Topic 1 Force & Motions

Velocity and acceleration

Speed: How fast you are going (Measured in M/s or KM/H or MPH)

Velocity: Is speed but must also have a direction (30mph north)

• Objects travelling at a constant speed with a change in velocity. This happens when object changes direction whilst staying the same Speed.

Average speed = Distance ÷ Time taken

Example: A cat Skulls 20 Min 35 seconds.

Find the average speed

Average speed = 20 ÷ 35 = 0.57

How long does it take to skull 75m

75 ÷ 0.57 = 1325

1325 seconds in minutes is = 2 mins 12 seconds

Acceleration:

• Measured in M/s²

• How Quickly the velocity changes (Increasing or decreasing)

• Change in velocity can be speed or Direction or both

Acceleration = Change in velocity ÷ Time taken

v² = u² +2as

• v: Final velocity

• U: Initial Velocity

• S: Is distance traveled while accelerating or deaccelerating

Distance Time and velocity time graph

Distance time graph:

• Flat: Stopped

• Steep: Going fast

• Curve: Positive or negative acceleration

Shows:

• Distance

• Time

• Speed

Velocity Time graph:

• Gradient: Acceleration

• Flat: Steady speed

• Steep: Greater acceleration or deacceleration

• Area under graph: Distance coverd

• Curve: Change in acceleration

Three Laws of motion

First Law - Balanced forces means no change in velocity

• As long as the forced of the object are balanced. Then it will stay still

• But if moving, It will carry the same velocity. As long as force is balanced

Second Law - A result force means acceleration

• If there is a unbalanced force, Then the object will accelerate in that direction

• It is called a resultant force , it always will produce acceleration or deacceleration

Never say : “If something’s moving there must be an overall resultant force.”

Resultant Force

• The bigger the force the greater the acceleration or deacceleration.

• The bigger the mass the smaller the accelerations.

• To get a big mass to accelerate as fast as a small mass it needs a bigger force than the mass.

• In most situations there are always at least 2 forces acting on an object in any direction. The overall affect of the 2 forces will change the objects motion.

Equation :

• Resultant Force = Mass X Acceleration

• Force = Mass X Acceleration

Third Law - Reaction forces

• If Object A exerts a force on object B then object B exerts on an equal and opposite force on object A

• Meaning if you push something it will push back against you just as hard. And when you stop pushing it, it will do the same.

Example:

1. Skater A pushed on Skater B, Their is an equal “Reaction force”

2. Both feel the same size of force in opposite directions so they will accelerate away from eachother

3. Although skater A will accelerate more than B becuase she has a smaller mass. And in this context (Stater A is 55 kg and skater B is 65 kg)

Combining Forces

Vector

• Physical Quantity that has size and direction (Magnitude and direction)

• Force, Velocity, Acceleration , Momentum ect…

Scalar

• Physical quantity have ONLY size NO direction (Magnitude)

• Mass, Temperature, Time, Length ect…

To work out a resultant force you must combine vectors by subtracting or Adding forces

Example:

Engine thrust + Friction = Resultant force

(22000 N, east) (8000 N, West) (14000 N, east)

Terminal Velocity

• Frictional force increases with speed. But only up to a certain point

1. When an object starts falling i has much more force accelerating it than resistance slowing it down

2. As the velocity increases, The resistance builds up

3. This resistance force gradually reduces the acceleration until the two forces (Resistance and acceleration) are equal. The object cannot accelerate after that meaning terminal velocity has been reached.

Effects on resistance

• small Surface area - Smaller area means that more force can pull the object down leading to its acceleration being faster

• Large surface area - Means there is more air resistance wich slows down the acceleration so terminal velocity is reached faster

Hook’s Law

1. Extension is proportional to force

• If metal wire is pulled down by a force, It stretches. The weight pulling down is equal to the support holding it at the top.

• The Extension of a stretched wire is proportional to the load or force

2. Hooks law stops working when force pulling is great enough

• There is a limit to extension

• When force is great enough the curve in a graph starts to form to represent its limit being reached

• This is called elastic limit and it will be permanently stretched and deformed

• A martial cannot return to its original shape after elastic deformation

Stopping distance

• Thinking distance: Distance car travels in the time between driver notching hazard and applying the brakes

• Braking distance: Distance car travels during its deacceleration whilst breaking

Effecting factors

Equation:

Stopping distance = Thinking distance + Braking distance

Momentum and Collisions

Momentum

• The greater the mass of an object and the greater its velocity , The more momentum the object has.

• Momentum is a vector quantity

Equation :

Momentum = Mass X Velocity

Momentum before is equal to momentum after

• Momentum is conserved when no external forces act - Total momentum after is the same as it was before.

How to calculate momentum

1. Choose which direction is positive (Positive means to the right)

2. Find the momentum before collision (Mass x Velocity) add them if needed

3. Total momentum after Collison (Add the masses)

4. Put it into an equation (Before = after) And solve for the missing units

Force causes change in momentum

• When a force acts on an object, it causes a change in momentum

• A larger force means a faster change of momentum ( and so a greater acceleration)

• Likewise, if someone’s momentum changes very quickly the force on the body will be very large, and more likley to cause injury

• This is why cares are designed to slow people down over a period of time.

Turning effect and center of gravity

1. The force of a spanner causes a turning effect or moment on the nut. A larger force would mean a larger momentum

2. Using a longer spanner, the same force can exert a larger moment becuase the distance from the pivot is greater.

3. To get maximum moment you need to push at a right angle perpendicular to the spanner.

The center of gravity hangs directly below the point of suspension

1. Think the center of gravity of any object as the point through which the weight of a body acts.

2. A freely suspended object will swing until its center of gravity is below the point of suspension

• You can find the center of gravity of a non symmetrical object by hanging it from the top. waiting for it to stop swinging. Once it stops it should be in the center directly under where it is hung.

Principle of moments

If an object is balanced then the total anticlockwise moments are equal to the total clockwise moments

Forces are not always equal

1. If a light rod is being supported at both ends, the upwards force provided by each support won’t always be the same.

2. If a heavy object is placed on the rod, the support closest to the object will provide a larger force

3. therefore if they have diffrent masses on each end, we can balance it by using distance.

If the total anticlockwise moments do not equal the total clockwise moments, there will be a resultant moment, so it will turn.