Transcript 1. newton`s laws

Newton’s Laws of Motion
Newton’s First Law
If there is no unbalanced force an object will move at
constant velocity or remain at rest.
Newton’s Second Law
Fu = ma
Newton’s Third Law
Every force has an equal but opposite reaction force.
Eg. Force of the rocket on the hot gases
= - Force of the hot gases on
the rocket
Balanced Forces
Forces are balanced when they are equal in size but
opposite in direction.
If everyone pulls with
the same strength there
will be no movement.
pull
pull
Forces are balanced.
Buoyancy of water
weight
Forces balanced so no up down movement
Bigger downward force
boat moves down
For balanced forces there is no change in movement.
Force due to water buoyancy
Force due to gravity weight
Going down
Going up
Balanced Forces
lift
Terminal speed
Air
resistance
thrust
weight
Object will be moving at
constant speed or be at rest.
Parachute
weight is constant
Sky diver
Draw a speed time graph for a parachute jump
No air resistance
Just left the plane
No speed, no air resistance.
Downward unbalanced force – accelerate down
weight
Air resistance Speeding up, air resistance increasing.
Unbalanced force is smaller so less
acceleration.
weight
Air resistance
Maximum speed ( terminal velocity)
Air resistance = - weight so no unbalanced force
No acceleration
weight
Air resistance
The parachute is opened and air resistance
increases.
Upward unbalanced force, so the parachute
decelerates and slows down
weight
Air resistance
As speed decreases so does air
resistance until the forces are balanced
and terminal velocity is reached.
weight
Land !!! Speed is now zero
Tutorial Qus page 5/6 Free falling objects.
Example
1. A car of mass 5000 kg is travelling at constant speed. The
engine exerts a force of 20 N.
(a) What is the magnitude of the friction forces?
(b) The engine force increases to 100N. What is the car’s
acceleration? Assume the friction forces stay constant.
Tutorial Questions pages 14 and 15
Qus 1 to 8
Rockets
2. A rocket of mass 10 000kg was launched from the Earth.
(a) If it had an initial acceleration of 2ms-2, what would the force
of the rockets be?
(b) As it moves away from the Earth, the acceleration increases,
give two reasons why.
Lifts
3. A person of mass 60 kg stands on scales inside a lift. The
lift accelerates at 4 ms-2 downwards. What will be the
reading on the scales?
Tutorial Questions 15 to17 Qus 9 to15
Purple book Chp 2.5
Connected Masses
4
.
A force of 20 N is used to pull the blue
box, mass 5 kg. Friction is 1N on each
mass.
The green box has a mass of 1.5 kg
and the grey box has a mass of 2 kg.
What is the acceleration of the blocks?
What will be the tension between the
green and blue boxes?
Tutorial Questions pages18 and 19 Qus 16 to 19
Connected Masses 2
0.5 kg
1 kg
5. Assume no friction
(a)What is the acceleration of the 0.5 kg trolley?
(b)What is the tension in the string?
Tutorial Questions page 19 Qu 20
Purple book Ex 2.2
Forces at Angles
40 N
450
6. If the object has mass, 1500 Kg, what will its acceleration be?
Tutorial Questions pages 19 to 22 Qus1 to 5
Forces and Slopes
Fc
200
Fw
200
7. A mass of 10 kg is sliding down a slope. If it accelerates at 2ms-2,
find:(a) The component of the weight acting down the slope.
(b) The friction between the mass and the slope.
Tutorial Questions pages 19 to 22 Qus1 to 9
Purple book Chp 2.4