P4 Explaining Motion
P4 Explaining Motion revision cards
- Created by: Molly Cutler
- Created on: 22-12-11 17:33
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
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
Friciton
- 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
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
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
Speed
- Average speed = distance / time
- Instantaneous speed – when average speed is measured over very short time intervals
- Speed cameras detect speeding cars
Motion Graphs
- Distance – time graph: gradient/slope shows speed
- Speed – time graph: gradient shows acceleration
- Velocity – time graph: also shows direction of motion
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
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
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.
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
Motion
- 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
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)
Change of energy
- The energy of a moving object is called kinetic energy
- As an object falls, its gravitational potential energy decreases
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)
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)
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.
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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