# Physics- Unit 2- Force and Acceleration

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Newton’s First Law of Motion;

Objects either stay at rest or in a uniform motion unless acted on by a force.

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What is an object moving at a constant velocity therefore?

Therefore, an object moving at constant velocity is either acted on by no forces or the forces acting on it are balanced (resultant force is zero)

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Newton’s Second Law of Motion;

Force= Mass x Acceleration (F=ma)

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Test for this?

Using a dynamics trolley and a motion sensor.

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How is the test completed?

The sensor sends messages to a computer to display how the velocity of the trolley changes over time. The trolley is pulled along a sloping runway (using an elastic band stretched to the same length).

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Why to the same lengh? Slope? To test?

To ensure the same force. Account for friction. Should move down the slope at constant speed after a push.

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What does the test show?

Velocity increases at a constant rate down the path with same mass and force. Force is proportional to mass x acceleration (increase in a when F and M increase)

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How can we use F=Ma to prove objects fall at same acceleration?

a = F/M. When gravity is only force acting on it, Mxg/M = a. Therefore a=g.

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What happens when an object is acted on by two unequal forces in opposite directions? (acceleration)

Accelerates in the direction of the larger force.

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Rocket problems?

We have two forces, the rocket thrust and the rockets weight. For it to fly thrust>weight. Therefore T-Weight= ma. Therefore thrust = ma + mg.

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Lift problems?

For a lift to move T-mg = ma.

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Lift moving at constant velocity?

T = mg

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Moving up and accelerating?whats its a like?

T= ma + mg (Tension will be larger then the mass x gravity) A>0

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Moving up and deaccelerating? whats its a like?

T= ma + mg (Mg is larger) A

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Moving down and accelerating?whats its a like?

T= ma + mg (Mg Larger) A

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Down and deaccelerating?whats its a like?

T= ma + mg (Mg is smaller). A>0

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Pulley Problems?

Mass that accelerates downwards = mg-t = ma and the one accelerating upwards = T-mg=ma.

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Slope? How to work out weight componant?

Weight down the slope = mgsin(angle)

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Equation for Force?

mgsin(angle)- friction force = ma.

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What is drag force dependant upon?

shape of the object, speed, viscosity of the fluid (How easilly the fluid flows past a surface).

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Describe the motion of a object falling in a fluid?

Speed of the object from rest increases as it falls so the drag force increases. Resultant force is the diffrence between the weight of the object and the drag force.

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what happens as the drag force increases?

As the drag force increases, the resultant force decreases (as the diffrence decreases) so acceleration becomes less.

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When does it reach terminal speed?

When the drag force is equal and opposite to the weight. Its acceleration is zero and its speed remains constant as it falls.

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Test to find out terminal velocity?

Object gets released with a thread attached which pulls a tape through a tickertimer, printing dots on the tape. Spaces between tapes get longer as speed increases as are taken in a constant rate. Tapes will be a constant size at terminal velocity.

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How to work out the resultant force?

F = mg -D

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How to find the acceleration therefore?

a = mg-D/m

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What happens at terminal speed (related to energy)?

Potential energy lost by the object is converted as it falls to internal energy of the fluid by the drag force.

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What is stopping distance (sum and explanation)?

Thinking distance + breaking distance. It is therfore the time for when a hazard is spotted to the time where the car reaches a full stop.

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Thinking distance?

Time taken for the driver to spot a hazard and then react. For a vehicle moving at constant velocity, it is speed x reaction time.

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effected by?

Alcohol, drugs, distractions (eg:phones).

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Breaking distance?

Distance travelled by the car from when the brakes are applied to when the car reaches a full stop.

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Equation?

v2/2a

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formula for impact force? also?

Change in kinetic energy/impact distance OR F=ma

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Name 5 car safety features?

Bumpers, crumple zones, seat belts, collapsible steering wheel, airbags

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Bumpers?

These give way a little during a collision which causes the impact time to increase at a low-speed impact. If impact time is reduced, impact force is reduced.

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Crumple zones?

Engine compartment gives way in a front-end impact. Increases the impact time which reduces the impact force.

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Seat belts?

The wearer stops more gradually then without them. This increases the impact time, decreasing impact force.

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Collapsible steering wheel?

Driver does not hit the wheel, reduces impact force.

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Airbags?

Acts as a cuhsion, which reduces impact force, and increases impact time. Force is also spread over a larger distance.

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## Other cards in this set

### Card 2

#### Front

What is an object moving at a constant velocity therefore?

#### Back

Therefore, an object moving at constant velocity is either acted on by no forces or the forces acting on it are balanced (resultant force is zero)

### Card 3

#### Front

Newton’s Second Law of Motion;

#### Back

### Card 4

#### Front

Test for this?

#### Back

### Card 5

#### Front

How is the test completed?

#### Back

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