P4 Explaining Motion

P4 Explaining Motion revision cards

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How forces arise

- Forces arise from an interaction between two objects

- Always come in pairs

- The two forces in an interaction pair are always equal and opposite and act on different objects

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How things start working

- To make a vehicle/person start moving it needs to push against the ground

- When it pushes on the ground the ground pushes back and it will start to move

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- Friction is an unusual force  

- It adjusts its size in response to the situation – up to a limit

- This limit depends on the objects and the surfaces involved

- The force of friction arises due to lots of tiny welds that have to be broken as an object slides against another

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Reaction of surfaces

- If an object is placed on a surface it squashes or distorts the surface

- The surface exerts a reaction force on the object

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Adding forces

- If there is a force acting on an object and it is not moving there must be another force balancing the first one  

- If they balance we say the “resultant force” is zero

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- Average speed = distance / time

- Instantaneous speed – when average speed is measured over very short time intervals

- Speed cameras detect speeding cars

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Motion Graphs

- Distance – time graph: gradient/slope shows speed

- Speed – time graph: gradient shows acceleration

- Velocity – time graph: also shows direction of motion

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Force and change of momentum

-  Momentum = mass x velocity 

-  Change of momentum caused by a force:

 Change of momentum = force x time

(time is for how long the force acts)

- Conservation of momentum – in an interaction the total change in momentum is zero

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Car Safety

In a collision the force on passengers can be great. Cars are designed to reduce these forces:

-  Crumple zones – increase the collision time

-  Seat belts – stretch to make the change of momentum longer

-  Air bags – cushion impact to reduce your momentum slowly

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Factors involved

- Collision time – the size of force on the car depends on the time the collision lasts

- Momentum – the bigger the time, the smaller the force

In summary, the longer it takes to reduce the passenger’s speed to zero, the smaller the force they experience.

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Laws of Motion

- Law 1 – if the resultant force acting on an object is zero, the momentum of the object does not change - Law 2 – if there is a resultant force acting on an object, the momentum will change (c.o.m.=r.f x time) and is in the same direction

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- Stationary objects have a resultant force that is zero

- Objects moving at a constant speed also have a resultant force that is zero  

- Speeding up or slowing down- overall resultant force exists

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Work done

- When a force causes movement of an object, work is done 

-  Use the equation:

work done by a force = force × distance     moved by     the force

  (joule, J)   (newton, N)   (metre, m)

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Change of energy

- The energy of a moving object is called kinetic energy

- As an object falls, its gravitational potential energy decreases

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Work and change of energy (cont.)

- Understand that when work is done on an object, the energy of the object increases and  

-  When work is done by an object, the energy of the object decreases according to the relationship:

change in energy = work done

  (joule, J)      (joule, J)

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From potential to kinetic energy

When an object is lifted to a higher position above the ground, work is done by the lifting force against the gravitational force acting on the object (its weight);

- this increases the object’s gravitational potential energy (GPE);

- use the equation:

change in GPE = weight × vertical height difference

(joule, J)   (newton, N) (metre, m)

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Changes in kinetic energy

- When work is done to make an object move faster the kinetic energy increase.

- Change in energy = work done

- So,

change in energy = force x distance

However, some work is wasted due to the force of friction.

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Conservation of energy

When an object falls it –

- Loses gravitational potential energy

- Gains kinetic energy

- If friction is small enough to ignore then

Amount of GPE lost = amount of KE gained

We use this formula to calculate KE:
Gain in KE = ½ mass x velocity squared

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