# SP2 Motion and Forces

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- Created by: AStarStudent2004
- Created on: 20-05-19 20:32

## SP2a Resultant Forces

**Scalar quantity**- only has**magnitude**.**Vector quantity**- has both**magnitude**and**direction**.**Resultant forces**are the**overall**force on an object.- If the resultant force is
**zero**, the object is**balanced**. - If there is a
**non-zero**resultant force, the forces are**unbalanced.**

To calculate the resulant force:

- acting in the
**same**direction -**add**forces together - acting in
**opposite**direction -**subtract**one force from another.

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## SP2b Newton's First Law

**Newton's 1st Law**- a object will remain**unchanged**unless a**resultant**force acts on it.**Balanced**forces -**doesn't**change velocity.**Unbalanced**forces -**changes**direction and/or speed.- Object moving in a
**circle**is constantly changing**velocity**, but speed**remains**the same. **Centripetal force**- resultant force that causes a**change**in direction of an object and acts towards the**centre**of the circle. e.g.**friction, gravity and tension**.

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## SP2c Mass and Weight

**Mass**- how much**matter**there is in an object,**(kg)**.**Weight**- measure of the**pull of gravity**on an object,**(N)**.- Measured using a
**force meter** - Gravitational Field Strength -
**10N/kg** **weight (N) = mass (kg) x gravity (N/kg)**

Forces on falling bodies

- Air resistance
**increase**with speed - resistance is**smaller**than weight & large**resultant**force - accelerate**downwards** - Air resistance
**larger**but weight is the**same**& resultant force**smaller**but accelerating (**not as much).** - Moving
**fast**- air resistance**balances**weight & falls at**same**speed.

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## SP2d Newton's 2nd Law

**Newtons's 2nd Law**- f=ma/**acceleration**depends on:**Size**of**force**(same mass, the**bigger**the**force**,**bigger**the**acceleration**).**Mass**of object (same force, the**bigger**the**object**,**smaller**the**acceleration**).**force (N) = mass (kg) x acceleration (m/s²).**- i
**nertial mass**= the force on it divided by the acceleration that force produces.

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## SP2d Core Practical

- Investigate the
**relationship**between**force**,**mass and acceleration**by varying the masses added to trolleys.

1. Prop up one end of the ramp so that the trolley just starts to move on **its own**. Set up the **light gates** and the pulley and string, so the masses are hanging **off** the table.

2. Release the trolley and calculate the **speed** from the **data logger**. 3.Put a mass ontop of the trolley and **repeat** steps.

- The investigation was how the
**mass**of the trolley affects the acceleration - To investigate the effect of
**force**on acceleration, keep the**mass**the**same**and change the**masses**at the end of the string.

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## SP2e Newton's Third Law

**Newton's 3rd Law**- for every action, there is an**equal**and**opposite**reaction. They are also the same type of force.**Equilibrium situation**-**nothing**is moving.- E.g The weight of a dog on ground = the force pushing up on the dog from ground & force from dog on rope = force from rope on dog.
**Action-reaction**pairs act on**different**objects**Balanced forces**all acto on the**same**object- E.g. Force of ground on dog = gravity on dog & force of rope on dog = force of friction on dog.
- In a
**collision**, action-reaction forces are the**same size**but**don't**have the**same affect**due to the objects having**different masses**.

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## SP2f Momentum

**Momentum**- tendency of an object to**keep moving**and how hard to stop moving.- Depends on mass and velocity, and is a vector.
**Momentum (kg m/s) = mass (kg) x velocity (m/s)**or**p= m x v****Force = change in momentum ÷ time or (mv-mu)÷ t****Conservation**of momentum -**total**momentum**before**=**total**momentum**after**.

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## SP2g Stopping Distances

**Thinking distance**- distance travelled while driver**reacts**to situation**Braking distance**- distance travelled while brake**works**to bring car to a stop.**Stopping time**=**thinking distance + braking distance****Reaction time**- time between person**detecting**stimulus and their**response**- Typical reaction time -
**0.25 seconds** - If a person is
**ill**, taking**drugs**,**drinking**, using**mobile phone**, all**increases**reaction times - If road is
**wet**, brakes are**worn**,**loose gravel**on road, tyres**worn**-**less friction**, braking distance**increases** - Vehicle with
**more mass**-**more force**needed to deacelerate - Same friction used to stop a vehicle - heavier vehicle will travel futher (greater braking distance)

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## SP2h Braking Distances and Energy

**work done**- energy transferred by a**force**acting over a**distance****work done (J) = force (N) x distance in direction of force (m)****kinetic energy**- energy stored in a**moving object****kinetic energy (J) = 0.5 x mass (kg) x (speed)² (m/s)²**- kinetic energy = energy transferred to accelerate
- when vehicle
**stops**- kinetic energy transfers to**other**energy stores by**braking force**

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## SP2i Crash Hazards

**Crumple zones**(front and back) - takes**time**to crumple,**deacceleration**is**less**, reduces force on passengers and force is less than car with solid front.**Seat belts**-**hold**passengers into car.**Air bags**- increase**time**for a**head**to**stop**in collision.- Force in collision depends on change in momentum
**F = mv-mu ÷ t**

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