Forces Y11

Newton’s 1st law extra

constant velocity = balanced forces

changing velocity = unbalanced forces

What is an action reaction pair?

• happens when things collide like a footballer’s head with a ball → (footballer’s head exerts a force on ball and ball exerts force on footballer’s head)

• action reaction forces are the same size but they don’t have the same effort on two objects because the objects are diff masses

stopping distance

the distance a vehicle travels between the driver seeing a hazard and the vehicle coming to a complete stop

equation for stopping distance

thinking distance + braking distance

(distance travelled during reaction time) (distance travelled between driver beginning to break and coming to a stop)

factors that increase thinking distance

• going fast (going faster increases the distance you travel per sec)

• tiredness

• alcohol

• depressant drugs

• distractions

(those factors increase the reaction time)

factors that increase braking distance

• going faster

• mass of vehicle

faster/heavier cars have more kinetic energy so the brakes will have to do more work to stop the car

• poor road conditions

• worn tyres

• worn brakes

reduce the friction so less work is done each second & the car takes longer to stop

Explain why braking causes an increase in temperature of the brakes (2)

• braking causes friction

• work done on brake which transfers energy to them which increases thermal energy store

speed time graph

• reaction time = time before brakes are hit (straight line )

• area= distance travelled

• diagonal line is braking time

• during the first 100s of the athlete was running at a constant speed of 4 m/s

• as they’re travelling in a circle the direction is changing so velocity changes

equation for acceleration

a = v - u /t

equation for acceleration and distance

2 a s = v²- u²

The skydiver accelerated until reaching terminal velocity

Explain why the skydiver reached terminal velocity. (4)

• as the skydiver fell velocity increases

• the faster the skydiver falls the higher amount of the air resistance

• the forces are balanced as the resultant force is 0 newtons so acceleration will be 0m/s

• the skydiver has reached terminal velocity as speed (velocity) is constant and they cannot go any faster

force and elasticity graph

shows how extension changes when force is applied to a spring

Hooke’s Law

• extension of spring is directly proportional to force applied (provided limit is not exceeded)

• when a force stretches/compresses spring → work is done on spring and e.p.e is stored in spring

elastic deformation

temporarily (changes shape)

plastic deformation

permanently (changes shape)

equation for force using spring constant and extension

F = k x e

equation for elastic potential energy

E = ½ k e²

Practical that shows how forces affect the extension of a spring

1. Secure a clamp stand to the bench using a G-clamp or a large mass on the base.

2. Use bosses to attach two clamps to the clamp stand.

3. Attach the spring to the top clamp, and a ruler to the bottom clamp.

4. Adjust the ruler so that it is vertical, and with its zero level with the top of the spring.

5. Measure and record the unloaded length of the spring.

6. Hang a 100 g slotted mass carrier - weight 0.98 newtons (N) - from the spring. Measure and record the new length of the spring.

7. Add a 100 g slotted mass to the carrier. Measure and record the new length of the spring.

8. Repeat step 7 until you have added a total of 1,000 g.

Steps to ensure accuracy on a practical that shows how forces affect the extension of a spring

• use higher resolution ruler

• hold ruler vertical

• use a pointer/marker at the end of the spring

resultant force

• single force acting on an object

• summarises all the forces acting on an object → can cause objects to change speed, direction or shape

when vectors are at a right angle

1. draw vectors tip-to-tail

2. draw in the resultant (complete triangle) 

3. use pythagoras to find magnitude of the resultant vector

4. use trig to find a relative direction (angle from horizontal/vertical)

momentum

• all moving objects have it 

• the greater the momentum the harder the object is to stop or change direction

momentum equation

p = m x v

conservation of momentum

in a closed system the total momentum before an event and a total momentum after an event are equal

Impact forces word equation

force = change in momentum / time

impact forces

the greater the rate of change of momentum then the greater the impact

Ft = m (change in) x v

inertia

is the property of an object that describes the resistance to change motion

car safety features

• front crumple zone → front of car designed crumple dissipating energy of impact slowing down the car

• anti-lock braking system 

• side impact bars

• air bag → automatically inflated with compressed air within 1/100th of a second on impact to cushion passengers

• safety belts → inertia reel safety belts lock to keep wears tightly pressed to their seats when a car decelerates rapidly

• rear crumple zone → trunk of saloon car crumples protects passengers from rear impact

Moment

turning effect of a force

moment of force about any point is → force x distance from turning point to line of action of force

equation for moments

moment = F x d

unit Nm

what is a turning point called?

pivot

centre of mass

• weight of object may be considered to act at a single point → object’s ‘centre of mass’

• centre of mass of symmetrical body is along axis of symmetry

• if suspended a body will come to rest with its centre of a mass directly below the point of suspension

principle of moments

• when an object isn’t turning(e.g) balanced the total clockwise moment equals total anticlockwise moment i.e equilibrium

lever (principle of moments)

applies a smaller effort force further from the pivot to create a larger output force closer to the pivot e.g hammer, car jack, arm

A student is struggling to loosen a bolt using a 10 cm spanner. Explain how they could increase the moment acting on the bolt and why this would help. (3)

• To increase the moment, the student should increase the distance from the pivot by using a longer spanner.

• A longer spanner means a larger perpendicular distance, so the same force produces a bigger moment.

• This larger moment makes it easier to turn the bolt and loosen it.

What are gears?

Gears are wheels with toothed edges that rotate on an axle or shaft. Their teeth interlock so one gear turns another.

How do gears affect direction of rotation?

 When two gears mesh, they always rotate in opposite directions.

What happens when the driven gear is larger?

It rotates more slowly but produces a greater moment (low gear ratio).

What happens when the driven gear is smaller?

It rotates faster but with a smaller moment (high gear ratio).

pressure in fluids ( liquids & gases)

causes a force normal to ( at right angles) any surface

the pressure at the surface of a fluid can be calculated using a diff equation

pressure = force normal to a surface / area of that surface

• pressure increases as depth increases

• this because the weight of the column of liquid above increases with depth

• therefore water at the bottom hole comes out with more force and shoots further

Why does the thickness of dam increase with depth? (3)

• pressure of water increases with depth → high amount of pressure at bottom of water

• the force acting at a right angle to the container to increase with depth

• wall at bottom of dam needs to be thicker to provide a larger reaction force inwards to balance outwards force → withstand higher amount of pressure

pressure due to a column of liquid can be calculated using the equation:

• pressure = height of column x density of liquid x gravitational field strength

• p=hpg or change in pressure = p x g x change in height

How does atmospheric pressure change with altitude?

Atmospheric pressure decreases as altitude increases.

Why is air pressure higher at ground level than at the top of a mountain?

• air molecules collide with surface creating pressure

•  here is more air (particles) above you at lower altitudes,

• creating a larger weight of air and therefore higher pressure.

• as pressure is caused by greater mass of air above

How is pressure in gases similar to pressure in liquids?

In both, pressure increases with depth because particles exert force from above.

What causes upthrust on a submerged object?

The pressure at the bottom of the object is greater than the pressure at the top, creating a resultant upward force called upthrust.

• pressure increase with depth

• higher pressure below the garlic than above garlic

• this means there’s a greater force on the bottom of the garlic than the top

• resultant force of these two is upwards causing upthrust