Physics paper 2: FORCES

CONTACT: Forces need to be touching.

e.g. friction

NON- CONTACT:  Forces don't need to touch.

e.g. gravitational force

SCALAR: Quantities that have magnitude but no direction.

e.g. speed

VECTOR:  Quantities that have magnitude and direction.

e.g. velocity

Gravity: - vector quantity

             - makes everything fall to the floor

             - gives everything a weight.

             - measured in N/Kg

Mass: 'amount of matter in an object.' Measured in Kilograms.

Weight: Force acting on an object. Measured in Newtons.

Mass and Weight are directly proportional.

WEIGHT = MASS x GRAVITATIONAL FIELD STRENGTH

Free body diagram: Shows the forces acting on an object.

Resultant Force: Overall force on an object.

* WHEN A FORCE MOVES AN OBJECT, ENERGY IS TRANSFERRED  AND WORK IS DONE.*

Work done is measured in Joules

Force is measured in Newtons

Distance is measued in Metres.

WORK DONE = FORCE X DISTANCE

-If object is at equilibrium, all the forces are balanced. So, it joins tip-to-tail...the resultant force is ZERO.

- If a force is at an awkward angle, split it into components.

Draw a scale drawing, then add the horizontal and vertical lines on the grid. Then, just measure it with a ruler.

http://alevelphysics.org.uk/forces.html => this site explains it all! 

On a spring, energy can transferred by compressing, bending and  stretching.

When an object is inelastically deformed, it doesn't go back to it's original shape.

Force and extension are directly proportional.

Force is measured in Newtons

Spring Constant is measured in N/m

Extension is measured in Metres

FORCE = SPRING CONSTANT X EXTENSION

Limit of proportionality => maximum force which causes graph to curve, force is no longer directly proportional to extension.

 - Measure the length of an unstretched spring.

 - On the end of a clamp, add the unstretched spring.

 - add a fixed ruler to the clamp to measure the extension.

 - at the end of the spring add masses one at a time, up to a total of five.

 - Remember to measure the length of the spring each time you add a mass.

 - Finally, plot your results on a force- extension graph

To find the energy stored:

ELASTIC ENERGY (IN JOULES)= 1/2 X SPRING CONSTANT X EXTENSION^2

- Moment is the turning effect of a force.

Moment is measured in N/m

MOMENTS = FORCE X DISTANCE

- If object is balanced, anticlockwise moment = clockwise moment.

- Levers: increase the distance so less force is needed to cause movement.

- Gears: transmit a rotational effect

PRESSURE: Force per unit area.

Pressure is measured in Pascals

Area in m^2

PRESSURE = FORCE ÷ AREA

- In a liquid pressure depends on density and depth.

The more dense a liquid is, the more particles it has in a space, this increases pressure.

As the depth increases, the particles also increase causing pressure to also increase.

PRESSURE = HEIGHT X GRAVITATIONAL FIELD STRENGTH X HEIGHT 

* DON'T NEED TO KNOW THIS EQUATION *

- Upthrust is the resultant force acting upwards.

If upthrust and the object's weight equals, then the object floats.

- Atmospheric pressure decreases as height increases.

As you get higher up, the atmoshere gets less dense, so there are fewer air molecules colliding which causes little pressure.

Distance: How far an object has moved.

Displacement: Measures the distance and direction in a straight line, of an object's movement.

Speed: How fast you're going.

Velocity: Speed in a direction.

Speed is measured in m/s.

Time is in seconds.

DISTANCE = SPEED X TIME

Acceleration: How quickly you're speeding up.

Acceleration is measured in m/s^2

Velocity is in m/s

ACCELERATION = CHANGE IN VELOCITY ÷ TIME

- Due to gravity, acceleration is uniform, so it's constant.

V^2 - U^2 = 2AS

* DON'T NEED TO KNOW THIS EQUATION *

DISTANCE

- Gradient = speed

- Flat = stationary

- straight line = speeding up

- curves = acceleration/ deceleration

Velocity

- Gradient = acceleration

- Flat = steady speed

- straight line = constant acceleration

- curves = changing acceleration

- Friction always slows things down.

- Drag is the reistance in fluid.

- As drag increases, speed increases

- Terminal velocity is the maximum speed

- Terminal velocity depends on shape and area

First law:

- A RESULTANT FORCE IS NEEDED TO MAKE AN OBJECT MOVE.

Second law:

- FORCE AND ACCELERATION ARE DIRECTLY PROPORTIONAL.

FORCE = MASS X ACCELERATION

Inertia: the tendency for motion to remain unchanged.

Inertial mass: How difficult it is to change velocity.

Newton's third law:

- WHEN OBJECTS INTERACT, THE FORCES EXERTED ARE EQUAL AND OPPOSITE.

1.Set up a trolley which holds a piece of card with a gap in the middle.

2.Measure card length and input this into a software. (The light gate will measure the velocity for each card)

3.Connect a string that goes over a pulley which is connected to a hook.

4.Weight of hook and mass will provide accelerating force.

5.Mark a starting line on the table and place the trolley there.

6.Hold the hook so the string is not loose, then release it.

7.Record acceleration measured by light gate.

8.Repeat to get mean acceleration.

Stopping distance= Distance it takes to stop. (surprisingly!)

Thinking distance= Distance the car travels during driver's reaction time.

STOPPING DISTANCE = THINKING DISTANCE + BRAKING DISTANCE

Thinking distance relies on: - speed     - reaction time

Braking distance relies on: - tyre condition   - speed    - road surface

Ruler drop test:

1. Sit with arm on the edge of the desk, ask someone to hold a ruler between their fingers.

2. Make sure to line the ruler at zero.

3. Without warning, drop the ruler. 

4. The other person should catch the ruler as quickly as possible.

5. The measurement on the ruler shows how far the ruler dropped during the person's reaction time.

6. Longer the distance, longer the reaction time.

7. Do lots of repeats and calculate the mean.

- Thinking distance increases at the same rate as speed.

( If you speed up, you cover more distance so you have more time to think.)

- Braking distance increases at a faster rate.

( If you speed up, it's more difficult to brake, so braking distance increases.)

Momentum is measured in Kg m/s

MOMENTUM = MASS X VELOCITY

Conservation of Momentum: Momentum before = Momentum after

- This normally occurs in a closed system.

- Forces cause a change in momentum

FORCE = CHANGE IN MOMENTUM ÷ TIME

- Larger the force, bigger the change.

Examples:

- Cars: have crumple zones to increase time for car to stop.

- Bike helmets: layer of foam increases time for head to stop in a crash.