Chapter 8

Question 1

Friction

Answer 1

Force opposing the motion of a surface that moves or tries to move across another surface

Question 2

Newton’s first law of motion

Answer 2

Objects either stay at rest or moves with constant velocity unless acted on by a force

Question 3

Resultant force equation

Answer 3

F = m x a 
F = resultant force N
M = mass kg 
A = acceleration ms-²

This equation is known as Newton’s second law for constant mass

Question 4

Newton’s second law

Answer 4

The rate of change of momentum of an object is proportional to the resultant force

Question 5

Weight

Answer 5

Weight w = m x g
M = mass
G = gravity
W = weight N

Question 6

Object in equilibrium

Answer 6

When an object is in equilbrium, the support torce on it is equal
and opposite to its weight. Therefore, an object placed on a
weighing balance exerts a force on the balance equal to the weight ot the object. Thus the balance measures the weight of the object.

Question 7

Gravitational field strength

Answer 7

Force of gravity per unit mass on a small object.

Question 8

Inertia

Answer 8

Resistance of an object to change of its motion

Question 9

Mass of an object

Answer 9

The mass of an object is a measure of its inertia, which is its
resistance to change of motion. More force is needed to give
an object a certain acceleration than to give an object with less
mass the same acceleration.

Question 10

Two forces in opposite direction

Answer 10

When an object is acted on by two unequal forces acting in opposite directions, the object accelerates in the direction of the larger force.
Resultant force = F1 - F2 = m x a

Question 11

Horizontal surface

Answer 11

If an object is on a horizontal surface and F1 and F2 are horizontal and in opposite directions the resultant force equation still applies. The support force on the object is equal and opposite to its weight

Question 12

Elevator

Answer 12

In a elevator which is falling the up force is the tension in the metal ropes and the downward force is mass of the elevator x gravity

Question 13

Towing a trailer

Answer 13

Car with mass of M
Trailer with a mass of m

When the car and the trailer accelerate, the car pulls the trailer forward and the trailer holds the car back. Assume the air resistance is negligible.

Engine thrust
And
Tension in the tow bar

The car is subjected to a driving force f pushing it forwards (engine thrust) and the tension T in the tow bar holding it back.
Therefore the resultant force on the car = F - T = m x a

The force on the trailer is due to the tension T in the tow bar pulling it forward
Therefore T = m x a
Combining the two equations together = F = M x a + m x a = (M+m) x a

Question 14

Rocket equation

Answer 14

T = m x g + m x a
T = thrust 
M = mass 
G = gravity 
A = acceleration

Question 15

Lift Problem

Answer 15

Using upwards is positive gives the resultant force on the lift as T - m x g
Where T is the tension in the lift cable and m is the total mass of the lift and occurring.

Question 16

Lift A

Answer 16

If the lift is moving at constant velocity then a = 0
So T = m x g
Tension = weight

Question 17

Lift B

Answer 17

If the lift is moving up and accelerating then a > 0

So T = m x g + m x a > m x g

Question 18

Lift C

Answer 18

If the lift is moving up and decelerating then a < 0

So T = m x g + m x a < m x g

Question 19

Lift D

Answer 19

If the lift is moving down and accelerating then a α < 0
(Velocity and acceleration are both downwards therefore negative)
So T = m x g + m x a < m x g

Question 20

Lift E

Answer 20

If the lift is moving down and decelerating then a > 0 (velocity downwards and acceleration upwards therefore positive)
So T = m x g + m x a > m x g

Question 21

The tension in the cable is less than the weight if

Answer 21

the lift is moving up and decelerating (velocity > 0 and
acceleration < 0)
the lift is moving down and accelerating (velocity <0 and
acceleration < 0).

Question 22

The tension in the cable is greater than the weight if

Answer 22

the lift is moving up and accelerating (velocity > 0 and
acceleration > 0)
the lift is moving down and decelerating (velocity <0 and
acceleration > 0).

Question 23

Drag force

Answer 23

The force of fluid resistance on an object moving through a liquid

The drag force
The shape of the object
It’s speed
The viscosity of the fluid which is measured of how easily the fluid flows past a surface
The faster an object travel is in a fluid the greater the drag force on it.

Question 24

Terminal speed

Answer 24

The maximum speed reached by an object when the drag force on it is equal and opposite to the force causing the motion of the object.

Question 25

Object falling in a liquid

Answer 25

The speed of an object released from rest in a fluid increases as it falls,
so the drag force on it due to the fluid increases. The resultant force
on the object is the difference between the force of gravity on it (its
weight) and the drag force. As the drag force increases, the resultant
force decreases, so the acceleration becomes less as it falls. If it
continues falling, it attains terminal speed, when the drag force on it
is equal and opposite to its weight. Its acceleration is then zero and its
speed remains constant as it falls

Question 26

Resultant force for an object falling

Answer 26

F = m x g - D
D = drag force

Question 27

Motive force

Answer 27

The force that drives a vehicle

Question 28

Motion of a powered vehicle

Answer 28

The top speed of a road vehicle or an aircraft depends on its engine
power and its shape. A vehicle with a streamlined shape can reach a
higher top speed than a vehicle with the same engine power that is
not streamlined.
For a powered vehicle of mass m moving on a level surface, it Fe
represents the motive force (driving force) provided by the engine,
the resultant force on it = Fe - Fr, where Fr is the resistive force
opposing the motion of the vehicle.
Fe = engine force
Fr v resistive force

Because the drag force increases with speed, the maximum speed (the
is reached when the resistive force terminal speed) of the vehicle max
becomes equal and opposite to the engine force, and a = 0.

Question 29

Acceleration with motive force

Answer 29

A = Fe - Fr / m 
Mass = m

Question 30

Explain how a car arrives at max speed.

Answer 30

The top speed of a road vehicle depends on its engine power and shape, with a vehicle with a streamlined shape can reach a higher top speed than a vehicle with the same engine power that is not streamlined.

Question 31

Free fall / parachute

Answer 31

1 Initially
V = 0ms-1
Fr = m x g
A = 9.81ms-2

2 Drag and m X g are involved
V > 0ms-1
Fr = m x g - drag (drag< m x g )
A < 9.81ms-2

3 eventually drag and mxg get bigger
V = max
Fr = 0N (mxg = drag)
A = 0ms-1

When parachute is deployed
Fr <0N (negative so upwards)
A < 0ms-2 (upwards)
For this The parachute is deployed and that drag is far bigger than m x g

Question 32

Thinking distance

Answer 32

Is the distance travelled by a vehicle it takes the driver to react

Equation 
U = s1 / t0 
U = initial speed 
S1 = thinking distance 
T0 = reaction time

Question 33

Stopping distance

Answer 33

Thinking + braking

Question 34

Braking distance

Answer 34

The distance travelled by a car in the time it takes top stop safely from when the brakes are first applied

V2 = U2 + 2 x a x s

Question 35

Damage to a passenger

Answer 35

Ina collision is caused by the impact force F = m x a

Question 36

Larger acceleration

Answer 36

Larger acceleration = a larger force

A = v-u / T
A similar time = a larger acceleration

Question 37

Smaller distance =

Answer 37

= smaller time

T = 2 x s / u + v

Question 38

Safety feature

Answer 38

Every safety feature in a car is designed to increase the impact time, therefore decreasing impact force on the passengers.

Seat belt = increases the distance to stop, increasing time decreases force.
Air bag
Reinforce chassis
Crumple zone