Importance of friction Friction between two surfaces may cause a moving object to slow down. For example, a book sliding along a table, or a hockey puck on the ice rink. Clearly, the nature of the two surfaces in contact, as well as the weight of the sliding body, determines the magnitude of the frictional force.
This type of friction can also allow people or cars to accelerate. Friction allows tyres to grip the road or the souls of your shoes to grip the path.
Ice hockey players make use of the fact that there is very little friction on ice
SAMPLE PROBLEM 3H
A sled of mass 60 kg is being pulled along the snow with a tractive (pulling) force of 500 N.The resistance to its motion is 80 N. (i) What is the resultant force on the sled?
(ii) What is its acceleration? SAMPLE ANSWER 3H
(i) Resultant force Tractive force Friction F T Fr F 500 80 420 Resultant force 420 N
(ii) Using the formula for force, making sure all values are in their standard unit: The acceleration of the sled is 7 m s2.
F maQ420 60aQa 7 EXERCISE 3.3 FORCE
Q1 A 2 400 kg car accelerates from 60 m s1 to 80 m s1 in 8 s. Calculate: (i) the change in momentum of the car (ii) the acceleration of the car (iii) the force required to cause this acceleration.
Q2 Calculate the force required to decelerate an aeroplane of mass 1.6 105 kg from 250 m s1 to 150 m s1 in 40 s.
Q3 A train of mass of 4.5 105 kg is accelerating at 0.6 m s2 while experiencing a constant frictional force of 8 104 N. What is the tractive force of the engines?
