1. Dynamics

2. Dynamics Glossary Slide 3-4: Scalar and Vector Quantities
Slide 5: Displacement
Slide 6: Speed and Velocity
Slide 7: Acceleration
Slide 8: Newton’s First Law
Slide 9: Newton’s Second Law
Slide 10: Newton’s 1st and 2nd Law
Slide 11: Newton’s Third Law
Slide 12: Mass and Weight
Slide 13: Gravitational Field Strength
Slide 14-15: Free-fall
Slide 16: Free-Fall and Terminal Velocity
Slide 17: Free-fall
Slide 18-20: Projectiles

3. Scalar and Vector Quantities
10 ms-1
A scalar quantity only has magnitude (size). A vector quantity has both magnitude and direction. You may be asked the difference between the two.
Speed (scalar) = 10 ms-1
Velocity (vector) = 10 ms-1, to the right

4. Scalar and Vector Quantities
SCALARS VECTORS Distance Displacement Time Acceleration Energy Velocity Mass Weight (force) Power Air Resistance (force) Speed Force (D.T.E.M.P.S) (D.A.V.W.A.F)

5. Displacement N S E W
Displacement is a measure of how far away something is from its starting point. Displacement is a vector so an accurate direction must be given. Displacement can be determined by drawing a scale diagram.
His displacement can then be measured as 5.8km
180o - 31o
149o
Start
A man travels 5km south
31o
5km
5.8km
3km
Finish
However, we also need a direction.
So we begin by measuring the angle at the starting point.
We are not finished yet. We still need to get a bearing. So we start at north then work our way around clockwise until we reach our displacement.
He then travels 3km east
Our final step,
Displacement = 149

6. Speed and Velocity
Say the man completed his journey in 2000 seconds. We could calculate his speed (scalar) but we could also calculate his velocity (vector).
149o
Start
31o
5km
5.8km
Velocity calculation,
s = vt
5800 = v x 2000
v = 5800/2000
v =
(direction must be mentioned for velocity)
3km
Finish
Speed calculation,
d = vt
8000 = v x 2000
v = 8000/2000
v = 4 ms-1
“s” is the symbol we use for displacement. The equation for velocity is on your equation sheet. Make sure you know it!
“v” is velocity in this equation.
Total distance travelled = 5km + 3km = 8km = 8000m

7. Acceleration
Acceleration is a measure of something’s change in velocity per second.
acceleration = change in velocity divided by time
acceleration = final velocity – initial velocity, divided by time

8. Newton’s First Law
“When the forces acting on an object are balanced then the velocity of the object does not change.” Balanced forces are equal in size but opposite in direction.
100 N
100 N
If both of the creatures push with 100 N from the beginning then the box will remain still (stationary).
The forces acting on the box are balanced as they are equal in size but opposite in direction.

9. Newton’s Second Law
“When the forces acting on an object are unbalanced then the velocity of the object does change.” Fun = ma Unbalanced forces are unequal in size and opposite in direction.
200 N
100 N
As the pushing force to the right is greater the box will accelerate to the right.

10. Newton’s 1st and 2nd Law 100 N 200 N 200 N 100 N
If there was an initial unbalanced force then the box would accelerate from rest. If the forces then became balanced the box would travel at a constant speed. The forces would need to become unbalanced in the opposite direction to get the box to stop.
100 N
200 N
200 N
100 N
When the forces start unbalanced the box accelerates from rest to the right.
When the forces become balanced the box travels at a constant speed.
When the forces become unbalanced in the opposite direction the box decelerates to a stop.

11. Newton’s Third Law
Examples include,
Force of man on chair
Force of chair on man =
“For every action there is an equal and opposite reaction.” This means however much force you apply to something, it applies the same amount of force back to you.
Force of fist on bag =
Force of bag on fist
Force of swimmer on wall =
Force of wall on swimmer

12. Mass and Weight
Mass is the amount of atoms that make something up. Weight is gravitational force which is dependent on the amount of mass and the strength of gravity. W = mg The larger the mass, the larger the weight (force). The larger the gravitational field strength, the larger the weight (force).

13. Gravitational Field Strength
g = 1.6 Nkg-1 on the moon.
g = 9.8 Nkg-1 on the Earth.
So you weigh about 6x less on the moon compared to Earth!
The amount of atoms you are made up of stays the same no matter where you are in space i.e. your mass stays the same. However, the gravitational field strength you experience is different depending where you are. As W = mg then your weight is different in different parts of space.

14. Free-fall
Air Resistance
Weight
When you jump out of an aeroplane you are in free-fall. There are two forces acting on you – weight and air resistance.

15. Free-fall
Air Resistance = 500N
When the forces are unbalanced downwardly then you are accelerating downwards.
Weight = 700N

16. Free-fall and Terminal Velocity
Air Resistance = 700N
When the forces become balanced then you are travelling at a constant speed downwards. We say you has reached a terminal velocity when the forces are balanced.
Weight = 700N

17. Free-fall
Air Resistance = 6000N
Air Resistance = 3000N
Air Resistance = 9000N
Air Resistance = 700N
When the parachute is deployed the air resistance increases suddenly. This causes the forces to be unbalanced upwardly so the sky diver decelerates downwards. When the forces become balanced again the sky diver will fall towards the ground with a much lower terminal velocity.
Weight = 700N

18. Projectiles
When considering projectiles in Physics we choose not to consider air resistance. We say it is negligible (can be neglected/ forgotten about).
We only think of air resistance for free-fall.

19. Projectiles
0 N
Direction of travel
The cannon is not pushing the cannon ball once fired so no forces this way.
There is no air resistance so no forces acting against the cannon ball this way.
The horizontal velocity of a projectile remains constant as there are no unbalanced forces acting horizontally.
0 N
Balanced Forces = Constant Speed
(Horizontally)
(Horizontally)
d = vt for horizontal motion

20. Projectiles
100 N (e.g.)
0 N
Direction of travel
Due to weight there is a force acting downwards.
No air resistance so no force acting upwards.
The vertical velocity of a projectile increases as there are unbalanced forces acting vertically.
Unbalanced Forces = Acceleration
(Vertically)
(Vertically)
for vertical motion