Forces

Question 1

Define resultant force

Answer 1

The total/overall force applied to a point (replaces a number of forces by simplification)

Question 2

What happens when the resultant force on an object is zero? Not zero?

Answer 2

Zero: The acceleration will be 0 - it will stay stationary if already stationary OR continue to move at the same velocity if already moving
Not zero: The object will accelerate in the direction of the resultant force

Question 3

Equation linking acceleration, force and mass?

In terms of proportion?

Answer 3

a = F ÷ m OR F = m x a [when F is in newtons, N]

a ∝ F AND a ∝ 1/m SO m x a ∝ F

Question 4

What happens when two objects interact?

Answer 4

They always exert equal and opposite forces on each other (when you punch someone it hurts you as well as that person)

Question 5

What effects might a force have on an object?

Answer 5

Change its shape, its motion or its state of rest

Question 6

Describe the forces present when a vehicle travels at constant speed

Answer 6

The resistive forces (mostly air resistance-caused, otherwise friction between car parts) balance the driving force (caused by friction between the wheels and the road)

Question 7

Define thinking distance, braking distance and stopping distance

Answer 7

Thinking: the distance travelled while during the driver’s reaction time
Braking: the distance travelled under the braking force (i.e. while the brakes are held)
Stopping: thinking distance + braking distance

Question 8

What might slow a driver’s reaction time? What else might increase the thinking distance?

Answer 8

Tiredness, drugs, alcohol

Distractions (children talking, music, looking out the window)

Question 9

What conditions might increase the braking distance?

Answer 9

Speed, road conditions (icy or wet) or car conditions (quality/condition of brakes and tyres)

Question 10

Equation linking momentum, mass and velocity?

Unit of momentum?

Answer 10

momentum = mass x velocity
P = m x v
Kgm/s

Question 11

What would a positive sign indicate when talking about momentum? Negative? Why is this?

Answer 11

+ve: movement from left to right
-ve: movement from right to left
Why?: momentum is a vector quantity

Question 12

Equation linking momentum, force and time?

Answer 12

force = change in momentum ÷ time taken
F = mv – mu ÷ t    OR   F = Δ(mv) ÷ t [Δ means change in]

Question 13

What does conservation of momentum mean?

Answer 13

In any collision in which no external forces act, momentum is conserved

Question 14

Why does a crumple zone on a car/cycle helmet/air bag prevent injury?

Answer 14

The time over which the force of impact is applied increases. t ∝ 1/F . When t increases, F decreases
Cushioning is NOT precise enough and NOTHING is absorbed

Question 15

Define weight and give its unit

Answer 15

Mass of an object x the gravitational field strength (W = mg)
Newtons, N

Question 16

What can be said about acceleration due to gravity?

Answer 16

It is numerically equal to the strength of the gravitational field and is independent of mass, IF gravity is the only active force

Question 17

How does speed relate to frictional force (friction/air resistance/drag)?

Answer 17

The higher the speed (the faster), the more frictional force

Question 18

Why does a person, falling through the air, stop accelerating after some time? What happens when they open a parachute?

Answer 18

Terminal velocity. Air resistance becomes equal to the weight, so the resultant force is 0.
Parachute: air resistance increases, so velocity (and therefore the air resistance) decreases, until the weight is equal to the AR again.

Question 19

What happens to a spring or elastic object when force is applied?

Answer 19

It will stretch and store elastic potential energy

Question 20

If an object can recover its original shape (e.g. a spring), when is elastic potential energy stored and what does it do to the object?

Answer 20

When work is done on it

It changes its shape

Question 21

What does Hooke’s law state (about proportionality with force and extension of a spring)?

Answer 21

Up to some point (limit of proportionality) the extension of the spring is directly proportional to the applied force.
F ∝ x therefore F = kx [k is the spring constant in newtons per metre, N/m]

Question 22

What happens if a spring is loaded beyond its limit of proportionality?

Answer 22

The spring is damaged/deformed so that its behaviour is no longer predictable.