thrust of 20,000N.
A 60kg student is standing in a lift of mass 200kg. Calculate the tension in the lift cable when
a. the lift is stationary
b. the lift is moving downwards at a constant speed.
c. the lift is moving downwards and decelerating at 1.0ms-2.
A boy on a sledge of total mass of 55kg is sliding down a 20° slope. The sledge accelerates at 3.0ms-2. Find the frictional force on the sledge.
A 45g golf ball is struck with a club with an average force of 3800N and moves off with a velocity of 45ms-1. For how long was the ball in contact with the club.
