Newton’s Second Law

Aims: To know Newton’s second law.

To be able to use it to solve problems.

Newton’s Second Law

Write your understanding of Newton’s second law on your white board.

DefinitionThe acceleration of an object is directly

proportional to the magnitude of the net force, in the same direction as the net

force, and inversely proportional to the mass of the object.F=ma

Combination of all forces

acting on the object

Newton’s Second Law

The acceleration of an object is directly proportional to the magnitude of the net force,

in the same direction as the net force, and inversely proportional to the mass of the object.

Forces are always measured in Newtons (N)

Multiple Choice Questions

• Try the 4 worksheet questions.

• For each question set up an equation using F=ma.

• Then solve your equation to find the unknown values.

Extra QuestionA water-skier of mass 80kg is towed over a straight 100m run of water. The tension in the horizontal towline is constant at 300N. The resistnace to the motion of the skier is 140N. Given that the skier takes 6.8 secs to complete the run calculate her speed at the start of the run.

Exam Question with Vectors

Weight and Mass• Mass measured in kg• Weight measured in N• Mass is the amount of matter contained in a

body.• Weight is the force which acts on a mass in a

gravitational field.• Weight = mass x gravitational acceleration• W = mg

mg

Weight, Mass and ElephantsWhat happens to their mass?

What happens to their weight?

If an elephant stepped on you on the moon what would the difference be to the Earth?

What if it charged you?

Normal Contact Force• If an object is at rest then the net forces must be

zero.• If weight is acting down then something must be

acting upwards.• If the object is on a surface this force is called the

Normal Reaction Force or Normal Contact Force.

mg

R

3. A package of mass 8 kg is lowered by means of a vertical cable with a downward acceleration of 2 ms-2. Find the tension in the cable.

4. Jack and Jill went up the hill to fetch a pail of water. The pail had a mass 5 kg when empty and 15 kg when full. The pail was lowered into the well at a constant acceleration of 4 ms-2. When the pail was full it was raised at a constant speed 3 ms-1. Neglecting the weight of the rope, find the tension in the rope (a) when the pail was being lowered, (b) when the full pail was being raised.

2. Find the acceleration of the block in each case.

More Contact ForcesThe pilot of a hot-air balloon has mass 85kg. As the balloon leave the ground the normal contact force between the pilot and the floor increases to 901N. Calculate the acceleration of the balloon.

And finally....

•http://www.youtube.com/watch?v=uaZT6iMogKs