Mechanics - Forces Flashcards
Define a force
Pushes, pulls or twists.
How do forces change objects?
Forces can change an object’s motion and shape.
- Change in motion = either change in speed or direction of motion. Change in speed = acceleration/deceleration.
- Sometimes forces can change both motion and direction. Eg when a ball is thrown up, gravity pulls downwards on the ball (weight is the force), so it slows down on the way up, changes direction and accelerate downwards back onto the ground.
How to describe forces
- label the force
- label/state direction of force (using an arrow)
- show the size of the force
- UNIT = N, newtons.
What is net force?
When there is more than one force acting on an object, it is the combination of forces that affects the object’s motion.
The combined force is called net force. F_net.
- Forces acting in the same direction are added together
- Forces acting in opposite directions are subtracted to give the net force.
What is thrust?
It is a mechanical force, meaning the propulsion system must be in physical contact with the object
Newton’s Third Law
If object A exerts a force on object B, then object B will exert an equal and opposite force on object A.
Eg. when you stand on the ground the force you are applying to the Earth is your weight, so Earth is also supporting you with an equal and opposite force so you aren’t moving vertically.
When you throw a ball up, you are exerting thrust onto the ball and we can see it accelerate due to this force. However, the ball also exerts an equal and opposite force to your hands, so your hands/arms (whole body) also move in the opposite direction. However, due to f=ma, and the two forces being equal, the ball has a smaller mass than your arm therefore it will accelerate more than your hand will.
When you push a trolley, the trolley is pushing you back with the equal and opposite force. However you do not move because the opposite force is not able to overcome your weight and friction (between you and the ground). Friction is a contact force which opposes motion, and in this case, the friction force is greater than the thrust force causing the motion so you do not move.
Newton’s second law
The acceleration of an object is directly proportional to the unbalanced external force and is in the same direction as that force.
F_net = ma
When the forces acting on an object are unbalanced, the net force will make the object accelerate. If the unbalanced force is in the same direction as motion, the accelerations will be positive. If the unbalanced force is against the motion, the acceleration will be negative, therefore a deceleration.
Units of F = ma
Force = N, newtons m = kg a = ms^-2
Mathematical relationship of net force (unbalanced force)
F_net is proportional to mass and acceleration. F_net increases as mass and acceleration increases.
This means that when different net forces are applied to objects of the same mass, a greater net force will produce a greater acceleration.
Also, when the same force is applied to objects of different masses, the smaller mass has the greater acceleration.
Labelling opposite forces
Use “-“ to represent force in the opposite direction but of the same size.
Eg upward force = -mg because weight is mg
Newton’s first law
An object will remain at rest or constant velocity unless acted upon by an UNBALANCED external force
What is friction?
A REACTION contact force which opposes motion.
Friction is created as soon as something starts SLIDING past another object. It is a reaction force as friction doesn’t exist until the sliding starts.
What is the difference between mass and weight?
Mass is the amount of material in an object (and therefore stays the same wherever the object is) whereas weight is the force with which an object pushes down on to whatever it is resting on. WEIGHT IS CAUSED BY GRAVITY PULLING AN OBJECT DOWNWARDS.
UNITS : mass in kg or g | weight in N
How to calculate weight?
weight = mg
This means weight is calculated by multiplying mass by the STRENGTH OF GRAVITY. The Earth attracts EACH kilogram towards itself with a force of 10N.
Therefore g = 10N kg^-1
The value of gravity is often called the acceleration of gravity. This is because if an objet is dropped it accelerates downwards with an acceleration that has the same value as gravity, ie at 10ms^-1.
Relationship between friction and an object
Friction always opposes the motion of the object.
The more the two sliding surfaces press together, the more friction generated. The rougher the sliding surfaces, the greater the friction that is generated (because there are more irregularities in the surfaces meaning the “peaks” of one surface can fall into the “valleys” of the other, meaning that to keep moving either something needs to break, or the surfaces would need to push apart briefly)
