Newton's Laws Revision Set 2

A parachutist is falling through the air at terminal velocity. Which statement about the parachutist is correct?

• Every force acting on the parachutist is equal to zero and his acceleration is equal to zero.

• Every force acting on the parachutist is equal to zero and his velocity is equal to zero.

• The resultant force acting on the parachutist is equal to zero and his acceleration is equal to zero.

• The resultant force acting on the parachutist is equal to zero and his velocity is equal to zero.

Answer explanation

At terminal velocity, drag = weight, so zero resultant force so zero acceleration

A stone of mass 0.12 kg is fired from a catapult. The velocity of the stone changes from 0 to 5.0 m / s in 0.60 s.

What is the average resultant force acting on the stone while it is being fired?

• 1.0 N

• 2.5 N

• 3.6 N

• 8.3 N

Answer explanation

acceleration = change in velocity ÷ time

5 m/s ÷ 0.6 s

Force = mass x acceleration

= 0.12 kg x 5 m/s ÷ 0.6 s

= 1 N

A ball of mass 0.16 kg is moving forward a velocity of 0.50 m/s. A second ball of mass 0.10 kg is stationary. The first ball strikes the second ball. The second ball moves forwards at a speed of 0.50 m/s. What is the velocity of the first ball after the collision? (Hint: conservation of momentum)

• 0.0 m/s

• 0.19 m/s

• 0.31 m/s

• 0.50 m/s

Answer explanation

Momentum before collision

= 0.16 kg x 0.5 m/s = 0.08 kgm/s

Momentum after collision = 0.08 kgm/s

(0.16 kg x ? m/s) + (0.1 kg x 0.05 m/s) =

(0.16 kg x ? m/s) + 0.05 kgm/s

0.16 kg x ? m/s = 0.03 kgm/s

velocity = momentum ÷ mass

= 0.03 kgm/s ÷ 0.16 kg

= 0.1875 m/s

A student cycles along a level road at a speed of 5.0 m/s. The total mass of the student and bicycle is 120 kg. The student applies the brakes and stops. The braking distance is 10 m. What is the average braking force? (Hint work out acceleration from v & s)

• 150 N

• 300 N

• 15 000 N

• 30 000 N

Answer explanation

(final velocity)²-(initial velocity)²=

2x acceleration x distance

Rearranging to make a the subject:

a = (v²- u²)/2s v = 0m/s, s = 10 m

a = -25/20 = 1.25 m/s²

force = mass x acceleration

= 120 kg x 1.25 m/s² = 150 N

What is the unit of power?

• joule

• newton

• pascal

• watt

Answer explanation

watt, (W) = J/s

A box of mass 2.0 kg is pulled across the floor by a force of 6.0 N. The frictional force acting on the box is 1.0 N.

What is the acceleration of the box?

• 0.40 m/s²

• 2.5 m/s²

• 3.0 m/s²

• 3.5 m/s²

Answer explanation

Resultant force on box = 5 N (6-1)

acceleration = force ÷ mass

Which moving body has a resultant force acting on it?

• a diver rising vertically through water at constant speed

• an aircraft circling an airport at constant speed

• a train going up a straight incline at constant speed

• a parachutist descending vertically at terminal velocity

Answer explanation

A resultant force is needed to change velocity (SPEED and/or DIRECTION)

A car, travelling on a straight horizontal road, has 1.6 MJ of kinetic energy. It accelerates for 20 s until it has 2.5 MJ of kinetic energy. What is the average power output used to increase the kinetic energy of the car?

• 45 W

• 205 W

• 45 kW

• 205 kW

Answer explanation

Power = energy transferred ÷ time

= 900 000 J ÷ 20 s = 45 000 W = 45 kW

A steel ball is dropped from the top floor of a building. Air resistance can be ignored. Which statement describes the motion of the ball?

• The ball falls with constant acceleration.

• The ball falls with constant speed.

• The ball falls with decreasing speed.

• The ball falls with increasing acceleration.

Answer explanation

If gravity is the only force acting then the object accelerates at a constant 10 m/s²

A car travels in a straight line along a horizontal road at constant speed. Three horizontal forces act on the car. The diagram shows two of these forces.

What is the size and the direction of the third horizontal force acting on the car?

• 1200 N backwards

• 1200 N forwards

• 1800 N backwards

• 1800 N forwards

Answer explanation

The car is moving at constant velocity so there must be zero resultant force - the total backward force must be the same size as the forward force.