Download P4 – Explaining Motion

P4 – Explaining Motion
Mechanics
Force
• Forces must always act in pairs!
• An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
• So remember only if the object is speeding up or slowing down is one force bigger than another!!!
Quick force pairs
Quick force pairs
Getting moving
• Draw both force arrows
• Write what the force is doing
– Eg. The foot pushes on the ground
Definitions
• Displacement
– Distance moved in a stated direction
• Speed
– Distance per unit time
• Velocity
– Displacement per unit time
• Instantaneous speed
– Speed at a given instant of time
• Acceleration
– Is the rate of change of velocity
Equations
• Average speed = Distance
Time
Example
Equations
• Average acceleration (a) = increase in velocity
Time
Remember that an increase in velocity can be negative, this means you have a negative acceleration (deceleration)
What causes friction?
• Friction is caused by irregular surfaces
• All surfaces are slightly irregular (except ice or frictionless
Remember we need friction
• Without it we can’t
push the ground Backwards.
But it stops us
• But is stops us
doing other thing
Quick Questions
Does the block move?
25N
62N
50N
50N
50N
50N
Momentum
• Momentum = mass x velocity
• Change in momentum = Force x Time
• Think of momentum as a property of a moving object.
Reducing force Safety Questions
Force = Change in momentum ÷ Time
If we want to reduce the force and you CANNOT change the change in momentum, what can you do? 6 Mark question
• Explain how seat belts reduce the injuries caused in a car crash?
[6 marks]
Steady State of motion
What does a steady state mean?
“A steady state is when an object travelling at a constant velocity, the resultant force on the object is zero”
This means that the forward force is balanced with the forces stopping the motion
Energy
• Energy is measured in
– joules, J
• Energy CANNOT be created or destroyed ONLY transferred
Different equations depending on type
Work Done
• Definition – Work is done whenever a force makes and object move. It is the energy transferred to move the object
Work done = Force x Distance moved
Work done = Fd
Energy transfers
Example
Lift up a 1kg bag of flour to a height of 2m.
change in gravitational potential energy
= weight  vertical height difference
= 10N  2m
= 20J
Example
All of the energy is transferred to kinetic
kinetic energy = 20 J
½  mass  speed2
mass  speed2
speed2
speed
speed
= 20J
= 40
= 40(m/s)
= √40m/s
= 6.32m/s
First drop is the tallest
• When doing maths questions in GCSE physics we imagine an ideal world.
• When explaining in a 6 mark question we explain the world how it actually is.
Big question
A rollercoaster will have a large drop at the start of the ride. Explain why will no other point on the roller coaster be as tall as the start of the first drop?
[6 marks]
Answering this question is all about loses.
Key steps
• Why is there a big lift, what is the car gaining?
• How is the energy transferred in the drop?
• Why is all of this energy not transferred?
– What are the loses
• Remember to use key words, they are on the wall if needed.
Kinetic Energy
• Definition
– The energy that something has owing to its motion
– The movement energy it has to keep moving
Kinetic Energy = ½ x mass x velocity2
KE = 1/2mv2
Gravitational Potential
• Definition
– The energy stored when an object is raised to a higher point in the Earth’s gravitational field
Gravitational Potential = Mass x gravity x height
GPE = mgh