P5 - Forces 3 Forces , Elasticity and Motion

Question 1

Explain the relationship between force applied and the extension of an elastic object

Answer 1

The extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded

Question 2

What is meant by an inelastic (plastic) deformation ?

Answer 2

• A deformation which results in the object being permanently stretched
• The object doesn’t return to its original shape when the force is removed

Question 3

State the equatiom relating force, spring constant and extension . Give appropriate units

Answer 3

Force = Spring Constant x Extension

Force (N) , Spring constant (N/m) , Extension (m)

Question 4

What type of energy is stored in a spring when it is stretched ?

Answer 4

Elastic potential energy

Question 5

What can extension be replaced with in the equation for spring force ?

Answer 5

Compression

Question 6

Does a distance quantity require a specific direction ? i.e. is it a scalar or vector quantity

Answer 6

No specific direction is required so it is a scalar quantity

Question 7

If an object moves 3 metres to the left and then 3 metres back to its initial position, what is the object’s total displacement?

Answer 7

• The object has zero displacement
• Displacement is a vector quantity so it also involves direction
• The object starts and ends at the same point

Question 8

State a typical value for the speed of sound

Answer 8

330 m/s

Question 9

What is a typical value for human walking speed ?

Answer 9

1.5 m/s

Question 10

What is a typical value for human running speed ?

Answer 10

3m/s

Question 11

What is a typical value for human cycling speed ?

Answer 11

6 m/s

Question 12

State the equation linking distance , speed and time . Give appropriate units

Answer 12

Distance = speed x Time

Diatance (m) , Speed (m/s) , Time (s)

Question 13

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

Answer 13

• Speed is a scalar quantity
• Velocity is a vector quantity which means it can only be constant if the direction is constant
• In circular motion, the direction is continuously changing

Question 14

How can speed be calculated from a distance-time graph ?

Answer 14

The speed is equal to rhe gradient of the graph

Question 15

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

Answer 15

• Drawing a tangent to the curve at the required time
• Calculating the gradient of the tangent

Question 16

State the equation for the average acceleration of an object. Give appropiate units.

Answer 16

Acceleration = (Change in Velocity)/(Time Taken)

Acceleration (m/s^2), Velocity (m/s) , Time (s)

Question 17

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

Answer 17

It is equal to the area under the graph

Question 18

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

Answer 18

• The resultant force is zero
• When at terminal velocity, the object is moving at a constant speed and so isn’t accelerating

Question 19

State Newton’s first law for a stationary object

Answer 19

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

Question 20

State Newton’s first law for a moving object

Answer 20

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

Question 21

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

Answer 21

The braking forces are equal to the driving forces

Question 22

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

Answer 22

Since there is a change in direction , there is a change in velocity and so there must be a resulatant force

Question 23

What is inertia ?

Answer 23

The tendency of an object to continue in the same state of motion

Question 24

State the defining equation for Newton’s Second Law

Answer 24

Resultant force = Mass x Acceleration

F = ma

Question 25

State Newton’s Second Law in words

Answer 25

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

Question 26

What is inertial mass ?

Answer 26

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

Question 27

State Newton’s third law

Answer 27

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

Question 28

What is the stopping distance of a vehicle equal to ?

Answer 28

The sum of thinking distance and braking distance

Question 29

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

Answer 29

The stopping distance is increased with an increase in speed

Question 30

Give a typical range of values for human reaction time

Answer 30

0.2 seconds - 0.9 seconds

Question 31

Give three factors which can affect a drivers reaction time

Answer 31

1. Tiredness
2. Drugs
3. Alcohol

Question 32

Give two factors which may affect braking diatance

Answer 32

1. Adverse (wet/icy) road conditions
2. Poor tyre/brake conditions

Question 33

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

Answer 33

• 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 theough the brake discs

Question 34

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

Answer 34

The breaking force must also be increased

Question 35

State two consequences of a vehicle undergoing very large declarations

Answer 35

1. Kinetic energy converted to heat is very high causing brakes to overheat
2. Loss of control of the vehicle