P4: Explaining Motion

Linking together forces, energy and

motion

Speed:

• How quickly an object covers a set distance

• Speed (m/s) = distance (m)

time (s)

dt x s

dirty toes smell!

Instantaneous speed… speed at a given point

Average speed… distance of whole journey divided by total time taken-which takes into account slowing down and speeding up.

Showing speeds with distance time graphs

Key features:• Flat = stationary• Straight diagonal

= constant speed• Increasingly

steeper curve = speeding up

• Decreasingly steep curve = slowing down

Gradient = speed

Time/ seconds

Distance/ metres

Graph for a 100m sprint:Can you find where the athlete…1. Slows down2. Speeds up3. Moves at a constant

speed?

Copy the graph then show a much faster runner’s graph on the same axes

Finding the speed using the gradient of distance- time graphs

Change in y/ change in x = speed

40/2 = 20 m/s

Time/ seconds

Distance/ metres

Change in distance=70-30= 40 m

Time for this change to occur= 6-4=2 s

4 6

70

30

(Gradient)

Velocity:

• Simply the speed of an object at a point of time PLUS the direction in which it travels (vector quantity)

• Units are the same as speed: m/s

The cheetah’s speed for the overall chase was 85 km/ h; its’ velocity reached 113 km/ h running north after 10.4 seconds.

If the cheetah then ran south at the same speed the velocity would be negative

e.g. -113 km/ h running south

Showing acceleration with velocity- time graphs

• Acceleration is a change in the velocity (either speeding up OR slowing down OR changing direction)

Is the roller coaster accelerating? Velocity

(m/ s west)

Time taken (s)

High positiveAcceleration(steep)

ConstantVelocity(level)

Low positive acceleration

Highnegativeacceleration

Where would you show a stationary object on this graph?

Forces

• Newton’s 3rd Law: “For every action there will be an equal but opposite reaction”

• Rephrased…when one object exerts a force on another, it always experiences a force in return

If the person jumps to shore what happens to the boat?

Describe and explain what happens when the balloon deflates

Examples…In which direction will be the reaction force?

Wings push downwards; Feet push backwards

Weight of cork pulls towards

the centre of the Earth

The magnet attracts the Iron objects(add both arrows)

Interactions at surfaces

The Sumo wrestler exerts a force downwards which deforms the floor surface slightly causing an equal force upwards (the reaction of the surface)

stationary

moving

For two surfaces sliding past each other…each surface experiences a force in the direction which prevents (or tends to prevent) relative movement; this interaction is called friction… in the case of the bear not very much!

Resultant forces• The overall effect of all

the forces acting on an object taking into account their sizes and directions.

e.g. For a pendulum swinging back…

In flight there are 4 main forces to consider!Which force must be largest to slow the plane down?

Momentum

• If there is a resultant force then there will be a change of momentum in the direction of the force

• Momentum (kg m/s) = mass (kg) x velocity (m/s)

• The change in momentum varies according to the resultant force and the time this force acts

• Change in (kg m/s) = resultant force (N) x time force acts (s)momentum

Mm x v

Mom mashes vegetables

Draw your own revision triangle for this equation and make up your own mnemonic

Changing momentum• If there is no change in momentum the object stays at rest OR

if moving stays at a constant speed in a straight line

• During collisions a big change in momentum occurs but since…

Change in (kg m/s) = resultant force (N) x time force momentum acts (s)

If we increase the time over which the force acts then the resultant force will be smaller (the change in momentum is unchanged!)

• This is the principle used in crash helmets, air bags, seat belts, climbing ropes and crumple zones on cars

Driving and counter forcesDriving force Counter force

(complete this section for each one)

Boat engine

Cyclists’ legs pedalling

Table tennis bat hitting ping pong ball

Wind blowing a sail on a yacht

Thrust from engine: driving force

Air resistance

frictionCounter forces

Using the car diagram below show the force arrows for when the car is

(a) speeding up (b) slowing down

and (c) at a constant

speed

Changing Energy

Work done (J) = force (N) x distance (m)

• Work is being done on the rock opposite so the energy of the rock increases

• Work is being done by the person so the energy of the person decreases

Change in energy (J) = work done (J)

• The falling rock will gain kinetic energy and lose gravitational potential energy

Gravitational Potential Energy

Change in

GPE (J) = weight (N) x vertical height difference (m)

2.5 m fall

800Nman

Show all the forces acting on the man (left) and make the resultant force clear.Calculate the change in GPE

In which position does the baseball have the greatest potential energy?

Kinetic Energy

• Value depends on the speed of an object and the mass

• Speed has a much greater effect than mass

KE (J) = ½ mass (kg) x velocity2 ((m/s)2)

A force which causes the object to speed up increases the kinetic energy.

Use this statement to explain why the speed limit in towns is 30 mph {“It’s 30 for a reason!”}

Conservation of energy

Energy input from fuel 150kJ

(A) Kinetic energy 100 kJ (useful)

(B) Heat and sound energy dissipated due to friction and air resistance (wasted) 50 kJ

Energy input will equal the energy output BUT the gain in kinetic energy of an object will be less than the work done on the object because of the forces of friction and air resistance.