T5 forces and motion

Question 1

does a distance quantity require a specific direction

Answer 1

no specific direction required so it is a scalar quantity

Question 2

typical value for the speed of sound

Answer 2

330 m/s

Question 3

typical value for human walking speed

Answer 3

1.5 m/s

Question 4

typical value for human running speed

Answer 4

3 m/s

Question 5

typical value for human cycling speed

Answer 5

6 m/s

Question 6

equation linking distance, speed and time

Answer 6

distance = speed x time

distance (m), speed (m/s), time (s)

Question 7

why can an object travelling at a constant speed in a circle not have a constant velocity

Answer 7

• speed is a scalar quantity
• velocity is a vector meaning, it can only be constant if the direction is constant
• in a circular motion, the direction is continuously changing

Question 8

how can speed be calculated from a distance-time graph

Answer 8

speed is equal to the gradient of the graph

Question 9

what must be done to calculate speed a given time from a distance-time graph for an accelerating object

Answer 9

• drawing a tangent to the curve at the required time

- calculating the gradient of the tangent

Question 10

equation for the average acceleration of an object

Answer 10

acceleration = (change in velocity) / (time taken)

Acceleration (m/s²), Velocity (m/s), Time (s)

Question 11

how can the distance travelled by an object be calculated from a velocity-time graph

Answer 11

equal to the area under the graph

Question 12

give an approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface

Answer 12

9.8 m/s²

Question 13

what can said about the resultant force acting on an object when it is falling at terminal velocity

Answer 13

• resultant force is zero

- when at terminal velocity, the object is moving at a constant speed and so isn’t accelerating

Question 14

state Newton’s first law for a stationary object

Answer 14

if the resultant force on a stationary object is zero, the object will remain at rest

Question 15

state Newton’s first law for a moving object

Answer 15

if the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction)

Question 16

what can be said about the braking forces and driving forces when a car is travelling at constant velocity

Answer 16

the braking forces are equal to the driving forces

Question 17

if an object changes direction but remains at a constant speed, is there a resultant force

Answer 17

since there is a change in direction, there is a change in velocity and so there must be a resultant force

Question 18

what is inertia

Answer 18

tendency of an object to continue in its state of rest or uniform motion

Question 19

state the defining equation for Newton’s second law

Answer 19

resultant force = mass x acceleration

F = ma

Question 20

state Newton’s second law in words

Answer 20

an object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass

Question 21

what is inertial mass

Answer 21

• measure of how difficult it is to change a given object’s velocity
• ratio of force over acceleration

Question 22

State Newton’s third law

Answer 22

whenever two objects interact, the forces that they exert on each other are always equal and opposite

Question 23

what is the stopping distance of a vehicle equal to

Answer 23

the sum of thinking distance and braking distance

Question 24

for a given braking distance, if the vehicle’s speed is increased, what can be said about its stopping distance

Answer 24

the stopping distance is increased with an increase in speed

Question 25

give a typical range of values for human reaction time

Answer 25

0.2 - 0.9 s

Question 26

3 factors which can affect a driver’s reaction time

Answer 26

1. tiredness
2. drugs
3. alcohol

Question 27

2 factors which may affect braking distance

Answer 27

1. adverse road conditions

2. poor tyre/brake conditions

Question 28

describe the energy transfers that take place when a car applies its brakes

Answer 28

• work is done by the friction force between the brakes and wheel
• kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs

Question 29

to stop a car in a given distance, if its velocity is increased, what must happen to the braking force applied

Answer 29

the braking force must also be increased

Question 30

2 consequences of a vehicle undergoing very large develerations

Answer 30

1. KE converted to heat is very high causing brakes to overheat
2. loss of control of the vehicle