# 2 Forces, movement and shape Flashcards

If spring A is a single spring, spring B must be..

1) stiffer spring

2) weaker spring

Answer: 2) weaker spring

B has a smaller gradient

It requires less force to stretch by the same amount.

Why is a double decker bus difficult to tip?

It has a low centre of gravity

- The weight acts from the centre of gravity.*
- It will not tip until the weight goes beyond the base*

Below is a velocity-time graph for a parachute jump.

What portion/s of the graph represents the constant speed? How do yoy know?

portion **c and e**

The velocity is constant. The graph is horizontal at c and e

gradient is zero = zero acceleration or constant speed

*The parachutist has reached terminal velocity at c and again at e (after he opens his parachute)*

How do you find the **centre of gravity** for a **symmetrical object**?

Draw lines of symmetry to find the centre of the object.

Below is a force extension graph for a wire. Does the wire obey Hook’e Law?

The wire __obeys Hooke’s Law__ during the __straight__ part of the graph. If the force were removed, the wire __behaves elastically__ and would __go back to its original length.__

*For larger forces the wire no longer obeys Hooke’s Law, the wire behaves plastically and will not go back to its original length.*

Define thinking distance

Thinking distance is the **distance** a car travel after you see the hazard to the point you put your foot on the brake - the distance travelled while you are reacting to the hazard

Below is a velocity-time graph for a parachute jump.

What portion/s of the graph represents the greatest deceleration?

portion d

The velocity is decreasing at d and the gradient is very steep representing a very large deceleration.

Why does the swinging basket finally come to rest?

Swung to one side, the line of the weight (the force) **does not act through the pivot** at the top of the hanger. M= F x d, **there is a moment** about the pivot making it swing.

When at rest the line of of the weight (the force) acts through the pivot at the top of the hanger. M = F x d. there is no moment about the pivot. No longer swings.

What is the Centre of Gravity?

The point on an object where the __weight appears to act__.

If one spring needs 5 N to stretch 10 cm. How far would this spring system stretch with 5 N pulling on it.

There are three springs in the series system. Each spring will stretch 10 cm. **The total stretch of the series spring system would be 30 cm.**

What factors affect thinking distance of a vehicle?

tiredness

drinking or drug taking

speed of vehicle

distractions such as mobile phone

Define braking distance

The __distance__ a vehicle travels while the __brakes are applied__ and the vehicle comes to rest.

How can the elephant balance on the ball?

The centre of gravtiy of the elephant is located over the ball

The weight of the elephant passes through the ball (base)

*There is no perpendiular distance to the pivot and no moment (turning effect)*

How do you find the stopping distance from thinking distance and braking distance?

stopping distance = thinking distance + braking distance

If the speed of a vehicle doubles, what happens to the braking distance?

As speed double the braking distance more than doubles- it actually quadruples!

If the single spring needs 2 N to stretch 10 cm, how many centimetres will the __series spring system__ stretch with the same force?

There are two springs in series with 2 N hanging on them. They will both stretch by 10 cm.

**The overall stretch will be 20 cm.**

Which spring is stiffer or has the greater spring constant?

Spring A needs more force to stretch the same distance as spring B.

Gradient for spring A is greater.

What is the equation that links moment, force and perpendicular distance?

Moment = force x perpendicular distance