CH 3-Momentum

Chapter: 3 Mass, weight and gravity
Gravity: The force that pulls masses towards one another
Gravitational field: a region where the Earth exerts a force on a body.
Gravitational field strength: The gravitational field strength at a point is the

gravitational force per unit mass on a small object placed at that point. It is a vector.
Mass is a measure of how much material an object contains. It is measured in

kilograms, kg.
Weight is a gravitational force on an object that has mass.

Chapter : 3 Momentum
Definition
 The momentum of an object is its mass multiplied by its velocity.
Key points
 The effect of a force F depends on- how big the force is, and the time interval.
 Momentum is calculated from velocity, and so it is a vector quantity since it has
both magnitude and direction. .
 The quantity mass x velocity is known as the momentum of the object, p=mv,
and is measured in kilogram metre per second (kg m/s) or newton second (N s)
 p=mv , this equation can be used to work out the momentum of a moving object.
A stationary object has no momentum.
 Both velocity and mass affect an object’s momentum.
 Momentum is always directed in the direction of velocity. The direction of
momentum is same as the direction of the velocity. So momentum is a vector.
 When a force acts on an object and changes its velocity, as velocity changes the
object’s momentum will also change. The change in momentum (p) is
calculated from the following formula:
Change∈momentum=final momentum−initial momentum
∆ p=mv−mu
 From Newton’s second law of motion:
change∈momentum
resultant force=
time
 Resultant force is the rate of change of momentum per unit time.
 The rate of change of momentum is called force.
∆ p mv−mu v −u
F= = =m× =ma
t t t
Impulse
The impulse of a force during a certain interval of time is the change in
momentum that the force produces, or the force acting on an object multiplied
by the time for which the force acts (Impulse= force x time).
impulse=change∈momentum∨force ×time
Key points
 Impulse is measured in kilogram metre per second (kg m/s) or newton second (N s).
 Momentum and the second law of motion: Newton noted that: when same force
acted for the same time on different masses, the change in momentum would be the
same for every case.
The law of conservation of momentum
The total momentum of the bodies in a closed system is constant before and after a
collision provided no external forces act (e.g. friction).
Key points
 When two of more objects act on each other, their total momentum remains
constant, provided no external forces are acting.
 Momentum is always conserved in an explosion/collision, so there is no change in

momentum.
 Momentum is conserved in an explosion such as occurs when a rifle is fired. Before

firing, the total momentum is zero since both rifle and bullet are at rest. During the
firing, the rifle and the bullet receive equal but opposite amounts of momentum so
that total momentum after firing is zero.
 Momentum and energy: Moving objects have kinetic energy. In a collision, some
of that energy may be changed into other forms. If collision is elastic, the total
kinetic energy of the moving object is the same. If the total kinetic energy is less
after a collision, the missing energy is converted into thermal energy (heat).
Chapter : 3 Turning and Circular motion
 Motion in a circular path is due to a force perpendicular to the motion.
 A resultant (unbalanced) force acting towards the centre of the circle is needed for
circular motion.
 When an object moves in a circle at a constant speed, its direction constantly changes. A
change in direction causes a change in velocity. A change in velocity results
in acceleration, so an object moving in a circle is accelerating even though its speed
may be constant.
 As the acceleration does not involve a change in the magnitude of the velocity, that is it
is directed towards the centre of the circle. This is called Centripetal acceleration and
it is caused by the resultant (unbalanced) forced towards the centre of the circle.
 An object will only accelerate if a resultant force acts on it. For an object moving in a
circle, this resultant force is the centripetal force that acts towards the middle of the
circle. Gravitational attraction provides the centripetal force needed to keep planets
and all types of satellite in orbit.
Examples Force provided by

Conker on a string Tension in the string on the conker
Car on a roundabout Friction from the road on the tyres
Satellite Gravitational pull of the Earth on the satellite
 The velocity of the object at any instant is along the tangent to the circle.
 Centripetal force: The resultant force on an object towards the centre of the circle
when the object is rotating round that circle at constant speed.
 The friction between the tyres and the road provides the centripetal force necessary for
the car’s circular motion.
 The size of the resultant force needed to make an object travel in a circular depends on
the object’s mass, its speed, and the radius of the circle in which it is moving. A bigger
force is needed if:
 object’s mass is bigger (speed and radius is constant)

 the object’s speed is bigger (mass and radius is constant)
 the radius of the circle is smaller (mass and speed is constant)
Scalars and Vectors
 Mass, energy and volume are the examples of scalar because they have only
magnitude without direction.
 Energy, density, temperature, and power are scalar quantities.
 Weight is an example of vector quantity because it is a force and therefore

has both magnitude and direction. Acceleration and momentum are also
examples of vector quantities.
 Displacement is a vector quantity, while distance is a scalar quantity.
 Displacement is the length and direction of a straight line drawn from the
starting point to the finishing point.
 The result of adding vectors together is called the resultant.

Chapter : 3 Scalar and Vector

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Chapter: 3 Mass, weight and gravity

Gravity: The force that pulls masses towards one another

Gravitational field: a region where the Earth exerts a force on a body.

Gravitational field strength: The gravitational field strength at a point is the

Mass is a measure of how much material an object contains. It is measured in

Weight is a gravitational force on an object that has mass.

 The momentum of an object is its mass multiplied by its velocity.

The law of conservation of momentum

 Momentum is always conserved in an explosion/collision, so there is no change in

 Momentum is conserved in an explosion such as occurs when a rifle is fired. Before

 Motion in a circular path is due to a force perpendicular to the motion.

Examples Force provided by

 object’s mass is bigger (speed and radius is constant)

 Energy, density, temperature, and power are scalar quantities.

 Weight is an example of vector quantity because it is a force and therefore

 Displacement is a vector quantity, while distance is a scalar quantity.

 The result of adding vectors together is called the resultant.

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