3 Motion

Question 1

What is instantaneous speed

Answer 1

Speed over a really short time, tangent of a graph at a particular time

Question 2

average speed and velocity equation

Answer 2

speed = distance/ time
velocity = displacement/ time

Question 3

define acceleration

Answer 3

rate of change of velocity m s^-2

Question 4

properties of a velocity-time graph - gradient, area

Answer 4

Gradient:
- positive - constant acceleration
- zero - constant velocity
negative - constant deceleration
- curve - changing acceleration
area: displacement

Question 5

Properties of an acceleration-time graph - gradient, area

Answer 5

Gradient:
value for the ‘jerk’ - rate of change of acceleration
Area: change in velocity

Question 6

Derive v=u+at from a velocity time graph

Answer 6

acceleration=gradient
a=(v-u)/t
rearranging: v=u+at

Question 7

Derive s=ut+1/2at^2 from a velocity time graph

Answer 7

displacement=area under graph
area = ut + (1/2(v-u)t)
(v-u=at)
s=ut+1/2at^2

Question 8

Derive s=1/2(u+v)t from a velocity time graph

Answer 8

displacement=area of a trapezium
s=1/2(v+u)t

Question 9

Derive v^2=u^2+2as from a velocity time graph

Answer 9

v=u+at – t=(v-u)/a
t is substituted into s=1/2(u+v)t
s=1/2(u+v)(v-u/a)
(u+v)(v-u)=2as
v^2-u^2=2as
v^2=u^2+2as

Question 10

Formula for stopping distance

Answer 10

Thinking + Braking Distance = Stopping Distance

Question 11

Thinking distance factors and equation

Answer 11

Tiredness, drugs, alcohol, distractions.
Thinking = reaction time x speed

Question 12

Braking distance factors

Answer 12

Speed of car, road conditions, tyre and brake conditions, weather conditions.

Question 13

What is the relationship between velocity and stopping distance?

Answer 13

braking distance is proportional to the square of the velocity - Ek=1/2mv^2

Question 14

Why does the value of g vary on Earth?

Answer 14

depends on factors including altitude, latitude and geology of an area

Question 15

Practical to determine g? (Electromagnets)

Answer 15

Electromagnet and trapdoor: current switched off, timer is triggered, electromagnet demagnetises and ball falls. Hits trapdoor, contact broken and timer stops.

Question 16

How to get value for g when plotting graph?

Answer 16

plot distance (y) against time^2 (x).
s=ut+1/2 at^2
Since object dropped from rest and a=g:
s=1/2gt^2
gradient = g/2

Question 17

Projectile motion - what happens to vertical and horizontal velocities?

Answer 17

Vertical velocity changes due to acceleration of free fall.
Horizontal velocity remains constant

Question 18

Projectile motion - Why does horizontal velocity remain constant?

Answer 18

horizontal acceleration - gcos90 = 0

Question 19

Projectile motion - How to calculate magnitude of actual velocity from Vx and Vy?

Answer 19

vertical and horizontal components - Pythagoras!
Actual velocity V=√Vx^2+Vy^2

Question 20

Projectile motion - How to calculate angle θ made by the velocity?

Answer 20

Velocity to the horizontal:
θ=tan^-1(Vy/Vx)

Question 21

Describe why the distance travelled by a ball A when thrown at a 30° angle with respect to the horizontal can be equal to the distance travelled by ball B thrown at the same velocity and angle of 60°, without involving any calculations.

Answer 21

Ball A has higher horizontal component of velocity than B. Ball A has lower vertical component of velocity than B.
Vertical component affects the time of flight. Ball A has higher horizontal velocity, but lower time of flight than B. Ball B has lower horizontal velocity, but longer time of flight than A.
It can lead to the range of a ball A thrown at an angle of 30° to the horizontal can be the same as the range of ball B thrown at the same speed at an angle of 60° to the horizontal.