1.1.1 An interaction pair of partner forces are when an object exerts a force
on another object, it always has a force in return: If you push against a
wall, the wall will push back with exactly the same force.
1.1.2 Forces can still cause things to move even if they are balanced, because
they act of different objects.. The forces on the gas and the forces on the
rocket are an interaction pair
1.1.3 An object resting on a surface has a reaction force. If you put a
book on a table, the book pushes down on the table with a force
equal to it's weight and the table exerts an equal and opposite
force upwards on the book. The upward force is a reaction force.
1.2.1 When an object is moving against another, both experience
friction, which is a reaction force. It happens because of an
1.2.2 Friction between solid surfaces which are gripping. This is a kind of friction that
lets you walk around. The friction between your shoes and the ground allow you to
push against it and move forwards. If there was no friction, you would slip
184.108.40.206 Friction between solid surfaces which a sliding past each other. For example,
the moving stuff in a car engine. You can reduce sliding and gripping friction
through the use of lubricants like oil between the surfaces.
220.127.116.11.1 Resistance or drag from fluids (liquids or gases). Drag is an object moving through a
fluid that has to force its way past all the molecules in that fluid which causes
friction. For example, a van driving would be an object moving through the air. There is
no drag in space because in space, there is no fluid.
1.3 Arrows show the size and direction of forces
1.3.1 The length of the arrow shows
the size of a force and the
direction fo the arrow shows the
direction of the force.
1.3.2 The reaction of a surface - BALANCED FORCES.
Steady speed - BALANCED FORCES
1.3.3 Resultant force - overall force acting on an object. Add up all the
individual forces and their directions. This forces decides the motion
of the object (accelerate, decelerate or stay at a steady speed). If
there is a resulting force acting upon an object, its speed or direction
(or both) changes.
18.104.22.168 Accelerate just means change velocity. Velocity has both speed
and direction. If a car is turning a corner, it is changing velocity
(accelerating), not necessarily speeding up.
2 MOMENTUM (kg/s)
2.1 Acceleration - UNBALANCED FORCES. If a car's
engine exerts a bigger driving force than the drag
counter force, the car will accelerate. The forward
arrow will therefore be bigger.
2.1.1 A rocket taking off accelerates away from the ground, so the upward force
(thrust) must be greater than the downward forces that are slowing it down.
There are two downward forces: gravity and drag. If the thrust stopped, that
the downward forces would be greater than the upward forces and the
rocket would slow down until it stopped and then accelerate downward.
22.214.171.124 The same thing happens when an object is tossed
in the air then it comes crashing down again. Their
motion is all about the relative sizes of the upward
and downward forces.
2.2 MOMENTUM = MASS X VELOCITY
2.2.1 EXAMPLE: A 90kg donkey is moving in a straight line at 10 m/s.
Calculate it's momentum
126.96.36.199 Momentum = mass x velocity
188.8.131.52.1 900 kg m/s = 90 x 10
2.3 Momentum is how hard it'd be to stop an object from moving. The
heavier the object is, and the faster it is moving, the harder it is to stop.
2.4 It is a vector quantity because it has size and direction like velocity, but not
speed. A resultant force of zero means that a stationary object will remain still.
If the object was moving, it stays at constant velocity (same speed in the same
direction) and constant momentum. If the resultant force on an object is not
zero, its momentum changes in the direction of force
2.5 When a resultant force acts on an object, it
causes a change in momentum in the direction of
2.5.1 EXAMPLE: A rock with mass 1kg is travelling
through space at 15 m/s. A coment hits giving it
a resultant force of 2000 N for 0.7 seconds.
Calculate the rocks inital momentum and then
the change in its momentum resulting from
the impact with the comet.
184.108.40.206 Initial momentum: mass x velocity = 1 x 15 = 15 kg m/s
220.127.116.11.1 Change of momentum: force x time = 2500 x 0.7 = 1750 kg m/s
2.5.2 The greater the time for a change in momentum, the smaller the force. If
your momentum changes slowly, like in a controlled braking in a car, the
forces acting on your body are small and your unlikely to be hurt
18.104.22.168 The average force on an object can be lowered by slowing the object down over a
longer time. You can't affect the change in momentum in a car collision so safety
features in a car increase the collision time
22.214.171.124.1 CRUMPLE ZONES crumple on
impact, increasing the time
taken for the car to stop
126.96.36.199.2 AIR BAGS also slow you down
188.8.131.52.3 SEAT BELTS stretch slightly,
increasing the time taken for
the wearer to stop. This
reduces the forces acting on
184.108.40.206.4 CYCLE AND MOTORCYCLE HELMETS
provide padding that increases the time
taken for your head to come to a stop
if it hits something hard
3.1 When a force moves an object it does work and energy is
transferred to the object.
