3.4-3.7 Forces and Newtons laws Part 1

Newton

One newton is the force that causes a mass of one kilogram to have an acceleration of one metre per second.

Weight

The weight of an object is the gravitational force acting on the object.

Drag (Force)

The resistive force experienced by an object moving through a fluid.

Terminal Velocity

The constant speed reached by an object when the drag force (and upthrust) is equal in magnitude and opposite in direction to the weight of the object.

(At terminal velocity, drag+ upthrust = weight.)

Moment of a Force

The force multiplied by the perpendicular distance from a specified point.

Principle of Moments

If an object is balanced, then the sum of the clockwise moments about a pivot is equal to the sum of the anticlockwise moments about the same pivot.

Equilibrium

At equilibrium, the resultant force on the body is zero and the resultant moment on the body is zero.

Couple

A couple is a pair of equal and parallel but opposite forces, which tends to produce rotation only.

Torque of a Couple

The force multiplied by the perpendicular distance between the forces.

Centre of Gravity

A point where the entire weight of an object appears to act.

Density

Mass per unit volume.

Pressure

Normal force exerted per unit cross-sectional area.

Upthrust

The upward buoyant force exerted on a body immersed in a fluid.

Archimedes’ Principle

The upthrust on an object in a fluid is equal to the weight of fluid it displaces.

The engines of a spaceship develop constant thrust. Explain why the acceleration of the spaceship increases after lift-off.

• Consider the equation F = ma;

• The mass (m) of the spaceship decreases as it burns fuel;

• The weight of the spaceship decreases as it burns fuel so resultant force (F) on spaceship increases;

• The weight of the spaceship decreases because g decreases with increasing altitude so resultant force (F) on spaceship increases.

Describe an experiment to determine the centre of gravity of a metal plate.

• Suspend the plate from a point;

• Use a plumb line to mark a vertical line on the plate from the point;

• Suspend the plate from another point;

• Use the plumb line to mark a vertical line on the plate from the second point;

  Where the lines intersect gives the position of the centre of gravity.

State why the equation 'F = ma' cannot be applied to a particle travelling at very high speeds.

• The mass of the particle increases:

• as its speed approaches the speed of light.

State the factors which affect the magnitude of the drag force acting on an object falling through air.

• Frontal area of object;

• Speed/velocity of object (relative to the air);

• Surface texture / aerodynamic shape/streamlining of object;

• Viscosity of air / air temperature / air density.

A skydiver jumps from a stationary hot-air balloon several kilometres above the ground.

Describe and explain the motion of the skydiver in terms of acceleration and forces:

[i] immediately after jumping;

• speed = 0;

• Therefore magnitude of drag = 0;

• Hence, resultant force = weight;

• and so acceleration = g, the acceleration due to gravity.

[ii] at a time before terminal velocity (during free-fall) is reached;

• speed has increased;

• the magnitude of drag has increased (drag is directly proportional to speed²);

• the magnitude of the resultant force has decreased (resultant force = weight - drag);

• resultant force F = ma therefore acceleration is less than g.

[iii] at terminal velocity (during free-fall).

• speed has increased to constant maximum value;

• the magnitude of drag has increased (drag is directly proportional to speed²);

• resultant force = 0 because weight = drag:

• resultant force F = ma therefore acceleration = 0.

The skydiver opens the parachute. Describe and explain the motion of the parachutist in terms of acceleration and forces:

[i] immediately after the parachute has opened;

• the magnitude of drag has greatly increased (because of large area of parachute);

• resultant force is large and vertically upwards (resultant force = weight - drag);

• resultant force F = ma therefore acceleration is large and vertically upwards.

[ii] at a time before terminal velocity (with parachute open) is reached;

• speed has decreased;

• the magnitude of drag has decreased (drag is directly proportional to speed²);

• the magnitude of the resultant force has decreased (resultant force = weight - drag);

• resultant force F = ma therefore magnitude of acceleration has decreased;

[iii] at terminal velocity (with parachute open).

• speed has reached a constant value;

• the magnitude of drag has reached a constant value

• resultant force = 0 because weight = drag;

• resultant force F = ma therefore acceleration = 0.

The graph of velocity against time for the free-fall period is shown below. Suggest the changes to the graph for a more massive skydiver of the same shape.

• The terminal velocity increases;

• Initial gradient is the same (initial a = g);

• Time taken to reach terminal velocity is longer.

State two factors that affect the braking distance of a car. Describe how each factor affects the braking distance.

• Greater speed means greater braking distance (braking distance proportional speed²);

• Greater mass means greater braking distance (braking distance proportional mass);

• Worn tyres / brakes implies less friction and greater braking distance;

• Wet/slippery/icy road means less friction and greater braking distance;

• Downhill means greater braking distance.

State two factors that affect the thinking distance of a driver. Describe how each factor affects the thinking distance

• Consumption of drugs / alcohol increases thinking distance;

• Driver fatigue increases thinking distance.

Describe how wearing a seat belt and having an airbag in a car can help to protect the driver from injury in a head on collision.

• Increasing the time/distance (for the driver) to stop;

• so smaller deceleration of the person;

• Decreases the (impact) forces on the driver because F = ma and a is smaller;

• Prevents driver from hitting the steering wheel / windscreen;

• (Deflates quickly to prevent whiplash);

• (Much wider area of the bag reduces pressure).