TOPIC 5 - FORCES

What is meant by ‘centre of mass’?

the point at which weight may be consider to act/ appears to be concentrated

Explain one property of the spring that makes it suitable for use in the balance

deformes elastically; so will return to its original length shape/ compression is directly proportional to the force, so gives a linear scale

What is the force in a string holding two things together called?

tension

Give one variable that should have been a control variable in this investigation with a trolley and pulley

the combined mass of the trolley + mass hanger

Describe how the acceleration of the trolley can be calculated using the time taken and distance travelled by the trolley

• distance/ time taken to give average velocity

• double mean velocity to give max velocity

• divide the change in velocity by time taken (to give acceleration)

Explain how the acceleration was affected by raising the end of the runway

a component of the weight of the trolley acts parallel to the runway; so resultant force increases so acceleration increases (or work is done by raising the trolley so trolley gains gravitational potential energy) - transferred to kinetic energy, increasing the final velocity)

Explain how drinking alcohol would affect the stopping distance of a train

stopping distance includes both braking + thinking distance; alcohol incerases driver's reaction time; which will increase the tinking distance so stoppingD increases

Explain why the distraction will increase the stopping distance

stopping distance includes thinking distance; additional time before driver applies brakes; thinking distance increases

Explain why the temperature of the brakes increases as they are used

work is done due to friction in the brake; causing an increase in thermal energy

What is Newton’s Third Law of motion?

Every action has an equal but opposite reaction

What is Newton's First Law of motion?

any object at rest or at constant velocity will continue that way unless a resultant force acts on it

What is Newton's 2nd Law of motion?

F=ma (acceleration directly proportional to force)

Explain how the flexible bumper reduces the risk of injury to the people in the bumper car during the collision

increases the time taken for the collision to occur; so the rate of change of momentum decreases; reducing the force

factors that increase the braking distance of a car

• speed
• wet/icy road conditions
• poor condition of brakes; tyres
• increased mass of car
• negative gradient of the road (downhill)

Explain why the driver’s reaction time affects the thinking distance of a car

Distance = speed x time; so longer reaction time = longer distance

The average speed of a car between the cameras and the average velocity of the car between the cameras are different - why?

velocity is a vector + speed is scalar; and the road isn't straight; so direction changes, velocity changes

Why does the hailstone accelerate while falling?

there is a resultant force acting; since weight is greater than air resistance

Explain why the hailstone reaches terminal velocity

• as the velocity of the hailstone increases, air resistance increases
• until the air resistance becomes equal to the weight of the hailstone
• so the resultant force is equal to 0

Explain the difference in the maximum kinetic energy of a hailstone with a mass of 10 g and a hailstone with a mass of 20 g

• KinE depends on mass + velocity
• as mass increases, so does terminal velocity
• so as mass doubles, kinetic energy increases more than a factor of 2 (since directly proportional to both)

How could the driver increase the moment about the pivot without increasing the size of the force?

apply the force further away from the pivot

The depth of the submarine increases. Explain what happens to the pressure on the submarine

• the height of the water above the submarine increases; which increases the force/ weight of the water acting on the submarine; so pressure increases

Which statement describes P-waves and S-waves?

P-waves are longitudinal; S-waves are transverse

Which seismic wave can travel through liquids?

only P-waves can

Describe the relationship between the distance from the earthquake to the seismometer and the time between the P-wave and the S-wave arriving

the distance is directly proportinal to the time between the two waves arriving at the resismometer

Explain why the forces on the brick at the bottom of the pool cause the brick to be stationary

upthrusts acts upwards on the brick; the normal contact force acts upwards on the brick; weight is equal to upthrust + normal contact force

Describe how the force on the pedal causes a moment about the rear axle

the force on the pedal causes a omement about the pedal axle; which exerts a force on the chain; which causes a moment about the rear axle

What is meant by ‘momentum is conserved’?

total momentum before = total momentum after

Explain how the protective pads help to reduce injury when the players collide

increase the time taken to stop; so the rate of change of momentum decreases; reducing the force

What is inertia?

the tendency of an object to continue in its state of rest or motion

Explain why the egg is less likely to break when dropped onto soft foam rather than onto a concrete floor

• the soft foam increases the time taken to stop
• decreases the rate of change in momentum
• reducing the force on the egg

Explain the changing motion of the skydiver in terms of the forces acting on the skydiver

• initially air resistance is less than weight; gravity so the skydiver accelerates
• acceleration causes the air resistance to increase
• resultant force decreases to 0
• so the skydiver falls at terminal velocity

Explain why jumping from a higher height allows a diver reach a greater speed?

the higher the altitude, the less dense the air; so the air resistance on the skydiver was less; so the skydiver was able to accelerate for longer before reaching a higher terminal velocity (accelerate for longer before AR = weight)

Explain why the window has been designed to have this curved shape

force from air pressure acting from inside to outside bigger than force acting inwards; so keeps the window in position

Explain why atmospheric pressure decreases with increasing altitude

air molecules colliding with a surface creates pressure; at increasing altitude, distance between the molecules increases/ fewer molecules; so number of collisions with surface decreases; so always less weight of air than below

Explain why it is easier to lower the ramp by pulling on the rope (above) rather than pulling on the handle (below)

• the perpendicular distance from pivot/ hinge to the force is greater; so a smaller force is required

The further a swimmer dives below the surface of the sea, the greater the pressure on the swimmer

increasing the depth, increases the height/ mass of water above the swimmer; increasing the weight/ force of water acting on the swimmer

Explain why adjusting the ruler to be straight was important

to reduce the error in measuring the extension of the spring; at an angle would make the measured extensions shorter

Describe one technique that you could have used to improve the accuracy of the measurements taken by the student with a spring and ruler

• use a set square to line up the bottom of the spring with the ruler scale
• attach a horizontal pointer to the bottom of the spring; so that the point goes across the ruler scale

In a weight against extension graph, when it stops being directly proportional, what happened?

the spring has been inelastically deformed; because went past its limit of proportionality

What is meant by the braking distance of a vehicle?

the distance travelled under the braking force

Explain why the stopping distance of the car increases if the driver is very tired

the reaction time will increase; increasing the thinking distance

thinking distance ∝ speed || braking distance ∝ speed - why is this wrong?

Yes - the speed increases the thinking distance by the same factor; but the braking distance increases by the squared factor? (10-20m/s -> 6 - 24m)

Why is it important that the sled is pulled at a constant speed?

If the sled accelerates the value for the constant of friction will be wrong

If the sled is pulled at an angle to the surface the value calculated for the constant of friction would not be appropriate - why?

only the horizontal component of the force would be pulling the sled forward; the vertical component lifts the sled, reducing the force of the surface on the sle