# P2.1 Forces and their Effects

## Speed, Distance and Time

The formula linking the three quantities is:

Speed(m/s) = Distance(m) / Time(s)

**Example**

1. Can you change 70km/hr into a speed of m/s?

70km/1hr

=70000m/3600s

=19.4444444444m/s

## Distance-Time Graph

A distance-time graph is a visual representation of an objects journey.

1. Straight sloping line = Constant Speed

2. Horizontal line = Stationary

3. The steeper the line, the faster the object is travelling

4. The gradient of the graph (Δy/Δx) is equal to the speed of the object.

## Velocity

**Velocity **is **speed** in a given **direction.**

Velocity quantities --> velocity, momentum **Have magnitude and direction**

Scalar quantities --> speed, energy **Have magnitude only**

Two objects can have the same speed but different velocities

If one car is travelling 30m/s right and another car is travelling 30m/s left, the car on the right would have a positive velocity of 30m/s and the car on the left would have a negative velocity of -30m/s.

## Acceleration

Acceleration **(deceleration) **is how quickly an objects velocity increases **(decreases)**.

Acceleration (m/s^2) = change in velocity (m/s)/time(s) or a = v - u / t

a = acceleration v = final velocity u = initial velocity t = time

Decreasing acceleration is called deceleration.

## Velocity-Time Graphs

Show the motion of an object. We can use the graphs to calculate the velocity, acceleration and distance travelled.

**They're often confused with distance-time graphs so check what's on the y axis.**

The steeper the line, the quicker the acceleration. **Area under the graph is the distance that the object has travelled. **The gradient of the velocity-time graph is

Acceleration = Change in velocity/time

## Resultant Forces

A resultant force is a single force which can replace all the forces acting on an object and have the same effect.

Example:

To work out the resultant force you add up the total force being exerted on each side and see which one is bigger.

Using example 2, the 10N is bigger than 5N so to find the resultant force you minus the smaller force from the bigger force and draw an arrow in the direction of the resultant force.

## Forces on the Road

**Stopping Distance **- the shortest distance a vehicle can safely stop in, and it is in two parts. Thinking Distance and Braking Distance.

## Stopping Distance and Braking Distance

**Factors that affect the Braking Distance**

- How wet the conditions are i.e. icy, wet (rain). Can double the time it takes to stop
- Worn brakes and/or tyres
- Speed of the car

**Factors that affect the Thinking Distance**

- Alcohol
- Speed of the car
- Tired
- Distracted
- Drugs

## Forces on the Road

Vehicles can be modified to air resistance by making the car **weigh less** or making the car **more streamlined**, which reduces air resistance.

**Speed Cameras**

They're calibrated so cars within speed limit are invisible. Only vehicles above the limit are seen and photographed. As the vehicle crosses the radar beam, Gatso camera are activated and takes 2 photographs half a second apart. An inbuilt computer works out the speed.

Calibration and checking regime is rigorous in the extreme to ensure no innocent drivers are photographed

- Average speeds fell by 17%
- Death and serious injuries fell by 55%
- There were no child deaths in road accidents for the first time since 1927

**Anti-skid Surface**they are high friction surfaces which greatly improve grip, particularly in the wet. They are often used to improve pedestrian safety - in factory walkways for example. Used on sharp corners or place you need to slow down.

## Force and Acceleration

Whenever an object accelerates, decelerates or changes direction, a resultant force is involved.

**Forces and Motion Examples**

1. *A stationary object remains stationary unless it is acted on by a resultant force*. The object remains stationary because there is no resultant force (forces are balanced).

2. *Any object travelling at constant speed will remain at this constant speed unless acted on by a resultant force. *The object will remain at a constant speed because there is no resultant force.

3. *A stationary object acted on by a resultant force will accelerate in the direction of the resultant force.* The object will accelerate to the direction of the resultant force.

4. *A moving object acted on by resultant force will accelerate in the direction of the resultant force. *The object will accelerate in the direction of the resultant force and the direction it is travelling because the resultant force is in that direction.

5. *A moving object acted on by a resultant force in the opposite direction to its movement will experience a deceleration.* The object has a resultant force in the opposite direction to the velocity, so the object will decelerate.

## Acceleration

Two factors affect the size of the objects **acceleration**:

1. Mass - *the lower the mass the bigger the acceleration.* This is an inversely proportional relationship

2. Resultant - *as the resultant force increases the acceleration also increases. *This is a proportional relationship

The equation linking these three quantities is:

F = m x a

F = Resultant Force (N)

m = Mass (kg)

a = acceleration (m/s^2)

## Changing your weight

If you wanted to lose weight, aside from diet and exercise, how could this be done?

Weight = mass x gravitational field strength

N = kg x N/kg

On earth g = 10N/kg

**Example**

On Neptune an 80kg astronaut would have a weight of 900N. What is the g.f.s of Neptune?

900/80 = 11.25 N/kg

## Forces and Streching

When we apply a force to an **elastic object** the object will stretch or extend.

We can see that the **force **and **extension of a spring **show a **directly proportional **relationship, As the line of best fit is a straight line through the origin. This is Hooke's Law. 'The extension of a spring is directly proportional to the force applied providing the **limit of proportionality** has not been exceeded.

The **limit of proportionality **is exceeded when an elastic object is stretched past its **elastic limit**. An object stretched past this point is permanently **deformed** and will not return to its **original shape**.

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