Physics OCR A Level: Forces In Motion

What is a resultant force?

The single force that replaces multiple forces acting at a single point/object

What is Newton's second law?

F = ma, force = mass x acceleration

What is the definition of weight?

The force acting on an object of mass due to gravity

What is the equation for weight?

Weight = mass x gravitational field strength (or acceleration of free fall), W = mg

In addition to gravitational, magnetic and electrical forces, what are the five main forces to know?

- Tension- Normal Contact Force- Upthrust- Friction- Drag

What is the definition of upthrust?

an upward force exerted on an object equal to the weight of fluid displaced when a body is completely or partially immersed in a fluid

What is the definition of friction?

The force that one surface exerts on another when the two surfaces rub against each other

What is terminal velocity?

the constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity

What should we consider when making a free-body diagram?

- The body on which the forces act.- The direction of application of each force.- The type of each force.- The size of each force.

What is the definition of terminal velocity?

This is the constant speed that a freely falling object eventually reaches when the resistance of the medium through which it is falling prevents further acceleration.

What direction does the effect of drag have on an object?

In the opposing direction to that of the moving object

Explain the experiment for determining terminal velocity of a ball bearing falling through a viscous liquid.

- Fill a tube with water and drop a ball bearing at the opening at the top.- As the ball falls, mark certain points where the ball is vertical using elastic bands. - Once the ball has reached the bottom, use a magnet to bring it back to the top.- Repeat to gain the following measurements: the time period between each band. Record the distance between the bands and use the data to calculate v.- Calculate the mean distance travelled for each time period, using this to find v of the ball for each time period.- Plot a graph of v on the y axis against t on the x axis. Draw a curve between each point and identify the time at which the ball reached its terminal velocity.

What is an equilibrium of two forces?

When both forces acting on an object are equal (or balanced) meaning there is no resultant force.

What does a triangle of forces represent?

The direction and magnitude of three coplanar forces that are acting on an object in equilibrium

Define the moment of a force

The product of the force and the perpendicular distance of the line of action of the force from the pivot

What does the principle of moments state?

That for an object to be in rotational equilibrium, the sum of all clockwise moments must be the sum of all anti-clockwise moments

Define a couple

When two equal forces travelling in opposite directions produce a rotation

Define torque

The rotary effect of a force

What is the centre of gravity?

The point through which the weight of a body acts

What is an investigation to find the centre of gravity of an object?

- Find an irregular shape, for example a jigsaw piece.- Make three holes in random places on the edge of the shape.- Hang the shape from the first hole placing a pin in it. Let it swing and come to rest. The piece should be suspended and will always come to rest with its centre of gravity below the point.- Have a string with a weight tied to the lower end.- Draw a straight vertical line down from the point of suspension to the bottom of the shape.- Repeat this from the two other holes and the lines should cross at a single point, which is the centre of mass.

What is the equation for density?

Density = mass/volume

What does Archimedes' principle state?

The upward buoyant force (upthrust) exerted on an object immersed in a fluid, whether partially or submerged, is equal to that of the weight displaced by the object.

What is the equation for pressure?

pressure = force/area

What is the definition of pressure?

Force per unit area.

What is the principle of conservation of energy?

Energy cannot be created or destroyed, only transformed from one type to another

What is a closed system?

A system where neither matter nor energy can enter or leave

Loss in potential energy = gain in kinetic energy, what is this in equation form?

Mgh = 1/2 mv ^2

What is the definition of power?

The rate at which energy is transferred or the rate at which work is done

What is the equation for power?

power = work/time

What is the equation for power of a moving object that is subject to a force?

Power = force x velocity (P = Fv)

What is the equation for working out efficiency?

efficiency = useful output energy transfer รท total input energy transfer or efficiency x 100