Chapter 7 - Kinematics

Question 1

Displacement defintion

Answer 1

Distance in a given direction

Question 2

Speed definition

Answer 2

Change of distance per unit time

Question 3

Velocity definition

Answer 3

Change of displacement per unit time
- Speed in a given direction
- Rate of change of displacement

Question 4

Speed equation

Answer 4

Speed = distance travelled/time
- v = s/t

Question 5

Equation for constant speed in a circular motion

Answer 5

Constant speed on a circle of radius, r = 2πr/T
- T = time taken to complete one rotation

Question 6

Shape of a distance-time graph for an object moving at constant speed

Answer 6

• Straight line with a constant gradient
• Speed = distance/time = gradient of the line

Question 7

Motion of an object moving with constant velocity

Answer 7

• Moves at the same speed without changing its direction of motion

Question 8

Acceleration definiton

Answer 8

Change of velocity per unit time
- Rate of change of velocity

Question 9

What is the gradient of a velocity-time graph equal to?

Answer 9

The acceleration

Question 10

What are the conditions of motion under which the kinematic equations apply?

Answer 10

• Uniform acceleration
• Motion along a straight line

Question 11

Equstion for motion without displacement (s)

Answer 11

v = u + at
- derived from y = mx + c → velocity time graph
- y-axis = v, x-axis = t
- y-intercept = c = u
- gradient = x = t

Question 12

What is non-uniform acceleration?

Answer 12

When the direction of motion of an object changes, or its speed changes, at a varying rate

Question 13

How is non-uniform acceleration seen on a velocity-time graph?

Answer 13

• Curve
• Gradient represents acceleration so increases/decreases with time
• Second derivative is rate of change of acceleration

Question 14

Equation for motion without acceleration (a)

Answer 14

s = ((u+v)t)/2
- derived from the area of a trapezium = 1/2(a+b)h → s = area under graph
- Can also be derived from average velocity = (u+v)/2 combined with s = vt

Question 15

Equation for motion without time (t)

Answer 15

v^2 = u^2 + 2as
- Derived by combining a = (v-u)/t and s = ((u+v)t)/2

Question 16

Equation for motion without final velocity (v)

Answer 16

s = ut + 1/2at^2
- Derived by combining s = ((u+v)t)/2 and v = u + at

Question 17

How is displacement represented on a velocity-time graph?

Answer 17

From the area under the graph

Question 18

Why does a heavy object not fall faster than a lighter one in a vacuum?

Answer 18

W = mg
- Acceleration due to gravity is constant
- g = W/m → When mass increases, weight also increases proportionally, so the increase cancels out in the equation for acceleration

Question 19

How does the steepness of a slope affect acceleration?

Answer 19

• Acceleration is greater the steeper the slope

Question 20

How to investigate the free fall of a ball?

Answer 20

• Take a multiflash photo or video of the ball’s flight as it falls after being released from rest
• Verticle metre rule provides scale
• Ordinary camera with slow shutter speed used to record ball’s descent in a dark room illuminated by a stroboscope flashing at a constant rate of about 20 flashes per second
• Record the distance fallen at a given time
• Calculate acceleration from s = ut + 1/2at^2
• Graph of s against t^2 will be a straight line through the origin with acceleration of 2 x gradient

Question 21

When is an object in free fall?

Answer 21

When there are no external forces acting on the object apart from the force of gravity

Question 22

What is the acceleration of free fall?

Answer 22

Acceleration due to gravity
- 9.81 ms^-2 when near the Earth’s surface

Question 23

Motion of a ball thrown directly upwards on a displacement-time graph

Answer 23

• Immediately after being thrown, the velocity is positive and large so the gradient is positive and large
• As the ball rises, velocity decreases so gradient decreases
• At max height, velocity is zero so gradient is zero
• As the ball descends, its velocity becomes increasingly negative, so gradient becomes increasingly negative, as its now moving in the opposite direction

Question 24

What is a projectile?

Answer 24

Any object acted upon only by the force of gravity

Question 25

Key prinicples that apply to all projectiles

Answer 25

1. Acceleration only affects the vertical motion of the object - acceleration always equal to g and acts downwards as force of gravity acts downwards.
2. Horizontal velocity is constant because the acceleration does not have a horizontal component
3. Motions in the horizontal and vertical directions are independent of each other

Question 26

When do two objects reach the ground, one dropped vertically and the other projected horizontally, relative to each other and why?

Answer 26

• Both reach the ground at the same time
• Both experience the same vertical acceleration and start from the same vertical velocity (zero)
• Horizontal velocity independent from (has no effect on) vertical velocity

Question 27

Why does an object projected velocity fall with a parabolic trajectory in the absence of air resistance?

Answer 27

• Horizontal velocity is constant - no horizontal velocity
• Vertical velocity increases at a constant rate - uniform vertical acceleration - vertical path gets steeper as it falls

Question 28

When will an object experience projectile-like motion?

Answer 28

When the object has constant acceleration in a different direction to its velocity

Question 29

What causes the drag force on a moving object?

Answer 29

• Air resistance - friction between the layers of air near the projectile’s surface where the air flows over the surface

Question 30

How does the drag force act?

Answer 30

• Acts in the opposite direction to the direction of motion of the projectile
• Drag force increases as projectile’s speed increases

Question 31

How does the drag force affect the horizontal motion of a projectile?

Answer 31

Horizontal component of drag force reduces the horizontal speed of the projectile and its range

Question 32

How does the drag force affect the vertical motion of a projectile?

Answer 32

• Reduces its maximum height of the projectile if its initial direction is above the horizontal
• Makes its descent steeper than its ascent

Question 33

How can the shape of the object cause a lift force?

Answer 33

When the shape casuses air to flow faster over the top of the object than underneath it
- Causes the pressure on the top of the surface to be less than on the bottom
- Pressure difference causes the lift force

Question 34

Equation for vertical displacement when air resistance is present

Answer 34

y = UtsinΘ - 1/2gt^2
- Vertical displacement = UtsinΘ without air resitance
- Displacement 1/2gt^2 lower due to drag

Question 35

Equation for vertical component of velocity when air resistance is present

Answer 35

V(y) = UsinΘ - gt
- Vertical velocity = UsinΘ without air resistance
- Velocity gt lower due to drag