Physics - Forces Part 1 Flashcards
Forces Part 1
What is a scalar quantity?
A scalar quantity has magnitude (size) only. Some examples are: speed, mass, time, energy.
What is a vector quantity?
A vector quantity has magnitude and direction. Some examples are: displacement, velocity,
acceleration, force, momentum.
What is a contact force?
A contact force occurs when two objects touch each other. Some examples are: Friction, normal contact force, tension, and air resistance.
What is a non-contact force?
A non-contact force does not require two objects to touch each other. Examples are: Gravity, Electrostatic force, magnetism.
What is Newton’s third law?
“Every action has an equal and opposite reaction” or in other words if object A exerts a force on object B then object B exerts an equal and opposite force on object A.
What is weight?
How is weight measured?
The gravitational force of attraction you experience because you are in the Earth’s gravitational field is also
known as your weight. Weight is a force and is measured in Newtons (N).
Weight is measured using a calibrated spring balance (Newton meter)
How would we calculate the weight of an object?
Weight = mass × gravitational field strength
W = mg
Weight (Newtons, N), Mass (kilograms, kg), Gravitational field strength (Newtons per kilogram, m/s)
The weight and mass of an object are directly proportional.
What is the centre of mass of an object?
The weight of an object may be considered to act from a single point referred to as the object’s centre of mass.
The mass of the object could be considered to be concentrated here.
What is a resultant force?
If two or more forces act on an object then these can be replaced by a single force arrow that we call the “resultant force”.
How do you calculate the size of a resultant force when two (or more) forces act along the same line?
If the forces act in the same direction we add the forces to work out the size of the resultant force. If the forces act in opposite directions then we take the smallest one away from the biggest to work out the size
of the resultant force.
How do you calculate the size of a resultant force when two forces act at right angles?
Draw a scale diagram: Make a parallelogram (rectangle here) with the two forces. Draw the resultant force and measure its length with a ruler. To get the direction measure the angle with a protractor.
What does it mean when we say we are going to resolve a force into two components?
Any force can be split into two components, one acting vertically and one acting horizontally. These can be determined again using
pythagoras and trigonometry.
How can you resolve the force of weight into two components, one parallel and one normal, when an object is on a ramp?
F normal = W cos(angle)
F parallel = W sin(angle)
Why does the moon orbit the
earth?
There is a resultant force acting on the moon towards the centre of the earth due to the earth’s
gravitational field. Because this is at right angles to the direction the moon is travelling it causes circular motion. There is also the same sized force acting on the earth, due to the moon’s gravitational field.
What is a moment?
If a force causes an object to rotate then it has a turning effect. We call this turning effect a moment.
Moment = force × perpendicular distance from pivot
Moment = Fd
Moment (Newton metres, Nm), Force (Newtons, N), Distance (Metres, m)
What is the principle of moments?
If an object is balanced then the sum of all the clockwise moments acting on it equals the sum of all the anticlockwise moments acting on it.
What is a gear?
Gears are devices that are used to transmit the rotational effects of forces from one place to another. E.g. a bike gear is used to transmit the rotational effect of the pedal to the wheels.
How do gears work?
Cog A will have a certain moment. The force from this moment is transmitted to cog B at the point of contact. Because cog B is a different size (so has a different perpendicular distance from the pivot) the turning effect of cog B will be different to cog A (momentB > moment A in this example because cog B is larger)
