Force ⇒ is a push or pull. ⇒ force is an interaction between objects to explain changes in motion When force is exerted on an object:
- object can either start or stop moving
- slow down or speed up
- change direction of motion of object
Types of Forces two types ⇒ contact and non contact. Non contact ⇒ do not require objects to be in contact to exist.
| Non Contact Forces | Contact Forces |
|---|---|
| Gravitational force: - pull exerted by earth’s gravity on an any object (weight) | Friction - force that tends to oppose or tends to oppose motion between surfaces in contact |
| Electrostatic force: - attractive (i.e. pull) or repulsive (i.e. push) forces between electric charges. (attractive forces between unlike charges) (repulsive forces between like charges) | Air Resistance - frictional force exerted by air that opposes motion of moving objects. |
| Magnetic force Attractive (i.e. pull) or repulsive (i.e. push) forces between magnets. (attractive magnetic forces between unlike poles) (repulsive magnetic forces between like poles) | Normal Force - push exerted by a surface on an object pressing on it - push is always perpendicular to the surface |
| Tension - pull exerted by a stretched spring, string, rope on an object attached to it |
Mass
Definition
Mass is a measure of amount of matter in a body #PhysicsDefinitions
SI unit: Kg
Mass ⇒ property of body ⇒ does not change with its location or shape Mass of body ⇒ depends on the number and composition of atoms and molecules that make up the body Scalar quantity
Weight
Definition
Weight is the gravitational force acting on an object that has mass #PhysicsDefinitions
SI unit: Newton(N)
Since weight is a force ⇒ it is a Vector quantity → with both magnitude and direction. Direction of weight ⇒ downwards ⇒ i.e. towards center of earth
Gravitational Field
Definition
gravitational field is a region in which a mass experiences a force due to gravitational attraction #PhysicsDefinitions
Example:
- earth with huge mass ⇒ has gravitational field surrounding it
- thus any object within earths gravitational field ⇒ will experience a force exerted by earth on it
- gravitational force experienced is strongest at surface of earth
- gets weaker further away due to decreasing gravitational field strength
Gravitational Field Strength
Weight of any object ⇒ depends on strength of gravitational force acting on it. Eg: objects weight less on the moon than on earth because moon’s gravitational field strength is weaker than earth’s gravitational field strength
Definition
Gravitational field strength, g is defined as the gravitational force per unit mass placed at one point #PhysicsDefinitions
Formula for gravitational field strength: Where g, is gravitational field strength (N/Kg) W = weight (N) M = mass of the object (Kg)
Earth - g is approximately 10N/Kg
- mass of 1kg on earths surface ⇒ experiences a force of 10N due to earths gravitational field
On Moon ⇒ same mass ⇒ experiences a gravitational force of only 1.6N → this is because gravitational field strength on moon is 1.6N/Kg
Relation between Mass and Weight
From equation , we have Weight or gravitational field strength W acting on an object ⇒ directly proportional to its mass m.
Gravitational Field Strength and acceleration due to gravity
if objects were to free-fall ⇒ under gravity without air resistance → we can find its acceleration using the equation:
F = resultant force
M = mass (Kg)
A = acceleration ()
Object of mass m free-falling under gravity ⇒ without air resistance will have acceleration of a due to its weight W
Common weighing instruments
spring balance, bathroom scale, electronic balance → all measure weight of object rather than mass. But they are calibrated to give readings in grams or kilograms.
Using these weighing instruments - objects will have diff. Mass readings at different gravitational field strengths. Refer to moon and earth example*
This means that a weighing scale calibrated for use on earth cannot be used on the moon. ⇒ the weighing scale must be calibrated to the moon’s gravitational field strength to give accurate mass measurements on the moon.
Scale measures the normal contact force of the feet pushing down on the scale. Normal force acts to compress the spring in the bathroom scale. When person is standing still, the normal force exerted on the spring is equal to the gravitational force of attraction (weight)
Measurement of Mass
To avoid having to calibrate weighing scales for different gravitational field strengths - mass of object can be measured using a beam balance.
Beam balance → compares the gravitational force acting on an object with that acting on standard masses. Both object and standard masses experience the same gravitational field strength - mass readings taken for a given object whether on earth or moon will be the same.
Differences Between Mass and Weight
| Mass | Weight |
|---|---|
| Amount of matter | Gravitational force |
| Scalar quantity (only has magnitude and no unit) | Vector quantity (both magnitude and direction) |
| SI unit: Kg | SI unit: Newton (N) |
| Independent of gravitational field strength | Dependent on the gravitational field strength |
| Measured with a beam balance or a calibrated electronic balance | measured with a spring balance |
Important Point
Explain what it is meant when an astronaut says he feels weightless:
- When the astronaut is in the satellite, the only force acting on him is the force of gravity, a non-contact force. (1) The astronaut cannot feel the force of gravity unless there are contact forces opposing it. (1) When the astronaut reports that he is weightless, he means he does not experience any external objects touching and exerting a push or pull on him (1)