Physics Forces Test
What is a scalar quantity and give examples | A quantity with magnitude (size) but no direction Some examples: speed, distance travelled, density, mass, energy, time, power |
What is a vector quantity and give examples | A quantity with magnitude (size) and direction Some examples: displacement, velocity, acceleration, force |
How can you represent a vector in a diagram? | As an arrow. The length of the arrow represents the size and the arrowhead shows the direction. |
How do you measure weight? | Using a Newton-meter/calibrated spring-balance |
What is weight? | The gravitational force acting on an object (Do not call it gravity! Gravity is not a force!). It always acts downwards. |
What is the centre of mass of an object | A point around which an object’s mass is equally distributed. Its weight acts downwards form this point. |
What is the difference between mass and weight | Weight: A force caused by gravity. It’s size depends on gravitational field strength and mass. Unit = Newton (all forces are measured in Newtons)
Mass: a measurement of how much of an object there is. It doesn’t change unless the object changes. Unit = kg |
What does the size of the weight force depend on? | The gravitational field strength and mass of the object. For a uniform gravitational field (so gravitational field strength is constant) weight is directly proportional to mass. |
What value does g take on Earth (including units) | g = 9.8N/kg |
What is the resultant force? | A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together. |
What are balanced forces and what does this do to an object? | When forces cancel out (usually equal size, opposite directions) so the resultant force = 0.
When forces are balanced, the object remains at a steady speed (or at rest) |
What are unbalanced force and what does this do to an object? | When forces do not cancel out completely there is a resultant force. This can cause the object to change: · Speed · Direction Shape (if there is more than one force) |
What is a free body diagram? | A diagram where object are simplified to a single circle/square and the forces acting on it are shown as inlabelled arrows |
What is tension? | When an object is stretched or squashed, there is a force trying to return it to its original shape, this is called tension. E.g. in stretched springs or wires |
What is the unit of all forces? | Newtons |
What is upthrust? | It is the force pushing up on objects that are in a fluid (floating) e.g. a hot air balloon in air or a boat in water.
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What does displacement mean? | The overall distance travelled and direction from a starting point to a finish point, measured in a straight line.
It is a vector quantity |
What are typical speed values for walking, running, cycling and sound? | walking 1.5 m/s running 3 m/s cycling 6 m/s sound 330m/s |
What does speed mean? Is it a vector or scalar and what are the units? | The distance an object travels in one second. Unit = m/s It is a scalar quantity |
What is velocity? Is it a vector or scalar and what are the units? | Velocity is the (overall) speed in a given direction. It is a vector quantity and is measured in m/s |
When can you use the equation speed = distance/time? | ONLY when there is a steady speed (balanced forces, no acceleration).
Otherwise you must use SUVAT equations. |
What is acceleration? Is it a vector or scalar and what are the units? | Acceleration = rate of change of velocity. It is a vector (a negative acceleration means its decelerating). It is measured in m/s2 |
How can an object be moving at a steady speed but still be accelerating? | If moving in a circle. The speed will be constant but the direction will change. Acceleration and velocity are vector quantities so if direction changes, acceleration changes. |
How do you find the speed from a distance-time graph? What if it is a curved line? | It is the gradient (rise/run)
If curved, you need to draw a tangent to the line and then use it to calculate the gradient. |
How do you calculate the acceleration from a velocity-time graph? (or speed-time) | It is the gradient (rise/run) |
How do you calculate the distance travelled (or displacement) from a velocity-time graph? (or speed-time) | Area underneath the graph |
What is Newton’s First Law? | If there is no resultant force on an object, the object will remain at rest or at a constant velocity (speed and direction) if it is already moving. |
What is drag? How can you make it bigger? | The resistive force that an object travelling in a fluid experiences (sometimes called air resistance or water resistance). Make it bigger by: Increasing speed, surface area or putting it through a denser fluid
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What is inertia? | The tendency of objects to continue in their state of rest or of uniform motion |
What is Newton’s Second Law? | The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object. |
What is inertial mass? | The ratio of force over acceleration. It is a measure of how difficult it is to change the velocity of an object |
What is Newton’s Third Law? | Whenever two objects interact, the forces they exert on each other are equal and opposite.
Note: the 2 forces must act on 2 different objects for it to be a third law pair e.g. a person’s feet push down on the ground, so the ground pushes up on the person’s feet by the same amount (the reaction force) |
What is the normal contact force? | If object A is pushing on object B, object B will push back on object A. The push back force is the normal contact force. It is always at 90° to the surface.
If object A pushes harder, object B will push back harder as well so forces are equal. |
What is friction? | The resistive force between two objects that are rubbing along each other (or potentially rubbing over each other if they are not moving).
It is always in the opposite direction to motion (or potential motion) |
What is thrust? | The force from an engine or another object e.g. sails, rowing ores, that pushes an object forwards. |
What is drag? How can you make it bigger? | The resistive force that an object travelling in a fluid experiences (sometimes called air resistance or water resistance). Make it bigger by: Increasing speed, cross-sectional area or putting it through a denser fluid |
What is an elastic material? | A material that returns to its original shape when the deforming force is removed |
What is an inelastic (plastic) material? | A material that does not return to its original shape when the deforming force is removed |
What is the limit of proportionality? | The point on a force-extension graph (for a spring) where the material stops behaving elastically and starts behaving inelastically. |
What is a directly proportional relationship and how is this shown on a graph? | The quantities go up in proportion to each other so if one doubles, the other doubles etc. 1. Straight line 2. Through (0,0) |
How do you find the spring constant from a force-extension graph? | The gradient |
What is the spring constant and what are its units? | It is a measurement of the stiffness of a spring.
Unit = N/m |