Forces (paper 2)

Vector quantity

Vector quantity

• Have both magnitude and direction

• eg. force, acceleration, momentum, etc

• represented by arrows, length of arrows show the size (magnitude)

Magnitude

a number/size

what are Scalar quantities

• have only magnitude but not direction

• eg. speed, distance, mass temperature, time, etc

A force

• push or pull on an object that is caused by it interacting with something

• either contact of non-contact

what is a contact force?

Two objects touching for a force to be acting

Examples of non-contact forces

• gravitational force

• magnetic force

• electrostatic force

what is Weight?

• The force exerted on a mass by the gravitational field

• measured in newtons, which can be calculated by a newtonmeter

• depends on the strength of the GP field

define Mass

The amount of matter in an object

Same person, on two different planets?

• Their mass is the same

• Their weight is different because the gravitational field strength is different on both planets

Centre of Mass

The single point where an object's weight is said to act (balanced in all directions)

What’s the gravitational field strength on Earth?

9.8 N/kg

what does Increasing the mass do to the weight?

increases the weight (directly proportional)

describe Free body diagrams

• weight acts pulling down

• drag acts upwards (air resistance)

• sizes of arrows show magnitudes

Resultant force

• the overall force on a point or object

• found by subtracting the opposite forces

The thing applying the force needs?

a source of energy (fuel or food)

What happens when a force moves an object through a distance?

• energy is transferred

• work is done on the object (movement)

what happens when you’re pushing something on a rough surface

• doing work against frictional forces

• energy is being transferred into kinetic stores

• then thermal stores due to friction causing temperature of the object to increase

In equilibrium

when all the forces acting on an object combine to give a resultant force of 0N

Elastically deformed

• object may stretch, compress or bend

• more than one force acting on the object

• it can go back to its original shape and length after force is removed

The extension of the spring is what in relation to the force applied?

directly proportional to the force applied (if one increases or decreases the other does the same)

what does a stiffer spring have?

greater spring constant

To work out the compression

difference between natural and compressed lengths

When maximum force is reached how could this be shown on a graph?

graph would curve (upwards or downwards) meaning extension is no longer proportional to force (limit of proportionality)

define Distance

• How far an object has moved

• is a scalar quantity (doesn’t involve direction)

define Displacement

• The change in position

• a vector quantity

• Distance and direction in a straight line

What do speed and velocity measure?

How fast you’re going

Whats the speed of an average person walking/running/cycling?

• 1.5 m/s

• 3 m/s

• 6 m/s

What’s the average speed of a car/train/plane?

• 25 m/s

• 30 m/s

• 250 m/s

What depends on a persons speed?

• fitness

• Age

Define acceleration

How quickly you’re speeding up (change in velocity)

What’s deceleration?

Something slowing down

What’s uniform acceleration?

Constant acceleration

What’s acceleration measured in?

M/s^2

describe Distance time graphs

• gradient = speed (steeper means faster)

• Flat line = stationary

• Straight uphill = steady speed

• Curves = acceleration or deceleration

• Levelling off = slowing down

Velocity time graphs

• gradient = acceleration

• Flat = steady speed

• Curve = change in direction

What does the section underneath the velocity time graph represent?

The distance travelled

Friction always acts…

In the opposite direction to movement

How can you travel at a steady speed?

Force needs to balance the frictional forces

What’s drag?

The resistance you get in a fluid or a gas

An example of drag?

Air resistance

If drag increases?

Resultant force from weight decreases, speed increases

What’s terminal velocity?

When the resultant force of all the forces acting on an object = 0

What is the accelerating force acting on all falling objects?

Gravity

Time line of a skydiver

1. As he falls, he accelerates, increasing his speed

2. As air resistance increases, the resultant force from weight decreases (parashoot)

3. acceleration decreases, so he is not speeding up as quickly

4. eventually the forces will balance reaching a resultant force of 0 (reaching terminal velocity)

Newton’s 1st law

A resultant force is needed to make something start moving (speed up, or slow down)/forces need to be balanced so that something can move

Newton’s 2nd law

• force = mass x acceleration

• greater force = greater acceleration

• greater mass = acceleration reduces

Objects that aren’t moving have no…

Momentum

What is momentum

The product of mass and velocity

What’s the conservation of momentum?

Total momentum before and after any event must be equal

Newtons 3rd law

When two objects interact, the forces they exert on each other are equal and opposite (pushing against a wall)

What’s the stopping distance?

The distance it takes to stop a car in an emergency

How do we find the stopping distance?

Thinking distance + braking distance

What’s the thinking distance?

The distance travelled by the vehicle in the time it takes for the driver to react

What is thinking distance affected by?

• speed - the faster you’re going the further you’ll travel during the time you react

• reaction time - longer reaction time = longer distance

• alcohol

• Drugs

• Fatigue

What’s braking distance?

The distance travelled by vehicle after breaks are applied

What’s braking distance affected by?

• speed - faster = longer to stop

• Weather/road surface - wet or icy/leaves or oil, less grip

• Conditions of tyres - can’t get rid of water, become slippery

• Brakes - worn or faulty = won’t be able to apply much force

The longer your stopping distance means?

The more space you need in front, in order to stop safely

Define a closed system

Only the exchange of energy is allowed

What are crumple zones?

• It absorbs energy to deform and compact

• It increases the time taken for the car to stop

• reduces acceleration and force on passengers

• Without them cars would immediately stop without softening the blow

what do Seat belts do?

• Stretch slightly

• Increasing time taken for wearer to stop

what do Air bags do?

• inflate before you hit dashboard

• Compressing air inside slows you down more gradually

How can you determine the extension of a spring?

Measure the spring before extension then measure it after and find the difference