2D Kinematics 4

1
2D Kinematics (4)

• Resultant Velocity
• Velocity problems

2
Resultant Velocity (1)
If the boat sails straight across, which
direction will it actually go?
A River
5ms-1
2ms-1
Direction of flow
3
Resultant Velocity (2)
The RESULTANT VELOCITY can be found by a triangle
of velocities
Resultant 5i 2j
Magnitude ?(52 22) ?29 5.4 ms-1
4
Resultant Velocity (3)
A plane flys with velocity 5i - 2j The wind
blows with velocity 3i 3j What is the
resultant velocity and direction?
Do a sketch
5
Resultant Velocity (3.5) - sketch
A plane flys with velocity 5i - 2j The wind
blows with velocity 3i 3j What is the
resultant velocity and direction?
You can sketch it either way round. The
resultant is identical in both cases.
6
Resultant Velocity (4)
A plane flys with velocity 5i - 2j The wind
blows with velocity 3i 3j What is the
resultant velocity and direction?
Wind
5i - 2j
3i 3j
v 3i 3j 5i - 2j 8i j
7
Resultant Velocity (5)
A plane flys with velocity 5i - 2j The wind
blows with velocity 3i 3j What is the
resultant velocity and direction?
resultant velocity v
v 8i j
Magnitude ?(82 12) ?65 8.1 ms-1
8
Activity

• Page 41 of your Mechanics 1 book and answer
• exercise E 1, 2, 4

9
Vector Problems (1)
The croc needs to swim directly across to get the
cow. He can swim at 2ms-1. What angle does he
need to swim at to go straight to the cow.
The river flows at ...
1ms-1
10
Vector Problems (2)
The croc needs to swim directly across to get the
cow. He can swim at 2ms-1. What angle does he
need to swim at to go straight to the cow.
1ms-1
River
resultant velocity
2ms-1
cos ? adj/hyp 1/2
? cos-1 (1/2)
? 60o
11
Velocity
a measure of speed and direction
An object moves 40ms-1 at 50o to the x-axis
N
What is its component in the East-West
direction? What is its component in the
North-South direction?
40ms-1
Cos 50o adj/hyp E/40 E 40 cos 50o
25.7ms-1 (1 d.p.)
N
50o
Sin 50o opp/hyp N/40 N 40 sin 50o
30.6ms-1 (1 d.p.)
E
12
Activity / Homework

• Page 43 of your Mechanics 1 book and answer
• F3, F4
• Homework
• page 41exercise E 1, 2, 4
• page 44 Ex F Q1, 3, 5