3.2 Whenever something moves, something else is providing some sort of
effort to move it. The thing putting the effort in, needs a supply of
energy. It then does work by moving the object - and one way or
another, it transfers the energy it receives into other forms. If energy
is transferred then the object doing the work loses energy.
3.3 Amount of energy transferred
(J) = Work Done (J)
3.4 Work done by a force (J) = Force (N) x
Distance moved in direction of force (m)
3.4.1 This formula only
works if the force is
in exactly the same
direction as the
3.4.2 EXAMPLE: Kids drag a tire 5m over flat ground. They pull with
a total force of 340 N. Find the work done.
220.127.116.11 W = F x D = 340 x 5 = 1700 J
4.1 Kinetic energy
4.1.1 Kinetic energy is energy of movement (K.E.). The only way to increase
kinetic energy is to apply a force to it (work). If you do work on an object
but it doesn't accelerate, then you haven't increaed its kinetic energy.
18.104.22.168 Increase in K.E = Work done
4.1.2 Kinetic energy = 1/2 x mass x velocity²
22.214.171.124 EXAMPLE: A car of mass 1450 kg is travelling at 28 m/s.
Calculate its kinetic enrgy.
126.96.36.199.1 1/2 x 1450 x 28² = 568 400 J
4.1.3 When work is done on an object, energy is transferred to that object, which is probably
going to make it start moving or move faster. You can't create or destroy energy, it just gets
transformed from one type to another. You would expect that kinetic energy = work done but
when it gets transferred, some of the energy gets wasted so the increase in an objects K.E. is
normally less than the amount of work done on it because some energy is wasted as heat.
188.8.131.52 But if there is no air resistance or friction acting on the object (vacuum) or you were told to
ignore it, then the increase in an objects kinetic energy is equal to the amount of work done on it.
4.2 Gravitational Potential Energy
4.2.1 Gravitational Potential energy is height energy (G.P.E). It is the energy stored in an
object when you raise it to a height against the force of gravity. When you lift an
object, its G.P.E increases as it's raised and vise versa. You increase G.P.E by doing work.
184.108.40.206 Increase in G.P.E = work done
4.2.2 Change in G.P.E. (J) = Weight (N) x Vertical height difference (m)
220.127.116.11 EXAMPLE: A 6000 N lamb is tossed up 10 m. Calculate its change in G.P.E
18.104.22.168.1 6000 x 10 = 60 000 J (or 60 kJ)
4.2.3 Falling objects convert G.P.E into K.E. meaning the further is falls, the faster it goes.
K.E. gained = G.P.E lost
5 SPEED AND DISTANCE
5.1 Speed (m/s) = Distance travelled (m) / Time taken (s)
5.1.1 EXAMPLE: A cat walks 20m in 40 seconds. Work out it's
speed and how long it'll take it to travel an extra 75 m.
22.214.171.124 Speed: 20/40 = 0.5 m/s
126.96.36.199.1 T = d/s = 75/0.5 = 150 s = 2 minutes 30 seconds
5.2 One direction is positive displacement whereas the opposite
direction is negative. If an object moves in one direction and then
back again, the distance travelled will be greater than the
5.2.1 EXAMPLE: A girl is playing with a yoyo, it drops from her hand 0.8 cm
down, She tries to pull it back up but it ends up dangling 0.2 below her
hand. What distance did the yoyo travel and what is its displacement
188.8.131.52 Distance: 0.8 + 0.6 = 1.4 m
184.108.40.206.1 Displacement: +0.2m. It is positive as the yoyo started off moving
downwards and has ended up lower from where it started
5.3 Distance Graphs
5.3.1 Horizontal = STOPPED
Shallow = SLOW
Steep = FAST
5.4.1 Sloping down = BACK TOWARDS starting point
Sloping up = AWAY FROM start point
220.127.116.11 If the line is sloping down and it
is getting steeper, the speed is
increasing. Getting shallower
means the speed is decreasing.
18.104.22.168.1 Displacement graphs show velocity (speed with direction). If two objects
are heading in opposite directions, you can say one had positive velocity
and the other had negative velocity. Acceleration is how quickly your
speeding up. Deceleration is negative acceleration.
5.5 Acceleration (m/s²) = Change in Velocity (m/s) /
Time taken (s)
5.5.1 EXAMPLE: A boy accelerates steadily from
2 m/s to 6 m/s in 5.6 s. Find it's accleration